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The First Systematic Survey for Lyman Alpha Emitters at z=7.3 with Red-sensitive Subaru/Suprime-Cam

We have performed deep imaging surveys for LyA emitters (LAEs) at redshift ~7.3 in two blank fields, the Subaru Deep Field (SDF) and the Subaru/XMM-Newton Deep survey Field (SXDF), using the Subaru/Suprime-Cam equipped with new red-sensitive CCDs and a new narrow-band filter, NB1006 (lambda_c=10052 Ang, FWHM=214 Ang). We identified four objects as LAE candidates that exhibit luminosity excess in NB1006. By carrying out deep follow-up spectroscopy for three of them using Subaru/FOCAS and Keck/DEIMOS, a definitively asymmetric emission line is detected for one of them, SXDF-NB1006-2. Assuming this line is LyA, this object is a LAE at z=7.215 which has luminosity of 1.2^{+1.5}_{-0.6} x 10^43 [erg s-1] and a weighted skewness S_w=4.90+-0.86. Another object, SDF-NB1006-2, shows variable photometry and is thus probably a quasar (QSO) or an active galactic nucleus (AGN). It shows an asymmetric emission line at 10076 Ang, which may be due to either LyA at z=7.288 or [OII] at z=1.703. The third object, SDF-NB1006-1, is likely a galaxy with temporal luminosity enhancement associated with a supernova explosion, as the brightness of this object varies between the observed epochs. Its spectrum does not show any emission lines. The inferred decrease in the number density of LAEs toward higher redshift is n_LyA(z=7.3)/n_LyA(z=5.7) = 0.05^+0.11_-0.05 from z=5.7 to 7.3 down to L(LyA)=1.0 x 10^43 [erg s-1]. The present result is consistent with the interpretation in previous studies that the neutral hydrogen fraction is rapidly increasing from z=5.7 to 7.3.


INTRODUCTION
Cosmic reionization is one of the most important events in the early history of the universe. It is the drastic phase transition of the intergalactic hydrogen medium (IGM) from neutral to ionized; therefore it is closely related to the formation and evolution of the first stars and galaxies. How and when this cosmic reionization process took place remains an issue of great interest in the studies of the early history of the universe and many observational studies have been made to probe early galaxies during the last decade (Iye 2011). The latest measurements of the polarization of the cosmic microwave HI ∼ 0.01 − 0.04 (Fan et al. 2006), where x HI is defined as x HI = n HI /n H with n HI and n H being the number density of neutral hydrogen and all hydrogen in the universe, respectively.
Another promising technique to probe the reionization process is to use Lyman α emitters (LAEs). Narrow-band (NB) (bandwidths of ∼ 100 − 200Å) imaging can efficiently isolate LAEs, which prominently emit Lyα from their interstellar gas ionized by massive stars. Comparing Lyα luminosity functions (LFs) of LAEs before and after cosmic reionization provides an approach to determine the epoch of end of reionization (Haiman & Spaans 1999;Malhotra & Rhoads 2004), because Lyman α photons are sensitive to neutral IGM abundances. In the reionization era, Lyman α photons would be scattered at higher redshifts due to the damping wing from the surrounding neutral IGM. Therefore, the observed abundance of LAEs at this epoch should vary with the neutral fraction of IGM hydrogen. Using LAEs to probe reionization is sensitive to neutral IGM fractions x HI > 10 −3 . This is an advantage over the GP test which is not sensitive at x HI > 10 −3 . LAE imaging surveys are a good observational method to probe the epoch of the cosmic reionization, although it is difficult to distinguish the effects of reionization from those of galaxy evolution.
Due to their very low number density, wide field deep sur-vey observations are essential to construct a statistical sample of high-redshift LAEs. Wide field coverage also allows us to overcome the uncertainties of cosmic variance, as well as potentially large cosmic variance. The Subaru Prime Focus Camera (Miyazaki et al. 2002, Suprime-Cam) has ten MIT/LL 2048 pix ×4096 pix CCDs, located at the prime focus of Subaru Telescope, covering a 34 ′ × 27 ′ field-of-view with a pixel scale of 0 ′′ .202. The CCDs of Suprime-Cam were replaced to new fully depleted CCDs made by Hamamatsu Photonics in 2008 (Kamata et al. 2008) (see the next section). Because the practical ranges of redshifts observable for LAEs are limited by OH airglow lines, some dedicated NB filters, corresponding to z = 3.1, 4.8, 5.7 and 6.6 were manufactured (e.g., Kashikawa et al. 2006Kashikawa et al. , 2011Ouchi et al. 2008Ouchi et al. , 2010Shimasaku et al. 2006;Taniguchi et al. 2005). Surveys performed using these NB filters revealed that the Lyα LF exhibits almost no evolution from z = 3.1 to 5.7, whereas the Lyα LF at z = 6.6 shows an apparent deficit, at least at its bright end, compared to that observed at z = 5.7, while the rest-UV LFs at z = 6.6 and 5.7 are comparable. This result strongly supports the suggestion that the decline in the LAE luminosity function at z > 6 was caused by the attenuation of the IGM rather than by the galaxy evolution. The derived L * at z = 6.6 is 30% − 60% lower than that at z = 5.7, indicating the luminosity attenuation toward higher redshift. On the other hand, Hu et al. (2010) reported that L * is unchanged from z = 5.7 to 6.5 based on their spectroscopically sample. Then, when and how the end of cosmic reionization was completed is still under debate. Similarly, discovery of an LAE at z ∼ 7.0, IOK-1 at z = 6.96, was made using the NB973 filter (λ c = 9755Å, ∆λ = 200Å) imaging (Iye et al. 2006;Ota et al. 2008), revealing that the Lyα luminosity density gradually decreases from z = 5.7 even to 7.0. In fact, the number density of z = 7 LAEs is only 18 − 36% of the density at z = 6.6. These results indicate a substantial transition in the IGM ionizing state during these early epochs. We might be observing the end of the cosmic reionization process.
