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Reionization Era Bright Emission Line Survey: Selection and Characterization of Luminous Interstellar Medium Reservoirs in the z>6.5 Universe

The Reionization Era Bright Emission Line Survey (REBELS) is a cycle-7 ALMA Large Program (LP) that is identifying and performing a first characterization of many of the most luminous star-forming galaxies known in the z>6.5 universe. REBELS is providing this probe by systematically scanning 40 of the brightest UV-selected galaxies identified over a 7-deg**2 area for bright 158-micron [CII] and 88-micron [OIII] lines and dust-continuum emission. Selection of the 40 REBELS targets was done by combining our own and other photometric selections, each of which is subject to extensive vetting using three completely independent sets of photometry and template-fitting codes. Building on the observational strategy deployed in two pilot programs, we are increasing the number of massive interstellar medium (ISM) reservoirs known at z>6.5 by ~4-5x to>30. In this manuscript, we motivate the observational strategy deployed in the REBELS program and present initial results. Based on the 60.6 hours of ALMA observations taken in the first year of the program (November 2019 to January 2020), 18 highly significant>~7sigma [CII] lines have already been discovered, the bulk of which (13/18) also show>~3.3 sigma dust-continuum emission. These newly discovered lines more than triple the number of bright ISM-cooling lines known in the z>6.5 universe, such that the number of ALMA-derived redshifts at z>6.5 already rival Lya redshift discoveries. An analysis of the completeness of our search results vs. star formation rate (SFR) suggests an ~79% efficiency in scanning for [CII] when the SFR(UV+IR) is in excess of 28 M_sol/yr. These new LP results further demonstrate ALMA's efficiency as a"redshift machine", particularly in the epoch of reionization.


INTRODUCTION
The first galaxies are thought to have started forming in the first 200-300 Myr of the Universe (Bromm & Yoshida 2011;Wise et al. 2012) and then rapidly built up their stellar mass. With existing telescopes, it has been possible to obtain a glimpse of galaxies forming close to this time (e.g., Zheng et al. 2012;Ellis et al. 2013;Oesch et al. 2014;Bouwens et al. 2015Bouwens et al. , 2016Bowler et al. 2015;Finkelstein et al. 2015;Stefanon et al. 2017a;Ono et al. 2018;Dayal & Ferrara 2018), with galaxies identified as far back as z ∼ 11 (e.g., Coe et al. 2013;Oesch et al. 2016), ∼400 Myr after the Big Bang.
The discovery of impressively massive galaxies in the first 1-2 billion years of the universe, including an especially massive 2 × 10 11 M passive galaxy at z=3.717 (Glazebrook et al. 2017), a ∼3×10 11 M star-forming galaxy at z = 6.9 (Strandet et al. 2017;Marrone et al. 2018), a z = 7.54 quasar with a supermassive black hole of mass 8×10 8 M (Bañados et al. 2018), and even a 10 9 M galaxy at z ∼ 11 Jiang et al. 2021), have reignited long-standing questions about how rapidly galaxies could have begun forming large numbers of stars. Besides these few well-known examples of z > 3 galaxies with particularly high masses (and other work by e.g. Díaz-Santos et al. 2016 andTanaka et al. 2019), other evidence for significant early star-formation include a few z > 6 galaxies with apparently older stellar populations (e.g., Hashimoto et al. 2018;Roberts-Borsani et al. 2020) and evidence for lower-mass z ∼ 4-8 galaxies with elevated stellar-mass-to-halo-mass ratios (e.g., Behroozi et al. 2013).
Until just recently, the only physical information we had on the early epochs of star formation for most massive galaxies is from sensitive near-IR observations from Hubble Space Telescope (HST), mid-IR observations from Spitzer, and some rest-UV spectroscopy from the ground. HST provides us with high spatial resolution images revealing newly formed stars not obscured by dust, while Spitzer probes line emission and the stellar population age in galaxies through features like the Balmer break. Quantifying the masses in z > 6 galaxies has proven to be particularly challenging due to the limited information in Spitzer/IRAC imaging observations and degeneracy between the impact of older stellar populations and strong nebular emission lines (e.g., Schaerer & de Barros 2009;Raiter et al. 2010;Roberts-Borsani et al. 2020). Rest-U V spectroscopy has resulted in the detection of Lyα and some high ionization U V lines in select z > 6 galaxies (e.g., Zitrin et al. 2015;Stark et al. 2017), suggestive of very young stellar populations with moderately hard radiation fields (Stark et al. 2015;Mainali et al. 2017;Schmidt et al. 2017;Mainali et al. 2018;Hutchison et al. 2019). While such radiation fields are likely important in driving the reionization of the Universe at z > 6, determining the typical stellar population properties of z > 6 galaxies has remained very challenging.
Fortunately, thanks to the improving capabilities of the Atacama Large Millimeter Array (ALMA), we can make significant progress characterizing the physical properties of massive star-forming sources in the early universe (e.g., Hodge & da Cunha 2020). ALMA efficiently allows for a probe of the redshift and dynamical state of galaxies in the reionization epoch using bright interstel-lar medium (ISM)-cooling lines like 157.74µm [CII] and 88.36µm [OIII] (e.g., Smit et al. 2018;Hashimoto et al. 2019;Tamura et al. 2019;Jones et al. 2021), while simultaneously probing the far-IR dust-continuum radiation (e.g., Capak et al. 2015;Bowler et al. 2018;Fudamoto et al. 2020;Schouws et al. 2021). This provides us with an essential measure of the star formation in galaxies obscured by dust. The effectiveness of ALMA in probing the physical properties of galaxies has been demonstrated by a few particularly noteworthy observational programs with ALMA, e.g., the ASPECS ALMA Large Program in cycle 4 probing the dust and gas in z > 0.5 galaxies over the Hubble Ultra Deep Field (e.g., González-López et al. 2020;Aravena et al. 2020;Bouwens et al. 2020;Decarli et al. 2020;Walter et al. 2020) and the ALPINE ALMA Large Program in cycle 5 targeting >110 z = 4-6 sources (Le Fevre et al. 2020;Béthermin et al. 2020;Faisst et al. 2020). By contrast, the number of sources in the reionization epoch which have been characterized in detail with ALMA has been modest (e.g., Watson et al. 2015;Maiolino et al. 2015;Inoue et al. 2016;Pentericci et al. 2016;Knudsen et al. 2017;Matthee et al. 2017Matthee et al. , 2019Laporte et al. 2017Laporte et al. , 2019Smit et al. 2018;Hashimoto et al. 2018Hashimoto et al. , 2019Tamura et al. 2019;Bakx et al. 2020;Schouws et al. 2021).
Until recently, the ALMA view of galaxies in the z > 6 universe has been restricted to those galaxies which show prominent Lyα emission, whether these sources be QSOs (e.g., Venemans et al. 2019Venemans et al. , 2020 or simply massive starforming galaxies, e.g., B14-65666 at z = 7.15 (Bowler et al. 2018;Hashimoto et al. 2019) or CR7 at z = 6.59 (Matthee et al. 2017). This is especially a concern for more massive galaxies ( 10 9 M ) which frequently do not show Lyα in emission at all (e.g., Stark et al. 2010;Schenker et al. 2014;Pentericci et al. 2018;Jung et al. 2020;Endsley et al. 2021b), giving us a potentially biased view of the characteristics of luminous galaxies at early times. Given such, it is clearly desirable for us to have an alternate strategy of selecting massive galaxies in the z > 6 universe.
Recently, it has been shown that scanning U Vbright galaxies for prominent ISM-cooling lines like [CII] 158µm or [OIII] 88µm can be a very efficient way to identify luminous galaxies in the z > 6 universe. This strategy appears to be most efficient (1) when targets are especially U V bright and (2) when the redshift of targets is particularly well constrained, e.g., from the position of the Lyman break or the position of strong nebular emission lines within the Spitzer/IRAC bands (Smit et al. 2015). Smit et al. (2018) used just an hour of ALMA observations to identify what were then two of the most luminous [CII] 158µm lines known at z > 6.5. Employing a similar spectral scan strategy to observe six more luminous z ∼ 7 galaxies with well-constrained redshifts, Schouws et al. (2022) found bright [CII] 158µm lines in three more z ∼ 7 galaxies with an 11-hour program (2018.1.00085.S). The Smit et al. (2018) and Schouws et al. (2022) program demonstrated how effective ALMA could be in scanning for extremely bright [CII] 158µm lines in U V -bright sources at z > 6.5 and effectively served as pilots for the ALMA Large Program we introduce here.
In parallel with these [CII] 158µm studies, there were simultaneous efforts exploring the use of the [OIII] 88µm line for spectral scans at z > 8, taking advantage of both the brightness of the line and its greater accessibility in band 7 at z > 8. The first detection of the [OIII] 88µm line in the z > 7 universe was by Inoue et al. (2016) at z = 7.21 in a Lyα-emitting galaxy. Hashimoto et al. (2018) then demonstrated the first successful scan for [OIII] 88µm at z > 8, securing a 7.4σ detection of line in the z = 9.1096 ± 0.0006 source MACS1149-JD (Zheng et al. 2012). Tamura et al. (2019) similarly made use of ALMA to successfully scan for the [OIII] 88µm line in a magnified Lyman-break galaxy behind MACS0416, MACS0416 Y1, finding a redshift of 8.3118±0.0003 based on a 6.3σ detection.
With multiple ALMA programs demonstrating the feasibility of using spectral scans to search for especially luminous ISM reservoirs at z > 6.5, the necessary preparatory work had been done to consider executing a much more significant survey program with ALMA for luminous ISM reservoirs in the z > 6.5 universe. It was in this environment that the Reionization Era Bright Emission Line Survey (REBELS) program (2019.1.01634.L) was approved for execution in cycle 7 as an extragalactic ALMA Large Program. The goal of the REBELS program is to create the first significant sample of especially luminous ISM reservoirs in the Reionization Epoch. REBELS is doing so by scanning for bright ISM cooling lines on a large sample of U V -bright (−23.0 < M U V,AB < −21.5) galaxies at z > 6.5, while simultaneously probing the dust-continuum flux of sources.
In this paper, we summarize the observational strategy employed by the REBELS ALMA Large Program and highlight some first results demonstrating the effectiveness of this strategy. This includes an overview of the target selection and major scientific goals. The standard concordance cosmology Ω 0 = 0.3, Ω Λ = 0.7, and H 0 = 70 km/s/Mpc is assumed for consistency with previous studies. SFR and stellar masses are quoted assuming a Chabrier (2003) IMF. All magnitudes are in the AB system (Oke & Gunn 1983).

U V -BRIGHT SAMPLE SELECTION
In this section, we motivate our use of a U V -bright selection for the REBELS ALMA LP, and then discuss procedurally how we identify the most robust set of targets for the program.
Briefly, the REBELS LP selects 40 U V -bright galaxies at z > 6.5 for spectral scan observations, drawing from a 7 deg 2 area with the deepest wide-area optical, near-IR, and Spitzer/IRAC imaging observations and features the 2 deg 2 COSMOS/UltraVISTA field, the 5 deg 2 VIDEO/XMM-LSS + UKIDSS/UDS fields, and an 0.2 deg 2 area composed of various HST legacy fields including CANDELS (Grogin et al. 2011;Koekemoer et al. 2011), CLASH , RELICS (Coe et al. 2019;Salmon et al. 2020), and various BoRG/HIPPIES pure parallel fields (Trenti et al. 2011;Yan et al. 2011;Bradley et al. 2012;Schmidt et al. 2014;Morishita et al. 2020;Roberts-Borsani et al. 2021). Sources are chosen both because of their U V luminosities ( Figure 1) and because of the tight constraints we have on their redshifts from the available imaging data ( Figure 2). Figure 3 illustrates the search fields used for constructing the REBELS sample as well as the distribution of the  -(upper ) Observed luminosity of the [CII] 158µm cooling line seen in galaxies at z ∼ 6-7 vs. the observed star formation rate of galaxies in the rest-UV. The plotted results are drawn from the ALPINE (Le Fevre et al. 2020;Béthermin et al. 2020; and Capak et al. (2015) z = 4-6 samples (blue circles) as well as the REBELS pilots Schouws et al. 2022) and other z > 6 sources (red circles: Matthee et al. 2019). The black solid line shows the et al. (2014) relation. Particularly noteworthy is the increase in the [CII] 158µm luminosities of galaxies as rest-U V SFRs increase from 5 to 30 M /yr. REBELS only includes sources with rest-U V SFRs in excess of 11 M /yr (dotted vertical line). The horizontal dotted line shows the approximate 5σ sensitivity limit adopted in the REBELS program. (lower ) The fraction of galaxies with [CII] 158µm luminosities in excess of 2 × 10 8 L (the typical sensitivity limit for observations taken as part of REBELS) vs. the unobscured SFR of galaxies at z > 6. A significant fraction of galaxies with rest-U V SFRs in excess of 10 M /yr and especially 20 M /yr have [CII] 158µm luminosities in excess of 2 × 10 8 L ( §2.1). It is these high SFR sources at z > 6.5 that the REBELS large program particularly targets in scanning for bright ISM cooling lines.
REBELS targets over the sky. Figure 4 illustrates the distribution of U V luminosities and redshifts for targets in the REBELS program and compares this distribution against those sources in the REBELS pilot programs, in the ALPINE program, galaxies with confirmed Lyα emission at z > 6.5, and sources in various HST legacy fields. A complete list of the targets in the program is provided in Table 1.

Prevalence of Massive Star-Forming Galaxies
Amongst U V -Bright Galaxies at z > 4 Over the last few years, it has become increasingly clear that many of the brightest known ISM-cooling lines at z 5.5 are found in especially U V -bright galaxies (M U V,AB −22 mag: ≥2 L * U V ). Examples include the ten z ∼ 5.5 sources targeted by Capak et al. (2015), two z ∼ 6.1 galaxies targeted by Willott et al. (2015), CR7 by Matthee et al. (2017), B14-65666 by , VR7 by Matthee et al. (2019), and the three z ∼ 6.1-6.2 galaxies targeted by Harikane et al. (2020). All show bright ( 2 mJy km/s) [CII] 158µm lines. Meanwhile, follow-up of z ≥ 6 galaxies with redshifts from Lyα have frequently revealed only moderate to low luminosity ( 1 mJy km/s) [CII] 158µm lines (Ouchi et al. 2013;Ota et al. 2014;Pentericci et al. 2016;Bradač et al. 2017;Carniani et al. 2018Carniani et al. , 2020, strongly suggesting that there may be an inverse correlation between Lyα equivalent width (EW) and the mass of the ISM reservoirs ( Figure  18 from Harikane et al. 2020). Given such, it seems the best way to obtain a representative sample of massive galaxies at z > 6 is to identify galaxies on the basis of their U V luminosity (rather than Lyα emission alone).
To quantify the relationship between the U V luminosity M U V of galaxies and their L [CII]158µm , we present a compilation of some recent results at z > 4 (Capak et al. 2015;Smit et al. 2018;Matthee et al. 2019;Béthermin et al. 2020;Schouws et al. 2022) in Figure 1. A conversion factor of 7.1 × 10 −29 L ν [M /yr/(ergs/s/Hz)] (M. Stefanon et al. 2022, in prep) is used to transform the U V luminosities of galaxies into SFRs. What is particularly striking here is the strong correlation we observe between L [CII]158µm and the unobscured SFR U V , with a particularly clear correlation around 10 M /yr. The fraction of sources with [CII] 158µm luminosities in excess of 2 × 10 8 L , the typical sensitivity limit achieved in REBELS observations, increases from 35% at 10 M /yr to 80% at 20 M /yr.
Because of the strong correlation between L [CII]158µm and M U V , one can very efficiently derive redshifts for galaxies which are bright in the U V by scanning for the [CII] 158µm ISM cooling line with ALMA, as demonstrated in the introduction by both pilot programs to REBELS Schouws et al. 2022).

Search Fields
In constructing a sample of 40 U V -bright galaxies to follow up with the REBELS LP, we made use of deep wide-area optical + near-IR observations over the ∼2 deg 2 COSMOS/UltraVISTA field (Scoville et al. 2007;McCracken et al. 2012), the ∼5 deg 2 UKIDSS/UDS + VIDEO/XMM-LSS fields (Lawrence et al. 2007), and a wide range of HST search fields, including CANDELS, CLASH, and the BoRG/HIPPIES pure parallel fields.
A significant fraction of the U V -bright targets for REBELS are drawn from the ∼2 deg 2 COS-MOS/UltraVISTA field. In constructing our source catalogs for this field, we made use of the very sensitive Y , J, H, and K s near-IR observations (both data release 3 and 4) from the UltraVISTA program (Mc-Cracken et al. 2012), the deep optical Subaru Suprime-Cam BgV riz observations from Taniguchi et al. (2005), the deep optical ugriyz observations from the CFHT Deep Legacy survey ), and the ugrizy Subaru Hyper Suprime-Cam (HSC) observations (Aihara et al. 2018a(Aihara et al. , 2018b. Use of the Spitzer/IRAC 3.6µm and 4.5µm observations from SCOSMOS (Sanders et al. 2007), Spitzer Large-Area Survey with HSC (SPLASH: Steinhardt et al. 2014), SMUVS (Caputi et al. 2017;Ashby et al. 2018), COM-PLETE COMPLETE2 (Stefanon et al. 2018) programs were also made.
The bulk of the remaining targets are drawn from the ∼5 deg 2 UKIDSS/UDS and VIDEO/XMM-LSS area. The UDS/XMM-LSS area includes sensitive near-IR observations from both the UKIDSS/UDS (Lawrence et al. 2007: J, H, and K bands) and VISTA Deep Extragalactic Observations (VIDEO) programs (Jarvis et al. 2013: Y , J, H, and K s ), sensitive optical observations from the Subaru Hyper Suprime-Cam deep and wide-area programs (Aihara et al. 2018a(Aihara et al. , 2018b, and Spitzer/IRAC observations from the SPLASH and Spitzer Extragalactic Representative Volume Survey (SERVS: Mauduit et al. 2012) programs.
We also considered including U V -bright z > 6.5 galaxies from the CANDELS, CLASH, and BoRG/HIPPIES fields as targets in the REBELS program. In total, the CANDELS, CLASH, and BoRG/HIPPIES fields that we used for selecting targets covered an area of 0.2 deg 2 (excluding those HST CANDELS fields within COSMOS and UDS).