To trace the neutral fraction as a function of redshift, it is necessary to survey for LAEs at higher-z. LAEs beyond z = 7 have been surveyed by NIR instruments covering wide field-of-view and equipped with NB filters including the 4 m Mayall Telescope/NEWFIRM (Hibon et al. 2010;Tilvi et al. 2010;Krug et al. 2012). They reported that the Lyα LF has not evolved beyond z = 6. These photometric LAE candidates have not yet been spectroscopically confirmed and possible presence of remaining contaminants cannot be ruled out. Therefore, photometric constraining of LFs at z > 7 is not yet robust, and spectroscopic confirmation of the Lyα line is required.
Another effective method is to measure the fraction of Lyα emitting galaxies among LBGs in order to understand the reionization process. Sizable LBG samples at z ∼ 7 found by the HST/WFC3 have been confirmed spectroscopically with the Keck/DEIMOS and the VLT/FORS2 (Fontana et al. 2010;Ono et al. 2012;Pentericci et al. 2011;Schenker et al. 2012;Vanzella et al. 2011). Their results suggest that the Lyα fraction at z ∼ 7 could decline from z ≤ 6 (Stark et al. 2010(Stark et al. , 2011 and it might be caused by the increased neutral fraction at z ∼ 7. Hence, the current suggested redshift range of the end of the cosmic reionization from ∼ 6 to ∼ 11 indicates that the details of the cosmic reionization process remain obscure. In this paper, we present the results of photometric and spectroscopic surveys for z = 7.3 LAEs and use this sample to constrain the history of cosmic reionization. The remainder of this paper is organized as follows. First we describe the imaging survey for z = 7.3 LAEs in two blank fields in §2. We also explain the selection criteria of z = 7.3 LAE candidates based on narrow-band and broad-band photometry in §3. Next, we present deep spectroscopic observations of our photometrically-selected LAE candidates in §4. We compare the estimated Lyα luminosity and SFR of z = 7.3 LAEs with those of z = 6.6 and 5.7 LAEs and discuss constraints on cosmic reionization in §5. In the last section, we summarize our results. Throughout the present paper, we adopt concordance cosmology with (Ω m , Ω Λ , h) = (0.3, 0.7, 0.7), (Spergel et al. 2007) and AB magnitudes with 2 ′′ diameter apertures for the NB1006 surveys unless otherwise specified.
2. IMAGING OBSERVATIONS The sensitivity at 1µm of the previous Suprime-Cam was only about 20%. The CCDs of Suprime-Cam were upgraded to new red-sensitive ones in 2008 (Kamata et al. 2008). As shown in Figure 1, these new CCDs have a quantum efficiency of 40% at 1µm, twice that of the previous CCDs and enable an effective survey for LAEs beyond z = 7. Actually, the survey using the new CCDs for LAEs at z = 7 have obtained a deeper limiting magnitude (Ota et al. 2010a) than that attained by the previous CCDs with same integration time.
We built a new NB filter, NB1006, to carry out a LAE survey at z = 7.3 with the new red-sensitive Suprime-Cam. The transmission band of NB1006 filter is centered at λ c = 10052Å, and has a FWHM ∆λ = 214Å to cover 9943 − 10142Å, a modest, but not perfectly clean, OH band gap. LAEs in the redshift range z = 7.261 ± 0.082 can be detected with this filter (see Figure 1).
We observed two sky regions, the Subaru Deep Field (SDF) and the Subaru XMM/Newton Deep survey Field (SXDF) using the new red-sensitive Suprime-Cam and the new filter, NB1006. The SDF is centered at 13 h 24 m 21. s 4, +27 d 29 ′ 23 ′′ (J2000) (Kashikawa et al. 2004). The field has the deepest imaging data from those having a single pointing of Suprime-Cam. Deep broadband B,V, R, i ′ , z ′ , and narrowband NB816 (λ c = 8160Å, ∆λ FW HM = 120Å) and NB921 (λ c = 9196Å, ∆λ FW HM = 132Å) filter images were taken (Kashikawa et al. 2004) and are made available for public access. The limiting magnitudes in a 2 ′′ aperture at 3σ are B = 28.45, V = 27.74, R = 27.80, i ′ = 27.43, z ′ = 26.62, NB816 = 26.63, NB921 = 26.54, respectively. All of the images were convolved to a common seeing size of 0 ′′ .98. These broad-band and narrow-band images can be downloaded from the official website of the SDF project j .