Selection of the Targets for REBELS
From each of these search fields, we considered a range of different catalogs in arriving at our final target list. Here we provide a brief summary of each sample we consider: Bowler et al. 2014Bowler et al. , 2017: The Bowler et al. (2014) z ∼ 7 selection was constructed based on deep z, Y , J, H, and K s observations obtained over the COSMOS/UltraVISTA and UKIDSS/UDS fields with Subaru Suprime-Cam + VISTA and Subaru, VISTA, and UKIRT, respectively. The deep Y + J and J band images over UltraVISTA/COSMOS and the UKIDSS/UDS fields, respectively, were used as the detection image in contructing source catalogs for the search. Then, after removing all sources detected at 2σ in the optical imaging data, the lephare photometric redshift software (Arnouts et al. 1999;Ilbert et al. 2006) was run, using Bruzual & Charlot (2003) models with a range of exponentially declining star-formation histories, metallicities of 0.2 Z and Z , a range of dust attenuation (A V < 4), Lyα EWs to 240Å, and Madau (1995)    , and REBELS-25 (right) by team members Rebecca Bowler, Ryan Endsley, and Mauro Stefanon (shown as the red, green, and blue lines, respectively) by applying the lephare, beagle, and eazy photometric redshift codes, respectively, to their own independent photometry PHOT1, PHOT2, and PHOT3, respectively. Three additional redshift likelihood distributions have been derived on the basis of each set of photometry by using the eazy photometric redshift code and a separate set of SED templates SED4 are shown as the dashed red, green, and blue lines, respectively. The thick black lines show the redshift likelihood distributions derived by averaging the results of the six separate likelihood distributions. galaxies in the neighborhood of their bright candidates and removing three cross-talk artefacts from their catalogs ( §3.2 of Bowler et al. 2017). In total, 22 bright z ∼ 7 galaxies were identified as part of the Bowler et al. (2014) and (2017) studies, with M U V luminosities ranging from −20.7 mag to −23.2 mag. Stefanon et al. 2017bStefanon et al. , 2019a: The selection of z ∼ 8-9 galaxies in Stefanon et al. (2017bStefanon et al. ( , 2019a was performed by first creating a large parent catalog over the COSMOS/UltraVISTA fields based on the Y , J, H, and K s images and then applying an optical non-detection and two color Lyman-break galaxy-like criteria. Stefanon et al. (2019a) then made use of the mophongo package (Labbé et al. 2006(Labbé et al. , 2013 to do careful optical, near-IR, and Spitzer/IRAC photometry for each source in the parent catalog, modeling and subtracting the flux from nearby neighbors to improve the robustness of the flux measurements. Candidate z ∼ 8-9 galaxies were then selected by running the eazy photometric redshift code (Brammer et al. 2008) on the optical+near-IR+Spitzer/IRAC photometry that had been derived. 18 z ∼ 8-9 candidate galaxies were identified by Stefanon et al. (2017bStefanon et al. ( , 2019a over the ∼2 deg 2 COSMOS field. Through similar fits of the source photometry to the SpecX dwarf star spectral libaries (Burgasser 2014), Stefanon et al. (2019a) explicitly verified that all sources were better fit (∆χ 2 > 1) by galaxy SED templates than dwarf star templates. Bowler et al. 2020 : Candidate z ∼ 8-11 galaxies were identified using the very deep ground-based optical and near-IR observations over ∼6 deg 2 the COSMOS/UltraVISTA and VIDEO/XMM-LSS + UKIDSS/UDS fields. Candidate z ∼ 8 galaxies were required to be detected at 5σ in either than J or H bands, while candidate z ∼ 9 galaxies were required to show a 5σ detection in either the H or K s /K bands. No detection (<2σ) was allowed for sources in all of the deep optical bands, as well as the Y band for z ∼ 9 candidates. The reality of sources showing 5σ detections in the UDS JHK data was tested by looking for similar 2σ detections in the VISTA data. The redshift likelihood distributions for candidate z ∼ 8-11 galaxies were then computed using a range of exponentially declining star formation histories, metallicities of 0.2 Z and Z , a range of dust attenuation (A V < 6), and the Madau (1995) IGM absorption. Bowler et al. (2020) identified 27 z ∼ 8-10 galaxies with U V luminosities M U V ranging from −23.7 to −21.2 mag. Endsley et al. 2021a: Candidate z ∼ 7 galaxies were identified over the deep ∼2 deg 2 COSMOS/UltraVISTA and ∼1 deg 2 UKIDSS UDS fields. Sources were detected from χ 2 detection images (Szalay et al. 1999) constructed from the deep yY JHK s HSC/UltraVISTA and yY JHK HSC/VIDEO/UKIDSS imaging observations. Endsley et al. (2021a) then applied the following color criteria: (1) z − y > 1.5, (2) z − Y > 1.5, (3) N B 921 − Y > 1.0, and (4) y − Y < 0.4. Sources were required to be detected at 5σ in at least one of the y, Y , and J bands and detected at 3σ in all three and undetected at <2σ in the HSC g and r bands. Possible T dwarf contaminants were removed by requiring either Y − J < 0.45 or both J − H > 0 and J − K s > 0. Endsley et al. (2021a) identified 50 sources over the COSMOS/UltraVISTA and UDS fields. Schouws et al. 2022: Candidate z ∼ 7 galaxies in Schouws et al. (2022) were selected from the ∼2 deg 2 UltraVISTA observations over the COSMOS field and required to show a >6σ detection in a stack of the Y , J, H, and K s images and a χ 2 opt < 4 (Bouwens et al. 2011). Best-fit photometric redshifts and redshift likelihood distributions were then derived with eazy using photometry of sources derived from the available CFHT + Subaru optical imaging data, UltraVISTA Y JHK s near-IR observations, and Spitzer/IRAC observations from SCOSMOS, SPLASH, and SMUVS programs.   The layout of the two wide-area ground-based fields + HST data sets used to identify z = 6.5-9.5 targets for the REBELS ALMA Large Program. The red circles indicate the position of REBELS targets, and the cyan circles indicate the position of targets in the REBELS pilot programs Schouws et al. 2022). The blue circles indicate the position of other sources in the literature that also meet the selection criteria for REBELS but already have substantial ALMA observations (Ouchi et al. 2013;Inoue et al. 2016;Matthee et al. 2017;Bowler et al. 2018;Hashimoto et al. 2019;Carniani et al. 2018). Also shown is the array of individual HST fields considered in the selection of targets for REBELS, including CANDELS GOODS South (Grogin et al. 2011), CLASH , various pure parallel fields (e.g., Morishita et al. 2018;Bridge et al. 2019), and RELICS (Coe et al. 2019). The position of the CANDELS COSMOS and CANDELS UDS fields within the COSMOS/UltraVISTA and UDS/XMM-LSS fields is indicated. The planned layout of the Webb-COSMOS survey (Kartaltepe et al. 2021) is also included on this figure.

CANDELS-UDS
sources were required to have an integrated z > 6 probability of >50% and to be better fit by a galaxy SED template than one of the SED templates from the SpecX dwarf star spectral library. Schouws et al. (2022) identified some 30 bright z ∼ 7 as part of their study, with M U V ranging from −23.0 mag to −21.4 mag.
This Paper (see also M. Stefanon et al. 2022, in prep): Finally, we considered separate selections of z ∼ 7 − 8 galaxies drawn from the ∼ 5 deg 2 VIDEO/XMM-LSS + UKIDSS/UDS fields generated using procedures similar to those described in Stefanon et al. (2019a). The detection was performed on the combination of the J, H, and K S mosaics, either from the UKIDSS/UDS or VIDEO programs. Candidates at z ∼ 7 were selected through the z − Y > 0.7 mag and Y J − JH < 0.5 mag color criteria, where .. denotes the average flux density in the two indicated bands. To identify the samples at z ∼ 8 we * Given that >15% of the integrated redshift likelihood distribution P (z) for the source is at z < 6, this is one of two targets in the REBELS selection which could correspond to a lower-redshift interloper. imposed Y − J > 0.7 mag and JH − HK S < 0.5 mag for z ∼ 8. These criteria were coupled with the requirement of non-detection (< 2σ) in all bands (CFHTLS, HSC, SUBARU) bluer than the nominal Lyman break, together with a 5σ detection in the combined J, H and K S bands, to increase the robustness of the detection. The selection was refined computing the photometric redshifts with eazy (Brammer et al. 2008), after adding the flux densities in the 3.6µm and 4.5µm Spitzer /IRAC bands from the SWIRE, SpUDS, and SEDS programs. Finally, the image stamps of the candidates were visually inspected, to remove contaminants and sources whose flux measurements are contaminated by poorly subtracted neighbors. Sources with better χ 2 from the templates of Burgasser (2014) were excluded as likely brown dwarf contaminants. This led to the identification of ∼ 40 candidates at z ∼ 7 and ∼ 25 at z ∼ 8 brighter than J ∼ 25 − 26 mag.
For each of the 60 candidates identified over the UltraVISTA/COSMOS, VIDEO/XMM-LSS, and UKIDSS/UDS fields, measurements of the optical, near-IR, and Spitzer/IRAC fluxes were made by three members of our team (Mauro Stefanon, Rebecca Bowler, and Ryan Endsley) applying their own photometric procedures to the imaging data sets each had compiled of the fields. We refer to these flux measurements as PHOT1, PHOT2, and PHOT3, respectively. Very briefly, PHOT1 is based on neighbor-subtracted aperture photometry of sources in 1.2 -diameter and 1.8 -diameter apertures in the optical/near-IR and Spitzer /IRAC data using mophongo (e.g., see Stefanon et al. 2019a), PHOT2 is based on aperture photometry in 1.8 -diameter apertures and neighbor-subtracted aperture photometry in 2.8 -diameter apertures with T-PHOT (Merlin et al. 2015) for the optical/near-IR and Spitzer /IRAC data, respectively (see e.g. Bowler et al. 2020), and PHOT3 is based on 1.2 -diameter aperture photometry for the optical/near-IR data, and in 2.8 -diameter apertures after subtracting neighbors using a mophongo/T-PHOTlike algorithm (see Endsley et al. 2021a). For each of these photometric catalogs, aperture measurements are corrected to total based on the curve of growth. More details on these photometric procedures will be provided in M. Stefanon et al. (2022, in prep). For sources found within various HST legacy fields, use was made of published photometry, except in the case of the Su-per8 sources where deeper IRAC imaging observations (Holwerda et al. 2018) were utilized.
Each of these members (MS, RB, RE) then made use of a separate photometric redshift code (eazy, lephare, and beagle [Chevallard & Charlot 2016], respectively) to derive a redshift likelihood distribution for each candidate on the basis of their photometry PHOT1, PHOT2, and PHOT3, respectively. A fourth member of the REBELS team (Rychard Bouwens) then derived a fourth, fifth, and sixth photometric redshift distribution for each source by running the eazy photometric redshift code a second time on the photometry derived by RB, RE, and MS, but using a different set of SED templates SED4. In particular, instead of using the EAZY v1.0 template set augmented by the Binary Population and Spectral Synthesis code (BPASS: Eldridge et al. 2017) v1.1 for sub-solar metallicity (Z = 0.2Z ), which include nebular emission from cloudy (Ferland et al. 2017), and 2Gyr-old passively evolving systems with varying amounts (A V = 0-8 mag) of dust extinction adopting the Calzetti et al. (2000) law, i.e., SED template set SED1, the second eazy runs use the EAZY v1.0 template set augmented by SED templates from the Galaxy Evolutionary Synthesis Models (GALEV: Kotulla et al. 2009), i.e., SED template set SED4. Nebular continuum and emission lines were included in the latter templates according to the prescription provided in Anders & Fritzev. Alvensleben (2003), a 0.2Z metallicity, and scaled to a rest-frame EW for Hα of 1300Å.
The mean of the six redshift likelihood distributions was then taken and a single distribution derived ( Figure 2). Care was taken to explicitly verify that none of the candidates considered resulted from the VISTA/VIRCAM electronic crosstalk artefact Bowler et al. (2017) identified. Bowler et al. (2017) found that such artefacts occurred at multiples of 128 pixel separations from saturated stars in the VISTA/VIRCAM data. Similar crosstalk artefacts are known to occur in the UKIDSS/UDS observations (Dye et al. 2006;Warren et al. 2007), and as such, care was taken to confirm the reality of sources based on multiple data sets, e.g., with VISTA/VIRCAM, UKIDSS/UDS, and Spitzer/IRAC data. Any source suspected to correspond to an artefact was excluded from consideration.
Sources were then ordered in terms of the likelihood of detecting an ISM cooling line in each. In computing these likelihoods, a SFR U V -to-L [CII]158µm conversion factor of 2×10 7 L /(M /yr) was assumed (de Looze et al. 2014), with a 0.3-dex scatter. Additionally, z = 6.5-7.2 candidate sources, z = 7.2-7.7 candidate sources, and z = 7.7-9.5 candidate sources were assumed to have an allocation of 2, 3, and 6 tunings, respectively. To ensure a good sampling of luminous ISM reservoirs as a function of redshift, 20 z = 6.5-7.2 targets, 16 z = 7.2-8.5 targets, and 4 z = 7.8-9.4 targets were chosen.

Characteristics of the REBELS Targets
The final set of 40 targets for the REBELS program is presented in Table 1. Along with the right ascension and declination for individual targets in our program, we also include the original source names used in the ALMA observations, U V luminosities, and photometric redshifts. U V -continuum slopes β, stellar mass estimates, [OIII] 4959,5007 +Hβ EWs, and SFRs are presented in Table 2. U V luminosities for sources and photometry are based on flux measurements made within 2.4 -diameter apertures. Fiducial stellar masses and [OIII] 4959,5007 +Hβ EWs for sources in REBELS are derived using the beagle photometric+spectroscopic stellar population fitting code (Chevallard & Charlot 2016), while our U V -continuum slopes β are derived based on power-law fits to the best-fit SED (as performed in Stefanon et al. 2019a). Our procedures for quantifying the U V -continuum slopes β, stellar masses, and [OIII] 4959,5007 +Hβ EWs for all 40 sources in the REBELS sample will be detailed in M. Stefanon et al. (2022, in prep). Unobscured and obscured SFRs for targets are derived from the rest-frame U V luminosities and IR luminosities using prescriptions given in §4.2.
In Figure 4, we present the U V luminosities M U V of sources in the 40 source REBELS selection vs. their photometric redshift. For context, the light blue circles are included to show the U V luminosities and redshifts of   Stefanon et al. (2022, in prep) for details. Note that alternate estimates of the EWs are provided assuming a delayed star formation history in Table 4 of Appendix B. † Derived from the measured U V luminosity using the prescription given in §4.2 ‡ Derived from the measured IR luminosity L IR (Inami et al. 2022, in prep) using the prescription given in §4.2. Note that no use of this SFR is made for Figure 5. Instead for that figure, SFR IR is taken to be (10 0.19 − 1)SFR U V as found for the average luminous z ∼ 7 source by Schouws et al. (2021). a Estimates of the stellar mass presented in Fudamoto et al. (2021) for REBELS-12 and REBELS-29 are higher by ∼0.7-1.0 dex than those quoted here using BEAGLE and assuming a constant star formation history and more in line with stellar mass estimates from Prospector relying on non-parameter star formation histories (Stefanon et al. 2022, in prep;Topping et al. 2022, in prep). Estimates for the obscured SFR estimates from Fudamoto et al. (2021) are almost identical to what we present here, while the U V SFRs are ∼0.1 dex higher. -U V luminosities and photometric redshifts of the new sample of bright z > 6.5 galaxies (red pentagons) being targeted with the REBELS ALMA Large program relative to z > 6.5 sources with spectroscopic redshifts from Lyα (dark blue circles), z > 6.5 sources from pilots to REBELS (light red pentagons), sources over various HST legacy fields : light blue circles), and sources with the ALPINE program (green circles). z > 6. S p e a g l e + 2 0 1 4 ( z = 7 ) S t e f a n o n + 2 0 2 2 ( z = 7 . 8 )  (Stefanon et al. 2022, in prep) and assuming a constant star formation history. Meanwhile, the SFRs shown in this figure are the unobscured SFRs derived from the rest-U V data corrected upwards by ∼0.19 dex to account for the average contribution of the obscured SF (prior to the execution of the REBELS program) to the total (as found by Schouws et al. 2021). Our assuming a constant star formation history may result in a significant underestimate of the stellar masses for sources with bursty star formation histories (Topping et al. 2022, in prep). An improved version of the presented relation will be presented in Topping et al. (2022, in prep).
sources from various legacy fields covering an area of ∼0.3 deg 2 . The light green hexagons indicate the U V luminosities and redshifts for other z > 6 sources with spectroscopic redshift determinations from ALMA. The solid and open blue circles are sources with redshift measurements from ≥ 7σ and <7σ Lyα lines, respectively Shibuya et al. 2012;Ono et al. 2012;Schenker et al. 2012;Pentericci et al. 2012;Jiang et al. 2013;Sobral et al. 2015;Zitrin et al. 2015;Song et al. 2016;Stark et al. 2017;Hoag et al. 2017;Larson et al. 2017;Pentericci et al. 2018;Fuller et al. 2020;Jung et al. 2020;Endsley et al. 2021;Pelliccia et al. 2021;Laporte et al. 2021). Figure 5 shows the approximate SFRs of targets in the REBELS program vs. their inferred stellar masses (solid red circles). These results are shown (shaded gray region) relative to the main sequence at z ∼ 7 implied by the Speagle et al. (2014) star-forming main sequence fitting formula log 10 SF R(M * , t) = (0.84 ± 0.02 − (0.026 ± 0.003)t) log 10 M * − (6.51 ± 0.24 − (0.11 ± 0.03)t) where t is the age of the universe in Gyr. Also shown for comparison is the main sequence relation shifted to match the specific star formation results from Stefanon et al. (2022) at z ∼ 7.8 (shaded blue region). Overall the REBELS targets are distributed both above and below the main sequence relation (particularly that derived by Stefanon et al. 2022). Our assuming a constant star formation history may result in a significant underestimate of the stellar masses for sources with bursty star formation histories (Topping et al Bakx et al. 2020;Carniani et al. 2020;Harikane et al. 2020) featuring ≥5σ line detections and where the emission is largely co-spatial (but see Carniani et al. 2017). For luminosity ratios in the red regions, [CII] 158µm scans are more efficient, while for luminosity ratios in the blue regions, [OIII] 88µm scans are more efficient. The breakeven ratios are computed based on the integration times required to detect [CII] 158µm and [OIII] 88µm lines at a given redshift and rely on the atmospheric transmission at a given frequency using the equations in the ALMA Technical Handbook (Cortes et al. 2020). Use of the [CII] 158µm line for spectral scans seems to be more efficient for galaxies at z = 6.5-8.2, while use of the [OIII] 88µm line for spectral scans at z > 8.5 seems more efficient.
al. (2022, in prep) will provide an improved discussion of where the REBELS targets fall on the SFR vs. stellar mass relation. Figure 17 from Appendix B shows the distribution of the REBELS targets in stellar mass and compare this distribution with that inferred  for the z = 4-6 ALPINE sample. The REBELS sample spans a very similar range in stellar mass as ALPINE. The median stellar mass for the REBELS sample inferred from beagle and Prospector stellar population fitting codes (assuming constant and non-parametric star formation histories, respectively) is 10 9.25 M and 10 9.79 M , ∼0.4 dex lower and ∼0.1 dex higher than inferred for ALPINE. Stellar mass estimates for sources in our sample from Prospector will be presented in Stefanon et al. (2022, in prep). Thanks to the similarities between the samples, ALPINE provides us with a convenient z = 4-6 comparison sample for assessing evolution in the galaxy population with redshift.
The distribution of REBELS targets in redshift, U V luminosity, U V -continuum slope β, and [OIII] 4959,5007 +Hβ EW is illustrated in Figure 18 of Appendix B. The latter two distributions appear to be completely consistent to that found for the z ∼ 7 galaxy population as a whole, suggesting that results derived from REBELS should be representative of the general population of massive star forming galaxies at z > 6.5. Despite the similar stellar mass characteristics of the two samples, REBELS does not include targets that extend as faint in U V luminosities as ALPINE does (as Figure 4 makes clear).

OBSERVATIONAL IMPLEMENTATION OF REBELS SURVEY
3.1. Choice of ISM Cooling Line for Spectral Scans Galaxies with the most luminous ISM reservoirs shine very brightly in both the 157.74µm [CII] and 88.36µm [OIII] ISM cooling lines and both lines are readily detectable with ALMA over a significant fraction of the redshift range between z ∼ 6 and z ∼ 10. As a result, ALMA has already been successful in scanning for the [CII] 158µm line in five sources at z ∼ 6-7 Schouws et al. 2022) as well as uncovering serendipitous galaxies in the immediate neighborhood of z ∼ 6-7 QSOs (Decarli et al. 2017;Venemans et al. 2019Venemans et al. , 2020. Similarly, spectral scans for the [OIII] 88µm line have been shown to allow for efficient redshift determinations for galaxies at z > 8 (e.g., Hashimoto et al. 2018;Tamura et al. 2019).
As such, we could potentially target either transition in searching for bright ISM cooling lines in z > 6 galaxies. To help decide which line would be more efficient, we calculated the approximate break-even luminosity L [OIII]88µm /L [CII]158µm ratio for [OIII] 88µm vs.
[CII] 158µm spectral scans as a function of redshift. In calculating this ratio, we expressly made use of the atmospheric transmission, using Eq. (9.8) from the ALMA Technical Handbook 28 and the specified inputs to this equation from chapter 9 of the handbook to derive sensitivities for a given integration time (Cortes et al. 2020). Observations were assumed to be conducted in precipitible water vapor (PWV) octile consistent with that shown in Figure 9.1 of the ALMA Technical Handbook, i.e., with band 6 observations being conducted in 5th octile weather and band 7 observations being conducted in generally better weather but depending on the observational frequency. As a check on the results from our sensitivity calculations, use was made of the ALMA sensitivity calculator. Also, account was made for the line scans with [OIII] 88µm covering a 1.8× smaller range in ∆z than [CII] 158µm (due to the higher frequency of [OIII] 88µm ). The FWHM of the [CII] 158µm and [OIII] 88µm lines was assumed to be the same, i.e., 250 km/s, for this calculation. Such a FWHM is fairly typical for ISM-cooling lines found for U V -bright z > 6 galaxies (e.g., Matthee et al. 2019;Harikane et al. 2020;Schouws et al. 2021) and is consistent with theoretical expectations (Kohandel et al. 2019(Kohandel et al. , 2020. We emphasize that other FWHMs should yield essentially identical results. The results are shown in Figure 6. For the typical luminosity ratios observed for z ∼ 6-8 galaxies, i.e., ∼4 (e.g., Inoue et al. 2016, Carniani et al. 2020Bakx et al. 2020;Harikane et al. 2020), not only is [CII] 158µm clearly the most efficient line to use for our spectral scans in the redshift range z ∼ 6.0 to z ∼ 8.5, but also is not subject to significant gaps in redshift coverage. For sources at z > 8.5, by contrast, [OIII]