The SXDF is centered at 02 h 18 m 00. s 0, −05 d 00 ′ 00 ′′ (J2000) (Furusawa et al. 2008;Ouchi et al. 2008Ouchi et al. , 2010, and covers ∼ 1.3 square degree region of sky by five pointings of Suprime-Cam (SXDF-Center, North, South, East, West). We chose a field to cover the southern half of SXDF-Center and the northern half of SXDF-South, where Spitzer SpUDS k , SEDS l , and HST CANDELS (Grogin et al. 2011;Koekemoer et al. 2011) data are also available. (see Figure.2 in Ota et al. (2010a) we matched the PSF of NB1006 images to the maximum PSF size of 0 ′′ .82 as of the SXDF-Center and South. The limiting magnitudes of available near infrared bands (J, H, and K) for SDF and SXDF are much shallower than those for optical bands and we did not use data from J, H, K images for the photometric selection of candidates. The transmission curves of the optical filters used in the observation of SDF and SXDF are shown in Figure 1. Spectroscopic standard stars GD153 and GD71 for SDF and SXDF were imaged during the observation to calibrate the photometric zeropoint.
The SDF and SXDF were observed using the NB1006 filter in February -April 2009 and October 2009 in 22.3 and 14.8 hour integrations, respectively. The observational log is summarized in Table 1. We removed some images with poor quality from final analysis and the actual integration times used in the present paper are 17.2 and 14.3 hours for the SDF and SXDF, respectively. After masking out those regions that have low S/N ratios, the final effective areas of the SDF and SXDF images used were 855 and 863 arcmin 2 , corresponding to the survey volumes of (2.96, 2.98)×10 5 Mpc 3 for the SDF and SXDF, respectively.

SELECTION OF PHOTOMETRIC CANDIDATES
3.1. Photometry Source detection and photometry were carried out using SExtractor version 2.8.6 (Bertin & Arnouts 1996). We searched for objects having an area larger than 5 contiguous pixels with a flux greater than 2σ excess over the sky surface brightness. Objects were chosen from the NB1006 image and their photometry was performed on the images in other wavebands at the same positions. The double-imaging mode of SExtractor was used for this task. The 2 ′′ -diameter aperture magnitudes of detected objects were measured using the MAG_APER parameter and the total magnitudes were measured using MAG_AUTO. When the LAE candidates were selected, the aperture magnitude was used as the magnitude of object. Astrometric calibration was performed for images Bright stellar halos, saturated CCD blooming, and pixels of abnormally high or low flux count spikes were masked out in the SDF and SXDF images in all wavebands. The low S/N regions at the edge of the image were also masked out. The final object catalog constructed from the NB1006 images contains 31, 168 objects for the SDF and 27, 565 objects for the SXDF down to NB1006< 24.83(5σ) for SDF and NB1006< 24.57(5σ) for SXDF.

Detection Completeness
Photometric detection completeness in photometry is defined as the ratio of the number of objects detected by the photometry to that of the objects actually present in the universe. In order to evaluate the detection completeness of our detections down to the limiting magnitude of our NB1006 images, the starlist and mkobjects tasks in IRAF were used. We created a sample 20, 000 artificial stellar objects having the same PSF as on the NB1006 images with a random spatial distribution over blank regions of the images and with magnitudes ranging from 21 to 27 mag. The SExtractor was run for source detection on these mockup image data in exactly the same way as we did in our actual photometry. We used the ASSOCiation mode of the SExtractor to avoid sky noise not located on the artificial stellar objects. We repeated this procedure five times and averaged the obtained detection completeness. The results are shown in Figure 2. It is likely that the detection completeness exceeds 90% at NB1006 < 23.7, and more than 50% even at the 5 sigma limiting magnitudes of NB1006 = 24.83, 24.57, for SDF and SXDF, respectively. This detection completeness factors were used to correct the number and luminosity densities of z = 7.3 LAEs discussed in §5.
3.3. Selection of z = 7.3 LAE candidates We select LAE candidates at z = 7.3 as those objects showing significant flux in NB1006 but not detected in any of the bluer bands. The (z ′ − NB1006, NB1006) color magnitude distribution of all the detected objects in the SDF and SXDF are shown in Figures 3 and 4, respectively. The average z ′ − NB1006 color excess in the magnitude range 22 < NB1006 < 24 was about +0.2. The dashed curves in these figures show 3σ boundaries of z ′ − NB1006 color distribution defined by the equation ±3σ BB−NB = −2.5 log 10 (1 ∓ ( f 2 3σNB + f 2 3σBB ) 0.5 / f NB ). As the diagram indicates, if z ′ − NB1006 ≥ 2.6 is used as the color selection criterion, then z = 7.3 LAEs can be identified.  It should be noted that the NB1006 band pass is located at the red edge of the CCD sensitive wavelengths; therefore, the UV continuum redder than Lyα emission is hardly detected even in the z ′ -band. Hence, we applied the following selection criteria to the detected objects in the same way as Ota et al. (2008): where all magnitudes were measured as 2 ′′ -diameter aperture magnitudes, MAG_APER.