Set Up of the Spectral Scan Windows
Here we describe our stategy for setting up the tunings for our spectral scans. For each target, we aim to optimally cover the redshift likelihood distribution derived combining the results from three independent sets of photometry (Figure 2).
For bright sources in the redshift range z = 6.5-7.2, the redshifts of sources can be accurately estimated thanks to the Lyman-break and high-EW [OIII] 4959,5007 doublet lying very close to edge of multiple spectral elements (z Subaru , z HSC , Y V IST A , y HSC , [3.6], and [4.5]) with deep coverage over the COSMOS/UltraVISTA, VIDEO/XMM-LSS, and UKIDSS/UDS fields. For z = 6.5-7.2 sources over the COSMOS/UltraVISTA field, the median ∆z uncertainty is ±0.11 while over the VIDEO/XMM-LSS + UKIDSS/UDS fields, the median ∆z uncertainty is ±0.15. For most z = 6.5-7.2 sources over the COSMOS/UltraVISTA field, we find that we can use [CII] 158µm scans to cover ∼89% of the likelihood distribution with two redshift tunings, each covering a contiguous 5.375 GHz frequency range (and a 10.75 GHz contiguous range in total). REBELS-08 in Figure 15 of Appendix A is one example of a source where we used this tuning strategy.
From z = 7.2 to z = 7.7, the redshifts of sources are largely constrained based on the position of the Lymanbreak in the Y band filter. Nevertheless, since there are fewer passbands with flux measurements providing direct constraints on the redshifts of sources, the median uncertainty on the redshifts of sources is larger, with a value of ±0.15. For sources in this redshift range, we find that we can use [CII] 158µm scans to cover the 91% of the redshift likelihood distribution in 3 tunings covering a contiguous frequency range of 20.375 GHz (5.375 + 7.5 + 7.5 GHz). REBELS-01 in Figure 15 of Appendix A is one example of a source where we used this tuning strategy.
For sources with photometric redshifts in excess of 7.7, the redshift uncertainties increase substantially due to the Lyman break falling between the ground-based Y and J bands. For these sources, the redshift likelihood distribution typically extended from z∼7.7 to 9.3, with a median uncertainty of ±0.44. To execute a spectral scan for ISM cooling line over this range, we made use of [CII] 158µm scans in cases where such a scan had already begun as part of a cycle-6 program (2018.1.00236.S, PI: Stefanon) probing the dust continuum. Otherwise, sources required six tunings in band 7 to search for [OIII] 88µm (probing the redshift range z = 8.10 to 9.39) and approximately three tunings in band 6 to search for [CII] 158µm (probing the range z = 7.37 to 8.00). 29 We present the spectral scan windows we use for the 40 targets in the REBELS LP in Figures 15 and 16 of Appendix A. In total, 16 of the targets from the program required 2 tunings to cover the redshift likelihood distribution, 17 targets required 3 tunings, 1 target required 4 tunings, 2 targets required 5 tunings, 1 target required 6 tunings, and 1 target required 8 tunings. For 2 targets, only 1 tuning was allocated to extend scans which already started in our second pilot program (Schouws et al. 2022). In total, the number of targets × tuning win-

Sensitivity Requirements for Spectral Scans
Our sensitivity requirements for REBELS relies on our experience with searches in our pilot programs Schouws et al. 2022). There the detected [CII] 158µm lines had peak fluxes of 1.5-4.0 mJy. To guarantee the selection of similar sources at z ∼ 7 with REBELS, we required that the peak flux sensitivity be ∼340µJy in a 66 km/s channel such that sources with a peak flux of 1 mJy can be detected at 5σ when combining multiple channels. This is equivalent to a 5σ limiting point-source luminosity of ∼ 3 × 10 8 L at z ∼ 7 (assuming a line width of 250 km/s), which requires ∼20 minutes of integration time per tuning. Reaching the same limiting luminosity at z ∼ 8 requires a 300µJy sensitivity and ∼30-40 minutes of integration time in band 5 or 6 (for z > 8 and z < 8 [CII] 158µm searches, respectively). In scanning for the bright [OIII] 88µm lines in our z ∼ 9 targets, the equivalent sensitivity requirement is 450µJy assuming an [OIII] 88µm /[CII] 158µm luminosity ratio of 3.5.
For the purposes of illustration, the limiting [CII] 158µm and [OIII] 88µm luminosities probed as a function of source redshift for the spectral scans planned for sources in REBELS is provided in Figure 7. This is for the requisite sensitivity specified for the REBELS program. In practice, the sensitivity achieved is typically ∼1.4× better than specified (Schouws et al. 2022, in prep), allowing us to detect even lower luminosity ISM cooling lines (indicated with the lighter shading in Figure 7). For context, we have included the [CII] 158µm luminosities and redshifts of detected sources from the literature (gray circles) and our pilot programs (filled stars). As should be clear from the figure, REBELS will detect line emission from sources if their [CII] 158µm luminosities exceed ∼ 3×10 8 L and ∼ 2×10 8 L (requested and typical sensitivities, respectively) and, for z > 8.5 targets, if their [OIII] 88µm luminosities exceed ∼ 1.1 × 10 9 L and 0.8 × 10 9 L (required and typical sensitivities, respectively).
To maximize the sensitivity of the REBELS spectral scan observations and not overresolve the [CII] 158µm line, observations were conducted in the lowest spatial resolution configurations (C43-1 and C43-2), with ∼1.2-1.6 FWHM. The REBELS observations were obtained in frequency domain mode (FDM) at a spectral resolution of 488 MHz and then spectrally averaged in bins of size 16, giving the output data a spectral resolution of 7.813 MHz, equivalent to a velocity resolution of ∼9 km/s for [CII] 158µm line at z ∼ 7. Given that [CII] 158µm lines in luminous z ∼ 7 galaxies have been found to have a minimum FWHM of ∼80 km/s and more typically 250 km/s, this was expected to be more than sufficient to study the kinematic structure of sources revealed by the program.
63% of the z ∼ 7 galaxies targeted with observations from our pilot programs yielded lines with luminosities in excess of 2 × 10 8 L . Given that similar selection criteria are used for the REBELS large program, we would expect a similar detection rate of [CII] 158µm for the REBELS program, suggesting we will detect lines in 25 out of 40 targets. Combining the expected results with previous [CII] 158µm and [OIII] 88µm line detections, we expected ≥35 z > 6.5 galaxies with ALMA line detections once the program was completed. This is a sufficient number for a detailed physical characterization of massive galaxies at z 6.5 and a study of their evolution.

Dust-Continuum Sensitivities
The detection of dust-continuum emission from starforming galaxies at z ∼ 5-8 with ALMA has been found to be much more difficult than detection of the [CII] 158µm or [OIII] 88µm ISM cooling lines. This basic difference in detectability was already evident in z ∼ 5.5 results by Capak et al. (2015), who were able to detect all 10 luminous z = 5.2-5.7 galaxies they targeted in lines and detection (3.3σ) of the dust continuum shown as a function of redshift. Also shown on this diagram are z ∼ 6-10 sources from the REBELS pilot programs Schouws et al. 2021Schouws et al. , 2022 and the literature Tamura et al. 2019;Bakx et al. 2020;Harikane et al. 2020). The dashed lines show the break-even ratios assuming three separate tunings for a spectral scan. The use of >3 tunings would shift the break-even ratio to even higher values. For the typical source, one can clearly consider up to 6 tunings per source before the line sensitivity of ALMA becomes the limiting factor in probing both line and continuum emission. This fits very well with the spectral scan strategy employed in REBELS where the aim is to probe both lines and the dust continuum.
[CII] 158µm , but only able to detect 4 of the 10 sources in the dust continuum. Other results available on z > 5 galaxies have been similar, with [CII] 158µm being detected in a much larger fraction of sources than the dust continuum (e.g., Willott et al. 2015;Maiolino et al. 2015;Inoue et al. 2016;Matthee et al. 2017Matthee et al. , 2019Harikane et al. 2020;Béthermin et al. 2020).
The relative detectability of ISM cooling lines like [CII] 158µm or [OIII] 88µm and the dust continuum can be quantified on the basis of their measured L [CII]158µm -to-L IR ratios or L [OIII]88µm -to-L IR ratios. Compiling previous results from Capak et al. (2015), ALPINE (Le Fevre et al. 2020;Béthermin et al. 2020;Faisst et al. 2020), Smit et al. (2018), Schouws et al. (2021Schouws et al. ( , 2022, we can calculate the relative integration time required to detect sources in the dust continuum and the integration time required to detect sources in [CII] 158µm . In performing this calculation, we assume a line width of 235 km/s (FWHM) for both the [CII] 158µm and [OIII] 88µm lines, and we assume a modified blackbody form for the dustcontinuum SED with a dust temperature of 50 K and dust emissivity index β of 1.6, which lies intermediate between the lowest and highest dust temperature measurements at z > 7 (e.g., Knudsen et al. 2017;Bakx et al. 2020). We present results in Figure 8.
Given the much longer exposure times required to detect sources in the dust continuum than in either [CII] 158µm or [OIII] 88µm , our use of multiple tuning windows for spectral scans really is an advantage in allowing us to probe the dust continuum. We have illustrated the approximate IR luminosities we are able to probe with the REBELS LP in Figure 9. Two spectral scan windows (10.75 GHz bandwidth) are assumed at z = 6.5-7.2, three spectral scan windows (20.375 GHz bandwidth) at z = 7.2-7.7, and six spectral scan windows (45 GHz bandwidth) at z > 7.7.
The REBELS dust-continuum probe also allows for a very valuable assessment of incompleteness in our spec-tral scan results. This is because incompleteness can arise as a result of (1) scan range not extending over a wide enough range in frequency to find the relevant ISM cooling line or (2) the relevant ISM cooling line being fainter than the 5σ detection limit for the spectral scan. Given the strong correlation between dustcontinuum luminosities of galaxies and the luminosities of [CII] 158µm and [OIII] 88µm , the detection of the dust continuum in a source strongly suggests that the associated ISM cooling is sufficiently bright to have been detected in our spectral scans. If the ISM line is not found but the dust continuum is, it strongly suggests the spectral scan did not extend broadly enough in frequency. In cases where neither the line nor the dust continuum is detected, it may mean that the line is fainter than the sensitivity limits of the scans (or in the worst case at lower redshift, but our careful selection suggests that the number of such targets in the REBELS program is small).

Summary and Execution of Program
The REBELS LP observations began on November 15, 2019 when ALMA was in the C43-2 configuration and continued until January 10, 2020 while ALMA was in configurations C43-1 and C43-2. 60.6 hours of ALMA observations have thus far been acquired, with 8 hours remaining to be observed. 34 targets from the program have now been fully observed. Observations are still incoming for REBELS-04, REBELS-06, . Two of the sources with incoming observations are part of our 33-target z = 6.5-7.7 sample. The remaining four are part of our 7-target z = 7.7-9.4 sample. The REBELS spectral scans for our z = 6.5-7.7 and z = 7.7-9.4 targets are therefore 94% and 43% complete, respectively.
Following the first year of observations from the REBELS program and successful detection of  Fig. 9.-The 3.3σ limiting IR luminosities probed by the REBELS scans for [CII] 158µm and [OIII] 88µm in 40 very bright reionization-era galaxies (light yellow and gold shaded regions, respectively). The red stars show the IR luminosities of six z = 7-8 galaxies from the REBELS pilot programs (Schouws et al. 2021;Smit et al. 2018) while the grey circles show the IR luminosities inferred for various dust detected galaxies in the literature. REBELS will detect sources in the dust continuum if their IR luminosities are greater than 3 × 10 11 L to z ∼ 7.2 and down to 2 × 10 11 L at z = 7.2-9.4 (equivalent to obscured SFRs of 36 M /yr and 24 M /yr, respectively). Extrapolating our pilot results, the REBELS program is projected to increase the number of dust detections by 3-4× at z > 6.5 and will allow us to test whether large dust reservoirs are common (as suggested by our detections and also the Watson et al. (2015) and Laporte et al. (2017) sources at z ∼ 7.5 and z = 8.38, respectively) or whether there is a rapid decline in IR bright galaxies at z > 7.5.
The remaining observations appear likely to be executed in March 2022 when the configuration of ALMA is in C43-1 or C43-2 according to the current JAO schedule.

SPECTRAL SCAN RESULTS
The purpose of this section is to provide a quick overview of some of the most prominent ISM cooling line detections obtained thus far with the REBELS LP observations and to assess how effective the observational strategy has been.

Processing of the ALMA Data and Initial Results
Here we provide a very brief summary of the procedures used to reduce and calibrate ALMA observations from the REBELS program. More details will be given in S. Schouws et al. (2022, in prep) andH. Inami et al. (2022, in prep).
Reduction and calibration of data from the program were performed using the standard ALMA calibration pipeline as implemented in the Common Astronomy Software Applications package (CASA) version 5.6.1. Data cubes were reimaged with the tclean task using a natural weighting to maximize our sensitivity for detecting ISM cooling lines in ALMA band-5/6/7 observations obtained from the program. Cleaning was done down to 2σ in producing the data cubes.
Line searches were performed using three different linesearch algorithms on the reduced data cubes. Searches for >5σ lines were perfomed within 1" of the target center in the rest-U V . Line widths from 80 km/s to 600 km/s were considered in searching for lines throughout our data cubes. S. Schouws et al. (2022, in prep) will provide a detailed description of our line search procedures and catalogs, while carefully quantifying both the completeness and purity of the line searches.
As an illustration of the effectiveness of the spectral scans employed in the REBELS program to-date, we include in Figure 10 the 18 [CII] 158µm lines detected with a significance of >7σ. Redshifts of the detected [CII] 158µm lines range from z = 6.496 to 7.677. 7σ was adopted as the detection threshold in this paper to keep the focus on the brightest and most significant lines found in the survey. Details on the purity of our ISM line searches, characteristics of the [CII] 158µm line detections as well as several additional lower S/N [CII] 158µm lines will be presented in S. Schouws et al. (2022, in prep).
Even if we consider only the >7σ line detections in the REBELS data obtained thus far, the program is already having a substantial impact on the number of spectroscopic redshifts derived with ALMA at z > 6.5. Figure 11 (top panel ) shows how the sources with ≥ 7σ ISM-cooling lines are distributed as a function of redshift and U V luminosity. Also shown in the top panel of Figure 11 are sources from the two pilot programs to REBELS Schouws et al. 2021Schouws et al. , 2022.
As the lower two panels of Figure 11 demonstrate, the number of spectroscopic redshift measurements from ALMA is already fairly similar to that available from similarly significant Lyα lines at z > 7, if one makes use of the most significant ISM-cooling line detections from the REBELS program and from the two pilot programs. This is especially the case for the brightest and most massive sources at z > 6.5 where the number of redshifts from ALMA already exceeds that available from Lyα by a factor of 2.
Simultaneous with the REBELS spectral scans for bright ISM cooling lines, we are able to probe dust continuum emission from sources in REBELS. Figure 12 shows the continuum observations available for first year data will be presented in S. Schouws et al. (2022, in prep). . Also shown (light red circles) are the U V luminosities and spectroscopic redshift for sources from the REBELS pilot programs Schouws et al. 2021Schouws et al. , 2022 and other spectral scan programs in the literature (Hashimoto et al. 2018;Tamura et al. 2019). For context, the U V luminosities and existing spectroscopic redshift measurements from the Lyα line are shown as the blue circles. z > 6.5 sources with redshift determinations based on Lyα and [OIII] 88µm lines where the signficance is <7σ are shown with open blue circles and open light red circles, respectively. The light blue circles are also shown for context and are based on the U V luminosities and photometric redshifts of sources over HST legacy fields . (middle panel) The number of spectroscopic redshifts available as a function of redshift relying on only the most prominent (>7σ) [CII] 158µm -detected galaxies in the REBELS large program and pilot programs (red and light red histogram, respectively). The green histogram shows the numbers based on other spectral scan programs in the literature (Hashimoto et al. 2018;Tamura et al. 2019). (lower panel) The number of spectroscopic redshifts available as a function of redshift relying on ≥ 7σ and 5-7σ Lyα-detected galaxies at z > 6.5 (blue and light blue hatched histogram, respectively). (right panel) Number of spectroscopic redshifts from ALMA (red histogram) and Lyα (blue histogram) on the basis of ≥7σ lines. Notice how there are already ≈2× as many spectroscopic redshifts available for the brightest (M U V,AB < −22) sources from ALMA as available from Lyα.  the 18 targets from REBELS with ≥ 7σ ISM-cooling line detections, and it is clear that the majority of these [CII] 158µm -detected sources (13/18) show nominal 3.3σ detections in the dust continuum. These dustcontinuum images were generated using the tclean task in CASA. Any channels containing emission from the [CII] 158µm line were excluded when producing the continuum maps. More details on our current dust-continuum results from REBELS will be presented in H. Inami et al. (2022, in prep).

Effectiveness of the REBELS Spectral Scan Strategy
We can already use the results from the REBELS firstyear data to assess the effectiveness of the spectral scan strategy. Perhaps the most relevant variable to utilize in gauging the effectiveness of the scans is in terms of the SFRs of individual targets in our program.
For our SFR estimates for individual sources in the REBELS program, we make use of the luminosities of sources in both the rest-U V and far-IR continuum. The unobscured SFR U V s for sources are estimated based on source U V luminosities using the relation SFR U V = 7.1 × 10 −29 L ν [ergs/s/Hz], while the obscured SFR IR s of sources are estimated based on source IR luminosities using the relation SFR IR = 1.2 × 10 −10 L IR /L . The inferred SFRs are ∼14% lower for a given U V /IR luminosity than the calibrations adopted for ALPINE (e.g. Schaerer et al. 2020). A detailed motivation and discussion of these relations will be provided in M. Stefanon et al. (2022, in prep), M. Topping et al. (2022, in prep), and H. Inami et al. (2022. In computing the obscured SFR IR of sources, the IR luminosities of sources L IR are taken to equal 14.2 +7.6 −4.7 νL ν where ν is the frequency of the [CII] 158µm line (H. Inami et al. 2022, in prep;Sommovigo et al. 2022). Using this scaling, sources in REBELS have L IR luminosi-ties ranging from 3×10 11 L to 1×10 12 L (equivalent to obscured SFRs of 36 M /yr to 120 M /yr, respectively). The precise scaling we use here is equivalent to that found from a modified blackbody SED with a dust temperature of 50 K and a dust opacity index β of 1.6. This is a slightly higher dust temperature than ALPINE use in the analysis of their z = 4-6 sample , but lower than is found by Bouwens et al. (2020) in modeling the dust temperature vs. redshift measurements available at the time. It is also consistent with the general range of dust temperatures found by semi-analytical models ) and numerical simulations (R. Schneider et al. 2022, in prep) for REBELS-like sources. A more extensive motivation for these conversion factors will be provided in H. Inami et al. (2022, in prep).
Using the above scalings to estimate the SFRs of individual sources, we present the number of sources showing prominent ISM-cooling line detections as a function of the total SFR of sources in Figure 13. We only include in the analysis the 32 sources for which our ISM cooling line scans are complete and which appear to be securely at z > 6. 30 Since only one of our [OIII] 88µm spectral scans is complete to present in the first year data, it makes sense to frame these search results in terms of our spectral scans for [CII] 158µm .
Looking over the results, the efficiency of our spectral 30 Significantly deeper Spitzer /IRAC observations became available for REBELS-10 following the selection of targets for the REBELS program (Stefanon et al. 2019b). Using the new photometry, REBELS-10 now appears to be more likely at z < 6 than at z > 6. In addition to REBELS-10, there are also concerns about the robustness of REBELS-13, given the formal integrated likelihood of its being at z < 6 and its lacking a red [3.6]−[4.5] color, as is typical for star-forming galaxies in the redshift range z = 7.0-9.1 due to the [OIII] 4959,5007 +Hβ emission lines (Roberts-Borsani et al. 2016). M. Stefanon et al. (2022, in prep) will discuss each of these cases in more detail.  [CII] 158µm is yet to be detected at 7σ in the REBELS program. Only sources where the spectral scans are complete -and where sources are considered to be securely at z > 6 -are included in the numbers. The upper horizontal axis shows the [CII] 158µm luminosity that is equivalent to a given SFR U V +IR s using the z ∼ 0 L [CII] 158µm -SFR relation from de Looze et al. (2014). Our prescriptions for computing SFR U V and SFR IR are given in §4.2. The detection rate of [CII] 158µm shows a dramatic increase at SFRs higher than 28 M /yr (equivalent to the approximate luminosity limit for searches for [CII] 158µm in REBELS adopting a 7σ detection threshold, i.e., 2.8 × 10 8 L [ Figure 7]).  Figure 13 but as a function of the unobscured SFR U V . While the [CII] 158µm -detected fraction of sources in REBELS does show some dependence on SFR U V , the dependence is significantly weaker than on SFR U V +IR (Figure 13). This demonstrates the essential value that ALMA observations have in helping us to characterize the ISM of star-forming galaxies at z > 6.5. scans for [CII] 158µm show a dramatic increase in efficiency above 28 M /yr. Fifteen of the 19 sources with SFRs in excess of 28 M /yr, i.e., 79%, show prominent 7σ ISMcooling lines in the observations taken to-date. Meanwhile, below a SFR of 28 M /yr, only three of thirteen sources show a prominent 7σ [CII] 158µm detection. We note that the 7σ limiting luminosity for [CII] 158µm scans in the REBELS program (Figure 7), 2.8×10 8 L /yr, is equivalent to a SFR threshold of 28 M /yr, using the z ∼ 0 relation from de Looze et al. (2014). This result is suggestive of only minimal evolution in the [CII] 158µm -SFR relation to z ∼ 0 (see also Matthee et al. 2017Matthee et al. , 2019Carniani et al. 2018Carniani et al. , 2020Harikane et al. 2020;Schaerer et al. 2020).
We also note that some sources with SFRs higher than 28 M /yr remain undetected in [CII] 158µm , while some sources below 28 M /yr are detected. This is suggestive of there being some intrinsic scatter in the L [CII]158µm vs. SFR relation at z ∼ 7, similar to the 0.27 dex scatter observed at z ∼ 0 by de Looze et al. (2014). We will further characterize both the evolution and the scatter in the L [CII]158µm vs. SFR relation in Schouws et al. (2022, in prep).
It is also interesting to present the efficiency of our [CII] 158µm scans in terms of the unobscured SFRs for sources. Figure 14 shows the number of sources where [CII] 158µm is prominently detected (>7σ: red histogram) as a function of the unobscured SFRs and also the number of sources where no prominent [CII] 158µm emission is found (gray histogram). While the [CII] 158µm detection fraction does show some dependence on the unobscured SFR U V s, the dependence is significantly less steep than on the total SFR U V +IR s ( Figure 13). This demonstrates that the essential value ALMA observations have for characterizing the ISM of star-forming galaxies at z > 6.5.
We emphasize that only the most prominent [CII] 158µm detections from the REBELS program are included here in evaluating the efficiency of the spectral scans.
There are indeed a larger number of [CII] 158µm detections in REBELS, but these detections require a more careful demonstration of their robustness. These line detections will be presented in detail in S. Schouws et al. (2022, in prep).