Color criterion, z ′ − NB1006 > 2.6, was determined by the model spectrum of a galaxy at z = 7.3. We created a spectral energy distribution (SED) of a starburst galaxy using stellar population synthesis model, GALAXEV (Bruzual & Charlot 2003), with a metallicity of Z = Z ⊙ = 0.02, an age of t = 1 Gyr, a Salpeter initial mass function with lower and upper mass cutoffs of m L = 0.1M ⊙ and m U = 100M ⊙ , and an exponentially decaying star formation history for τ = 1 Gyr. These parameters were selected to be the same as those used in Taniguchi et al. (2005); Ota et al. (2008), for consistency. The absorption by IGM was applied to the generated spectrum using the model of Madau (1995). The flux of a Lyα emission line with EW 0 = 0Å was added to the SED at (1 + 7.26)1216Å. In other words, we create a SED of LBG with no Lyα emission line. Hence, the presently adopted color criterion, z ′ − NB1006 > 2.6, corresponds to the rest-frame equivalent width of Lyα of EW 0 > 0. Moreover, the photometric error may cause interlopers at lower-z to happen to satisfy our color selection criteria. We carried out the following test in the same way as did in Bouwens et al. (2011b); Wilkins et al. (2011). First, we choose brighter sources in magnitude range of 22.5 > NB1006 > 23.5 from our two fields and then dim these bright sources to match to the same magnitudes as of our LAE candidates by scaling the optical BB and NB fluxes. We then distribute these objects over the real images. The number of these artificial objects is the same as the number of observed objects in the magnitude range from NB1006 < 23.5 down to each NB1006 limiting magnitude. We applied exact the same detection and selection criteria as the candidate selection. As a result, we found no objects happen to come into our criteria, suggesting that our selection criteria can robustly avoid any nearby red and line emitting interlopers.
As a consequence of selection, we found no candidates meeting only criterion (1) and two objects satisfying both criteria (1) and (2) in each field. The images and photometric properties of these LAE candidates are listed in Figure 5, and Table 2. We detected no flux even in the stacked optical and NIR images of all the candidates, as shown in the bottoms of  4. SPECTROSCOPY We carried out optical deep spectroscopy for three of our four photometric LAE candidates. We used the Faint Object Camera and Spectrograph (FOCAS) in Multi-Object Spectroscopy (MOS) mode mounted on the Subaru telescope on 2010 March 18, 19, and 21 to follow the two candidates in the SDF, SDF-NB1006-1 and SDF-NB1006-2. The observations were made with a 555 mm −1 grating and an O58 order-cut filter, giving spectral coverage of 7500 − 10450Å with a dispersion of 0.74Åpix −1 . The 0 ′′ .8-wide slit used gave a spectroscopic resolution of 5.7Å (R∼ 1500) which is sufficient to distinguish [O II] doublet lines from possible contaminant galaxies at z=1.7. However, it is likely that [O II] doublet lines are not resolved in this resolution if the emission lines are too faint or are affected by OH sky lines. The observing nights were photometric, with good seeing of ∼ 0 ′′ .6 − 1 ′′ .0. Because no emission line feature was visible for the brighter candidate, SDF-NB1006-1, in the entire transmission range of the NB1006 filter in 5 hours integration, this object was abandoned. We switched on to the second candidate, SDF-NB1006-2, and secured spectra for a total integration time of 11.5 hours.
In addition to the observations made in March, we observed SDF-NB1006-2 for a half night on 2010 May 5. The seeing 0 ′′ .8 − 1 ′′ .0 was somewhat poor but the weather condition was photometric. The setup of the instrument was exactly the same as that in March. The integration time in May was 4.5 hours, and total integration time for the SDF-NB1006-2 spectroscopy is 16 hours.
Each of the 30 minute exposures was taken by dithering the telescope pointing along the slit by ±1 ′′ .0 under good seeing conditions and ±1 ′′ .5 under moderate seeing conditions to enable more accurate sky subtraction. The standard star Feige 34 and Hz 44 were taken at the beginning and end of each observed night except on May 5 when only Feige 34 was observed (Massey & Gronwall 1990). Feige 34 and Hz 44 were used for flux calibration of May and March data, respectively. The object and standard star data were reduced in a standard manner with IRAF. Also, we evaluated the slit loss for the Lyα flux measurements calculated from the spectra. The targets were assumed to be almost perfectly centered in the slit and point sources. The slit loss was calculated by putting a Gaussian profile with FWHM derived from the average seeing size in the FOCAS spectroscopic observation on the 0 ′′ .8-wide slit. The slit loss of the FOCAS spectroscopy was evaluated to be ∼ 23%. The resultant slit loss was corrected for the spectroscopically measured Lyα for the object. The stacked oneand two-dimensional spectra of SDF-NB1006-2 are shown in Figure 6. A prominent emission line is seen at 10075.8Å.