SCIENCE OBJECTIVES
In facilitating the construction of a significant sample of especially luminous ISM reservoirs in the z > 6.5 universe, the REBELS Large Program enables us to pursue a wide variety of different scientific objectives.
These include the following: Significantly Expand the Overall Number of Sources in Current Spectroscopic Samples at z 7: Based on the success rate of observations from various pilot programs Hashimoto et al. 2018;Schouws et al. 2022), REBELS is providing new redshift measurements for 25-30 galaxies at z ∼ 7-9 in the reionization epoch, almost tripling the number of such measurements available now with ALMA at z > 7. See histograms in the lower two panels of Figure 11. In addition to the increase in numbers, the new spectroscopic redshifts we are obtaining with ALMA sample much more massive sources than has generally been identified thus far with rest-U V lines like Lyα.
Probe the Emergence of Dust in Bright, Massive z > 6.5 Galaxies: REBELS is providing a sensitive probe of dust-continuum emission in z > 6 galaxies due to our scan strategy for finding [CII] 158µm +[OIII] 88µm line emission. As Figure 9 shows, our scan strategy allows us to observe faint enough in the far-IR continuum to probe dust growth over most of the reionization era, effectively providing the community with a z = 6-10 extension to z = 4-6 results from ALPINE (Le Fevre et al. 2020;Béthermin et al. 2020;Faisst et al. 2020). Probing dust emission in z > 7 galaxies is very interesting, given the huge uncertainties that exist regarding the origin and production of early dust in the universe. In particular, both the relative contribution of dust creation from high-z supernovae (SNe) or Asymptotic Giant-branch (AGB) stars, as well as grain growth and dust destruction, are poorly constrained and continue to be actively debated (e.g., Mancini et al. 2015;Michalowski 2015;Popping et al. 2017;Graziani et al. 2020). Of considerable interest to such discussions has been the detection and IR luminosities of z > 6 galaxies like A1689-zD1 (z ∼ 7.5: Watson et al. 2015;Knudsen et al. 2017) and A2744 YD4 (z = 8.38: Laporte et al. 2017), which have been argued to show too much dust mass/emission to match most models (e.g. Mancini et al. 2015;Michalowski 2015). One exception is Behrens et al. (2018), who reproduced the observed SED from their simulations ) finding a SFR ≈4 higher than deduced from SED fitting by Laporte et al. (2017), as well as a low dust-to-metal ratio (implying inefficient early dust formation), and higher dust temperatures (as high as Td ∼ 91 K: e.g., Ferrara et al. 2016Ferrara et al. , 2021Sommovigo et al. 2020Sommovigo et al. , 2021. Whatever the resolution of this issue, REBELS is allowing for a characterization of the build-up of dust in galaxies, based on comparisons it allows from galaxies at z ∼ 8.5-9.5 to z ∼ 6, 7, and 8 (Dayal et al. 2022;H. Inami et al. 2022, in prep;R. Schneider et al. 2022, in prep). Follow-up of bright sources from our sample should allow for a definitive measurement of the dust temperature, the emissivity, as well as evolution in the IRX-β relation, as well as the evolution of these quantities with cosmic time. This will be valuable for determining whether the dust temperatures of galaxies evolve strongly in the first 2 billion years of the universe (Béthermin et al. 2015;Schreiber et al. 2018;Bouwens et al. 2020) or whether the evolution is relatively mild . The REBELS probe of the dust content in galaxies is also allowing for a definitive determination of the obscured SFR density in U V bright galaxies in the epoch of reionization as well as a search for highly dust obscured galaxies in the immediate neighborhood of the U V -bright population as was recently reported in Fudamoto et al. (2021) using the REBELS data set. This will allow for an investigation into whether the cosmic star formation rate density flattens, as found in FIR and radio observations dusty UV-obscured systems (Novak et al. 2017;Gruppioni et al. 2020;Talia et al. 2021).

Provide First Significant Probe of the High-Luminosity
End of the IR and [CII] 158µm luminosity functions in the Reionization Epoch: Using the luminosities of the [CII] 158µm lines we detect in REBELS and volume densities of the UV samples from which our targets are drawn, we are deriving the first significant [CII] 158µm and IR luminosity functions at z > 6 at the high luminosity end, which provide a valuable constraint for theoretical models that look at [CII] 158µm emission from galaxies (e.g., Vallini et al. 2015Vallini et al. , 2017Vallini et al. , 2020Pallottini et al. 2017Pallottini et al. , 2019Olsen et al. 2017;Katz et al. 2017;Popping et al. 2017;Lagache et al. 2018;Kohandel et al. 2019;Arata et al. 2020). Having constraints on the IR luminosity function provides us with a direct pathway to compute the SFR function itself (= SF R U V +SF R IR ) at z > 6.5.
Rest-optical Emission Line EWs, stellar masses, specific star formation rates, and the galaxy stellar-mass function at z ∼ 7: Our estimates on the stellar masses of z > 6 galaxies are poor due to the impact of very strong optical nebular line emission which have a huge (factor of ∼1.7) impact on observed 3-5µm fluxes of galaxies at z > 6 (e.g., Schaerer & de Barros 2009;Raiter et al. 2010;Stark et al. 2013;Smit et al. 2014;Roberts-Borsani et al. 2016). Our ALMA scans give us the precise redshift of sources, enabling us to model the impact of line emission on the Spitzer/IRAC fluxes much more precisely. These data are thus allowing us to dramatically improve the robustness of the high-mass end of the galaxy stellar mass function at z ∼ 7.
Dynamical mass estimates: To reveal the physics behind early massive galaxy build-up, we are deriving dynamical mass estimates from the emission-line widths of our sample (following, e.g., Capak et al. 2015). These estimates can then be related to the stellar mass estimates that we can uniquely derive for this sample, allowing us to investigate if the stellar mass in these early galaxies builds up smoothly as their gas reservoirs grow or if feedback processes plays a significant role in the conversion from gas to stars, based on the scatter in the stellar-to-dynamical mass ratio.

Major mergers and rotation-dominated systems:
Our observations are also providing robust detections of the [CII] 158µm or [OIII] 88µm lines in our galaxies, which reveal if major mergers are ongoing in our sources by fitting for double-peaked line profiles (such as observed by Willott et al. 2015, andMatthee et al. 2017). We are comparing the major merger rate found in our program with recent HST imaging analysis of major merger rates between redshift 1 < z < 7 (e.g., Duncan et al. 2019), which suggest a steady increase in the major merger rate with redshift, such that at z > 6 roughly ∼50% of galaxy growth is contributed by merger activity (but see however Dayal et al. 2013). We are testing these claims in a statistically significant sample at z 7, using the first such analysis from spectroscopy. Furthermore, we are performing a Briggs-weighted analysis of the data to investigate the low-resolution kinematics of our sources and derive velocity gradients (following e.g. Smit et al. 2018). Forster-Schreiber et al. (2009) estimate the level of rotational support in a galaxy based on the observed velocity gradient and the integrated velocity dispersion of the source, defining v obs /2σ int > 0.4 as rotation-dominated systems. Using sources with single peaked line profiles and monotonically rising velocity gradients (i.e. sources without major mergers), we aim to obtain a census of the rotation-dominated fraction of the z 7 galaxy population.
Outflows and [CII] 158µm halos: We are performing a stacking analysis of [CII] 158µm (in the uv plane) to reveal the low surface brightness components of [CII] 158µm and search for broad line emission in the stacked spectra. By comparing the outflow velocities to SFRs, dynamical, and stellar masses, we are investigating if outflow velocities increase with redshift to z ∼ 7, as has been found at z ∼ 0-2 (e.g. Sugahara et al. 2019). Recent observations furthermore suggest that diffuse and extended '[CII] 158µm halos' might be present in z ∼ 3-7 star-forming galaxies (Fujimoto et al. 2019;Rybak et al. 2019;Herrera-Camus et al. 2021). Our stacking analysis is revealing if such diffuse components are ubiquitous in the earliest galaxies, providing the first insight into the physical properties of the circumgalactic medium at z 7.
Legacy value: REBELS is explicitly designed to deliver a large sample of very high luminosity [CII] 158µm and [OIII] 88µm -emitting galaxies at z 7, at a modest cost. The REBELS line emitters are prime targets for deep spectroscopic follow-up with JWST, given the clear value of these galaxies for understanding the early build-up of luminous ISM reservoirs, and in fact one such program has been approved for execution in cycle 1 (Stefanon et al. 2021). Additionally, the identification of such line emitters is absolutely essential for future work probing the kinematics of z 7 galaxies at high resolution. This will be essential for robustly discriminating between rotation-dominated galaxies and those undergoing mergers -which can be degenerate for some choices of parameters at the REBELS spatial resolution.
Furthermore, the characterisation of the physical conditions of the gas in high redshift galaxies through observations of [NII]  is only feasible for the brightest galaxies in the Epoch of Reionization. New systemic redshift measurements from REBELS would have huge value for studies of low-S/N rest-UV lines in these bright galaxies (including constraints on the neutral Hydrogen fraction of the IGM, based on the declining prevalence of Lyα as a function of redshift), as is being presented e.g. in Endsley et al. (2022). After shifting to a common rest-frame wavelength, archival/future spectra of these galaxies can be immediately stacked.