We also used DEep ImagingMulti-Object Spectrograph (DEIMOS) in MOS mode at the Nasmyth focus of the Keck II telescope to take spectroscopy for the candidate, SXDF-NB1006-2 as described in (Ota et al. in prep). The seeing was ∼ 0 ′′ .6 − 1 ′′ .0. We used the GG495 filter and the 830 lines mm −1 grating, which was tilted to place a central wavelength of 9000Å on the detectors. This configuration provided spectral coverage between 7000Å and 10400Å. The spatial pixel scale was 0 ′′ .1185 pix −1 , and the spectral dispersion was 0.47Åpix −1 . The slit widths were 1 ′′ .0, which gives the spectral resolution of 3.3Å. The total integration time of DEIMOS is 3.5 hours.
The reduction of the spectra for SXDF-NB1006-2 was performed using spec2d IDL pipeline developed by the DEEP2 Redshift Survey Team (Davis et al. 2003). The final onedimensional data output from the spec2d, however, show apparent over-subtraction of the foreground OH sky lines. In order to solve this difficulty, we modified the code in the process of calibrating the wavelength using arc lamp and subtracted OH sky lines carefully in a manner similar to that employed in the FOCAS data reduction with IRAF.
Although we also observed the standard star Feige 110 for flux calibration with DEIMOS, these data were not reduced due to a computational problem. Hence, the flux of the SXDF-NB1006-2 was calibrated using the spectra of five late-type stars observed simultaneously in the same slit mask mounted on DEIMOS. We compared their spectroscopically measured fluxes with those expected from their photometry convolved with the filter (NB1006) transmission and derived the zero-point for the spectroscopic flux calibration. The statistical fluctuation in the above flux calibration with five latetype stars was used as the estimated flux error of the SXDF-NB1006-2. The slit loss has already been corrected in this calibration. The stacked one-and two-dimensional spectrum of the SXDF-NB1006-2 are shown in Figure 7.
We measured the weighted skewness of the detected emission lines, which can be used as a criterion to identify high-z Lyα emission lines, (Kashikawa et al. 2006). The weighted skewness was calculated in the wavelength range where the flux is higher than 10% of the peak value, ranging from 10068Å to 10088Å, and from 9980Å to 9998Å, for SDF-NB1006-2 and SXDF-NB1006-2, respectively. They were S ω = 4.59 ± 1.25 for SDF-NB1006-2 and 4.90 ± 0.86 for SXDF-NB1006-2. These measured skewness values satisfy the usually adopted criterion for the presence of asymmetry of high-z Lyα emission, S ω > 3 for typical LAEs at z > 5.7. Note however that the red side of emission line in the SDF-NB1006-2 spectrum has an overlap with a weak OH sky line, and the derived skewness value might be overestimated.
We had no chance to observe the remaining fourth candidate in the SXDF, SXDF-NB1006-1 during our allocated time. The spectroscopic properties of the line emitting objects are summarized in Table 4. 5. DISCUSSION 5.1. Check for the Variability of the Candidates Since the NB1006 imaging observations we used for detecting our z = 7.3 LAE candidates were made a few years after the imaging observations for other bands, some of the objects which are only visible in NB1006 could be variable objects, such as galaxies with supernovae or enhanced active galactic nuclei (AGNs), which happened to increase their luminosities during our NB1006 imaging observations. Since our SDF imaging was made in three epochs, we created three stacked NB1006 SDF images, one for February 2009, one for March 2009, and one for April 2009 to check for any variability. The SXDF observations were taken on two successive nights on October 15 and October 16. Although the time interval is short for SXDF, we also examined possible photometric variability.
Objects were chosen from the stacked NB1006 image in all epochs and photometry was performed on the images in the  24.26 ± 0.07 a The 3σ limiting magnitudes of the candidates in z ′ -band were derived from the 2 ′′ φ-aperture photometry. b The NB1006 magnitude of the SDF-NB1006-2 was derived by 3 ′′ diameter aperture photometry instead of MAG_AUTO to avoid contamination from the flux of a nearby object. individual epochs at exactly the same positions as the objects in the NB1006 filter using the double-imaging mode of SExtractor. The results of independent brightness measurements at different epochs of observation are summarized in Table 5 and shown as postage stamp images of the individual epochs are shown in Figure 8.
As shown in the table, the magnitudes of candidates SDF-NB1006-1, and 2 vary by more than ∼ 0.5 mag over two months, while the average magnitude inconsistency of other objects in the SDF NB1006 images which have the similar magnitudes as the LAE candidates is less than 0.1 ± 0.6 mag.
In particular, SDF-NB1006-1, the brightest among our z = 7.3 LAE candidates, appears to show a significant luminosity decline of 1.8 mag as shown in Figure 8. The brightness change corresponds to 7σ phot , where σ phot is the 1σ photometric error of the object. This amplitude of variability is roughly consistent with the J-band light-curve of a type Ia supernova, depending on the phase of its explosion (Burns et al. 2011). We conclude that SDF-NB1006-1 is a variable object which brightened suddenly in February and then gradually faded. It may be a galaxy with a supernova, which is consistent with the fact that no obvious emission line was confirmed in the  FOCAS spectroscopy.
The possible magnitude variation of SDF-NB1006-2, which shows an emission line with an asymmetric profile is a bit more enigmatic. The emission line indicates that it could be a LAE at z = 7.288 considering its high weighted skewness (S ω = 4.59 ± 1.25). On the other hand, because the NB1006 magnitude of this object changed by ±0.8 − 0.9 mag (3.3 − 7.9σ phot ) over about one month, it could also be an AGN or a QSO. This suggestion might be supported by the high UV magnitude of SDF-NB1006-2 derived by subtracting the Lyα flux from the total flux in the NB1006 magnitude. Considering that the line width is narrower than that of typical QSOs, it is unlikely to be a QSO. change (Kaspi et al. 2007). Besides, the change of UV continuum flux radiated from the BLR is investigated by the structure function of the photometric variability of AGNs/QSOs (de Vries et al. 2005), and is ∼ 0.55 mag for a year. The structure function is the ensemble average of QSO variability as function of time and has been used as a tool to characterize the QSO variability. Since the UV continuum flux of the SDF-NB1006-1 is only ∼ 70% of the NB1006 flux, its large variability might not be due to AGNs/QSOs, though the variation of the investigated structure function in the literature is relatively large. Hence, this object might be an AGN/QSO at z = 7.288 if this line is Lyα, or at z = 1.703 if that is [O II], rather than a LAE, nevertheless the large amplitude of the variability cannot be explained adequately.
By contrast, the magnitude of the candidates in the SXDF, SXDF-NB1006-1 and 2, did not show any significant variation that would lead us to suspect their AGN nature. The brightness changes of SXDF-NB1006-1 and 2 correspond to 0.2 mag (1.1σ phot ) and 0.42 mag (2.2σ phot ), respectively. The reality of the SXDF-NB1006-2 at z = 7.215 is also supported by this result of checking variability.
Moreover, we investigate the possibility that the detected lines could be Hα, Hβ, or [O III] in terms of metallicities estimated from diagnostic line ratios. The same case test for z ∼ 6.9 − 7.2 LBGs was also discussed in Ono et al. (2012). Any other emission line features were not observed in the spectra; therefore the metallicity is only given as an upper limit by using the flux limit. If the detected lines were Hβ, [O III]λ5007 line would fall at 10378Å and 10288Å for SDF-NB1006-2 and SXDF-NB1006-2. The upper limits of [O III]λ5007/Hβ line ratios are calculated to be ≤ 0.1 − 0.4, corresponding to the metallicities of 12+log(O/H) ≥ 9.0 − 9.5 based on the Figure 17 of Nagao et al. (2006). In the case of such high metallicities, they should be very massive and bright taking account the mass-metallicity relation. However, they are not detected in our deep broadband images. If the detected lines were [O III]λ5007, the Hβ line would be detected at 9782Å and 9697Å for SDF-NB1006-2 and SXDF-NB1006-2, and the lower limits of [O III]λ5007/Hβ are given as ≥ 11 and ≥ 2, respectively. The detected line of the SDF-NB1006-2 is unlikely to be [O III], because no outlier with [O III]λ5007/Hβ > 10 in Nagao et al. (2006). The deep FOCAS spectroscopy for SDF-NB1006-2 completely rules out the possibility of other lines. On the other hand, the lower limits for SXDF-NB1006-2 corresponds to subsolar oxygen abundances, which may be typical of low-mass galaxies. Hence, the non-detection of Hβ cannot strongly rule out the possibility of the detected line in the SXDF-NB1006-2 spectrum being [O III]λ5007. The possibility of Hα can be ruled out because either [O III] or Hβ is expected to be simultaneously detected at the observed wavelength for both targets. The possibility that the detected line is the unresolved [O II] doublet can be ruled out for SXDF-NB1006-2 due to the spectral resolution of DEIMOS. On the other hand, for SDF-NB1006-2, we cannot deny the possibility of [O II]. After all, we could completely rule out the possibility of the detected emission lines being other nebular emission lines, i.e. Hα, Hβ, and [O III]. One possibility that the detected line of SXDF-NB1006-2 is Hβ could not be completely ruled out based on the diagnostic line