SUMMARY
The purpose of this paper is to summarize the scientific motivation, observational strategy, and sample selection of the 70-hour Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Program (2019.1.01634.L), while showcasing some of the most exciting initial results from the program.
The motivation for the REBELS program has been to construct and to perform a first characterization of a significant sample of especially luminous ISM reservoirs in the z > 6.5 Universe. To achieve this, REBELS has been systematically scanning 40 of the brightest z > 6.5 galaxies identified over 7 deg 2 for bright ISM cooling lines, while probing dust-continuum emission from the same sources.
The utility of targeting U V -bright galaxies with ALMA has become increasingly clear from work over the past few years, with many of brightest ISM-cooling lines at z > 4 being present among the bright population (Ca-pak et al. 2015;Willott et al. 2015;Smit et al. 2018;Matthee et al. 2019;Harikane et al. 2020;Schouws et al. 2022). In fact, 79% of the sources with SFRs of >28 M /yr show ≥ 7σ [CII] 158µm lines, which given the typical sensitivites of the REBELS spectral scans would give the sources [CII] 158µm luminosities in excess of 2.8 × 10 8 L (Figure 1).
To maximize the impact of the REBELS program, considerable effort was thus devoted towards targeting the brightest and most robust selection of z > 6.5 galaxies visible to ALMA ( §2). The use of targets from at least eight different high-redshift selections (e.g., Bowler et al. 2014Bowler et al. , 2017Bowler et al. , 2020Stefanon et al. 2017bStefanon et al. , 2019aEndsley et al. 2021Endsley et al. , 2022M. Stefanon et al. 2022, in prep) was considered. For each potential target, photometry was done independently by at least three different members of the REBELS team, and redshift likelihood distributions were derived from three independent codes. Targets were then ranked according to their brightness and robustness, and ultimately the likelihood of detecting an ISM cooling line in a spectral scan. Only the highest ranked sources were included in our final set of 40 targets.
Consideration was given both to spectral scans targeting the 157.74µm [CII] line and 88.36µm [OIII] line for the REBELS program. We computed the limiting luminosity to which we could probe each line as a function of redshift to assess the relative efficiency. After considering the line ratios observed for z > 6 galaxies in the literature (Figure 6), we decided that [CII] 158µm line would be best choice for executing line scans out to z ∼ 8.5 and that the [OIII] 88µm line would be the best choice for sources with likely redshifts above z ∼ 8.5 ( §3.1).
For each target in our sample, we set up the tunings for our spectral scans to cover the bulk of the redshift likelihood distribution for each source (typically >80-90%). Results obtained from three independent sets of photometry were used in deriving this likelihood distribution ( §3.2). With 1, 2, and 3 tunings, we can cover a contiguous frequency range of 5.375 GHz, 10.75 GHz, and 20.375 GHz, respectively, if the spectral scan is in band 6. We have arranged the tunings for sources to optimize the number of ISM cooling lines we can detect for a ∼70hour ALMA allocation. Appendix A illustrates in detail the layout of the tunings that make up the spectral scans for the 40 targets in our program.
The integration time for each tuning is set by the requirement that we detect a 3 × 10 8 L [CII] 158µm line at 5σ when executing our band-6/5 scans and a 1.1 × 10 9 L [OIII] 88µm line at 5σ when executing our band-7 scans. Those sensitivity requirements translated into ∼20 minute integration times for REBELS [CII] 158µm searches to z ∼ 7.2, ∼30-40 minute integration times for [CII] 158µm searches to z ∼ 8.5, and ∼20-40 minute integration times in searches for the [OIII] 88µm line at z > 8.2.
Simultaneous with the REBELS scans for [CII] 158µm and [OIII] 88µm , REBELS probes the IR luminosities of our targets based on their dust-continuum emission. Given the poorer sensitivity of REBELS for dust continuum (Figure 8), our use of multiple spectral scan tunings to search for ISM cooling lines really is an advantage in allowing us to probe the dust continuum.
As shown in Figure 9, REBELS probes to 3 × 10 11 L in galaxies out to z ∼ 7.2. At z > 7.2, it probes even deeper to 2 × 10 11 L . This is equivalent to probing to obscured SFRs of 36 M /yr and 24 M /yr, respectively.
During the first year of observations from the REBELS program (November 2019 to January 2020), 60.6 out of the total 69.6 hours allocated to REBELS have been executed. A search for prominent cooling lines in the data revealed 18 prominent ≥ 7σ [CII] 158µm lines (Figure 10 and Table 2). No especially significant ≥ 7σ [OIII] 88µm lines have been identified in the existing data from the REBELS program, but observations are complete for only one of four sources using [OIII] 88µm scans. The majority of the sources showing ≥7σ detections of [CII] 158µm (13/18) also show ≥3.3σ dust-continuum emission ( Figure 12). Remarkably, adding the newly identified [CII] 158µm lines to those from the literature and pilot programs to REBELS Schouws et al. 2022), the number of redshift determinations from ALMA is already starting to be comparable to the number of Lyα-derived redshifts at z > 7 ( Figure 11).
It is interesting to already make use of the prominent 7σ [CII] 158µm line detections to quantify the efficiency of spectral scans with ALMA. Looking specifically at the fraction of [CII] 158µm -detected galaxies as a function of their total SFR, we find a dramatic increase in the fraction above a SFR of 28 M /yr ( Figure 13). Fifteen of the 19 sources with SFRs in excess of 28 M /yr, i.e., 79%, show prominent 7σ [CII] 158µm -cooling lines in the observations taken to date. Meanwhile, below a SFR of 28 M /yr, only three of thirteen sources shows a prominent [CII] 158µm detection. We note that a SFR threshold of 28 M /yr corresponds to a [CII] 158µm luminosity of 2.8×10 8 L using the z ∼ 0 De Looze et al. (2014) relation. Since 2.8×10 8 L is also the approximate 7σ limit for our [CII] 158µm scans in the REBELS program (Figure 7), this is suggestive of minimal evolution in the [CII] 158µm -SFR relation from z ∼ 8 to z ∼ 0, as Schaerer et al. (2020) also conclude on the basis of the ALPINE program (see Matthee et al. 2017Matthee et al. , 2019Carniani et al. 2018Carniani et al. , 2020Harikane et al. 2020).
In this paper, we have presented the motivation, observational strategy, and some initial observational results from a cycle-7 ALMA large program known as REBELS.
In the future, we look forward to the completion of spectral scans for the final seven targets in the program, the bulk of which probe galaxies at z ≥ 8. Also of considerable importance will be an exciting array of followup observations being acquired on targets from our program, including from JWST, JVLA, ALMA, Keck, and the VLT. With the exception of four targets in our program (REBELS-04,  where the bulk of the redshift likelihood distribution is z > 8, the [CII] 158µm line is targeted with the REBELS spectral scans. For REBELS-04, REBELS-11, and REBELS-13, the [OIII] 88µm line is targeted. For REBELS-37, both the [CII] 158µm line (at z < 8) and [OIII] 88µm line (at z > 8) are targeted as part of the spectral scans.
Following the first year of observations from the REBELS program and successful detection of the [CII] 158µm ISM cooling line in REBELS-18 and REBELS-36, several tunings from REBELS-18 and REBELS-36 were shifted to REBELS-06, REBELS-16, and REBELS-37 to extend the redshift range of spectral scans for [CII] 158µm in these sources.

B. DISTRIBUTION OF THE REBELS SAMPLE IN PARAMETER SPACE
The purpose of Appendix B is to illustrate the distribution of REBELS targets in parameter space. Figure 17 shows the stellar mass distribution of sources in the REBELS sample inferred using beagle and compares it against the stellar mass distribution inferred for ALPINE . Overall the two distributions span a fairly similar mass range.
The median stellar mass we estimate for the REBELS sample from beagle is 10 9.25 M (shown in Figure 17  -Distribution of stellar masses (orange histogram) inferred using beagle for the 40 targets in the REBELS LP assuming a constant star formation history. For context, the green histogram shows the stellar mass distribution for z = 4-6 targets in the ALPINE program . The median stellar masses inferred using beagle and using Prospector (Leja et al. 2017: assuming constant and non-parametric star formation histories, respectively) are shown as downward pointing filled and open orangle triangles, respectively. Given the overall similarity of the two distributions and similar medians, the ALPINE program provides us with a convenient lower redshift sample to characterize evolution in galaxies from z > 6.5 to z = 4-6. A similar presentation of the REBELS sample in redshift, U V luminosity, U V -continuum slope β, and the [OIII] 4959,5007 +Hβ EW is provided in Figure 18.
filled downward-pointing orange triangle) assuming a constant star formation rate, while using prospector (Stefanon et al. 2022, in prep), the median stellar mass we estimate is 10 9.8 M (shown in Figure 17 as an open downwardpointing orange triangle). These masses are ∼0.4 dex lower and ∼0.1 dex higher, respectively, than inferred by Faisst et al. (2020) for ALPINE. Figure 18 shows the distribution of galaxies in redshift, U V luminosity, U V -continuum slope β, and [OIII] 4959,5007 +Hβ EW. The EW distribution shown in the lower right panel is derived using a similar procedure to what Endsley et al. (2021a) use to derive [OIII] 4959,5007 +Hβ EWs in their paper, i.e., assuming a delayed star formation history (SFR∝ te −t/τ ) and a SMC extinction curve (Pei 1992). This is to ensure the comparison with Endsley et al. (2021a) is done in the most fair way possible. For completeness, we include the EWs derived both assuming a constant star formation history, as shown in Table 2, and assuming a delayed star formation history in Table 4. The median photometric redshift, U V luminosity, β, and [OIII] 4959,5007 +Hβ EW of the REBELS sources is 6.99, −22.2, −1.98, and 638Å, respectively.
Meanwhile, the median [OIII] 4959,5007 +Hβ EW is consistent with the median EW 759 +112 −113Å derived by Endsley et al. (2021a) at z ∼ 6.75 derived using an analogous procedure. The similarity of the U V -continuum slope β and [OIII] 4959,5007 +Hβ EW distributions seen in REBELS to the z ∼ 7 population suggest that the results derived from REBELS should be reasonably representative of the overall galaxy population at z ∼ 7.  Bowler et al. (2017) for a similar luminosity sample of z ∼ 7 galaxies identified over the COSMOS and UKIDSS/UDS regions, while the shaded region shown in the lower right region is the [OIII] 4959,5007 +Hβ EW distribution derived by Endsley et al. (2021a) at z ∼ 6.75. The median photometric redshift and U V luminosity are 6.99 and −22.2 mag, respectively and indicated by the orange downwardpointing triangles. The median U V -continuum slope β is −1.98 (indicated by the orange downward-pointing triangle) and is consistent with the mean β, i.e., −1.98 ± 0.07, for the Bowler et al. (2017) z ∼ 7 sample as well as the biweight mean β, i.e., −1.75±0.18±0.13, derived by Bouwens et al. (2014) in their highest luminosity z ∼ 7 bin (black circle with 1σ uncertainties). The median [OIII] 4959,5007 +Hβ EW for targets in the REBELS program using a delayed star formation history is 638Å (indicated by the orange downward-pointing triangle) and is consistent with the median EW 759 +112 −113Å (lower black circle with 1σ uncertainties) derived by Endsley et al. (2021a) at z ∼ 6.75 using a similar fitting procedure, as well as an sSFR-evolution-corrected EW of 820 +119 −120Å (upper black circle with 1σ uncertainties) at z ∼ 6.99. The similarity of the U V -continuum slopes β and the [OIII] 4959,5007 +Hβ distribution to the population averages at z ∼ 7 suggest that the results derived from REBELS should be reasonably representative of the general galaxy population at z ∼ 7.