ratios. However, we should note that the asymmetric line profile strongly implies the line to be Lyα emission. The SDF-NB1006-1 and 2, which have significant variability amplitude more than 3σ phot , are concluded as variable objects. We conclude that the SXDF-NB1006-2 is a plausible LAE at z = 7.215, but the SDF-NB1006-2 is likely to be a AGN/QSO either at z = 7.288 or at z = 1.703. Our results indicate that checking the time variability is crucial to strictly verify the LAE photometric candidates if they have been detected in only a single band. The checking the variability is also attempted in Bouwens et al. (2011a). This check implies a possibility that two of the LAE candidates in the SDF could be transient objects such as an AGN/QSO or supernova, even though an asymmetric line was detected. However, we should note that the time span in SXDF was only one day delay which was too short to completely trace the typical variability. In Figure 9, we compare the luminosities of our spectroscopically confirmed LAE at z = 7.215 with the previous LF results obtained at z ≤ 7.0. The SDF-NB1006-2 is also included as a preliminary measurement. The number densities of both objects are corrected for the detection completeness derived in §3.2. The error of the number density of the z = 7.3 LAEs includes the possible cosmic variance and the Poissonian error for small number statistics. The values in Table. 1 and 2 of Gehrels (1986) are used as the upper and lower limit of Poissonian errors. The relative cosmic (root) variance σ v is calculated by σ v = bσ DM , where b and σ DM are the bias parameter and the (root) variance of dark halo, respectively (Somerville et al. 2004). The σ DM at z = 7.3 with our survey volume of 5.94 × 10 5 Mpc 3 is calculated to be 0.033 by the model of Sheth & Tormen (1999). The bias parameter is adopted to be b max = 9 derived from the LAE sample at z = 6.6 detected in the full SXDF, which is ∼ 5 times wider than the SDF . The resultant cosmic variance σ v is ∼ 30%. The error of Lyα flux of the SXDF-NB1006-2 includes the error in the flux calibration from using the dwarf stars (see §4) in addition to the spectral sky noise. The cumulative Lyα LFs derived from the photometric LAE candidates at z = 7 and 7.7 (Hu et al. 2010;Hibon et al. 2010Hibon et al. , 2012Krug et al. 2012;Tilvi et al. 2010) are also plotted in Figure 9. Their photometric Lyα LFs at z = 7 and 7.7 in Hibon et al. (2010); Ota et al. (2010a) is based on Lyα luminosity of each LAE candidate directly converted from Lyα flux by assuming f Lyα = f NB . However, Ota et al. (2010b) estimated that ∼ 77% and 23% of NB973 total flux of a z = 6.96 LAE comes from Lyα and UV continuum fluxes, respectively, using imaging and spectroscopy data. Hence, in Figure 9 we shifted the photometric z = 7 and 7.7 Lyα LFs faintward with 0.77 × L Lyα for fair comparison with other spectroscopic Lyα LFs. As shown in Figure 9, the luminosity of SXDF-NB1006-2 is fainter than those of the LFs at z = 5.7 and 6.6, though their errors are large. Furthermore, the number density of the SXDF-NB1006-2 is slightly smaller than that of the LAE at z = 6.96, IOK-1, which was selected a single pointing of NB973 imaging (only SDF) with the Suprime-Cam, while their Lyα luminosities are similar. What needs to be emphasized is that secure LAE samples can be constructed by spectroscopy, line-asymmetry investigation, and checking the magnitude variability, and compared appropriately with LFs at z ≤ 7.0. While the previous photometric LFs at z = 7 and 7.7 do not change significantly from z < 7, the number density of the LAE at z = 7.3 shows deficit.
We also compare the number and SFR densities at z = 7.3 with those of other LAE surveys at z ≤ 7 down to L Lyα = 1.0 × 10 43 erg s −1 in Figure 10 in the same way as Ota et al. (2008) to investigate the redshift-evolution of these densities. The densities of LAEs at z = 5.7 and 6.6 in Kashikawa FIG. 9.-Cumulative Lyα LFs at z ≤ 7 and the number densities of z = 7.3 LAE, the SXDF-NB1006-2 (filled square) and the SDF-NB1006-2 (open square). The number densities are corrected using the detection completeness derived in §3.2. The error of the number densities includes cosmic variance and Poissonian error. The error of the Lyα luminosity includes the spectral sky noise of both objects in the z = 7.3 survey, and the error in the flux calibration of the DEIMOS data in addition to the sky noise of SXDF-NB1006-2 (see §4). The filled circle is the LAE at z = 6.96 identified spectroscopically (Ota et al. 2008). The dashed blue and red curves represent the Lyα LF fitted by Schechter function of LAEs at z = 5.7 and 6.6 in the SDF, respectively (Kashikawa et al. 2011). The solid blue and red curves are those at z = 5.7 and 6.6 in the full SXDF, which is ∼ 5 wider than the SDF . The dark-green triangles, bold-and dashed-curves are photometric z = 7 LAE samples in Ota et al. (2010a), fitted Schechter functions in the D33 and D41 field of the CFHTLS in Hibon et al. (2012) derived by using the Suprime-Cam equipped with NB973. The orange filled squares, filled circles, and curve are photometric z = 7.7 LAE samples derived by using the Mayall 4m telescope/NEWFIRM camera (Krug et al. 2012;Tilvi et al. 2010;Hibon et al. 2010). The slope parameter α of all the fitted Schechter functions are fixed to be −1.5. If SXDF-NB1006-1 is a real LAE at z = 7.3 with brighter L Lyα than one of the SXDF-NB1006-2, the cumulative number densities of SXDF-NB1006-2 and SDF-NB1006-1 become the small filled square and the open square without L Lyα error slightly shifted in the Lyα luminosity direction for clarification. et al. (2006) are updated to those of the ∼ 80% spectroscopically identified samples (Kashikawa et al. 2011), and the one of LAEs at z = 6.6 selected from the ∼ 5 times wider survey area ) than SDF. It is notable that the Lyα LF of Ouchi et al. (2010) is mainly from the photometric LAE sample. The density errors are included as in Figure 9. As shown in this figure, compared with number at z = 5.7 and 6.6, the LAE number density at z = 7.3 decreases by factors of n z=7.3 Lyα /n z=5.7 Lyα = 0.05 +0.11 −0.05 from z = 5.7 to 7.3 and n z=7.3 Lyα /n z=6.6 Lyα = 0.16 +0.38 −0.16 from z = 6.6 to 7.3, although the statistical uncertainty at z = 7.3 is large. The number and SFR density might decrease, n z=7.3 Lyα /n z=7.0 Lyα = 0.72 +0.28 −0.72 from z = 7.0 to 7.3. Figure 10 indicates that the number and SFR densities monotonically decrease clearly from z = 5.7 to 7.3, while they do not change from z = 3 to 6. In contrast, the number density of the rest-UV continuum of the SXDF-NB1006-2 at z = 7.3 might be comparable with the bright end of the rest-UV continuum cumulative LFs at z = 5.7 and 6.6 as shown in Figure 11. These UV continuum LFs were constructed from LAE samples in SDF (Kashikawa et al. 2011). The UV magnitude of SXDF-NB1006-2 was derived by subtracting the Lyα flux from the total flux in the NB1006 magnitude. This Lyα deficit might be caused by the gradual and continuous reionizing process from z = 5.7 to 7.3, because the density of the rest-UV continuum is insensitive to -Number density n Lyα , and star formation rate density SFRD Lyα of the LAE at z = 7.215, SXDF-NB1006-2 confirmed spectroscopically in this study (filled square) and those at 2.3 ≤ z ≤ 7 from the spectroscopic studies down to L Lyα = 1.0 × 10 43 erg s −1 . The filled circles are those of the LAE at z = 6.96 (Ota et al. 2008). The crosses show the densities at z = 5.7 and 6.6 in the SDF calculated from the samples of (Kashikawa et al. 2011). Densities at 2.3 ≤ z ≤ 4.5 and z ∼ 4.5 are calculated using the best-fit Lyα Schechter LFs from van Breukelen et al. (2005) (inverse triangles) and Dawson et al. (2007) (pentagons). Also, densities at z = 3.1, 3.7, 5.7 (diamonds) and 6.6 (triangles) in the ∼ 1.0 deg 2 of SXDF are calculated using the best-fit Lyα Schechter LFs from Ouchi et al. (2008Ouchi et al. ( , 2010. Each horizontal error bar shows the redshift range of each survey. The vertical error bars at z = 5.7, 6.6, 7 and 7.3 include both cosmic variance and Poissonian errors for small number statistics. We correct for the detection completeness of the z = 7.3 LAE survey. If SXDF-NB1006-1 is a real LAE at z = 7.3 with a brighter L Lyα than one of SXDF-NB1006-2, the number and SFR densities of SXDF-NB1006-2 becomes small the open square slightly shifted in the redshift direction for clarification. The filled circle and square is the rest-UV number density of the z = 6.96 (Ota et al. 2008) LAE and the SXDF-NB1006-2, respectively. The UV magnitude of the SXDF-NB1006-2 is comparable with that of the rest-UV bright end at z = 5.7 and 6.6. the surrounding HI clouds and might not evolve. Note however that the UV magnitude of SXDF-NB1006-2 has a large error caused by Lyα flux error (see Table 4), and the Lyα deficit might arise from the galaxy evolution as well as HI clouds. This tendency is consistent with the results of the previous surveys for LAEs z ≤ 7 (Ota et al. 2008;Kashikawa et al. 2006Kashikawa et al. , 2011Ouchi et al. 2010) and our results might show for the first time that the trend continues up to z ∼ 7.3. On the contrary, our result is inconsistent with the results of Hu et al. (2010); Tilvi et al. (2010); Krug et al. (2012); Hibon et al. (2010) which reported that Lyα LFs do not evolve from z = 5.7 to 7.7. If the decline of Lyα luminosity is caused only by IGM attenuation, the neutral fraction x HI of hydrogen gas at z = 7.3 can be estimated to be ∼ 0.79 using a radiative transfer simulation based on McQuinn et al. (2007) which calculate the number density of LAE at the epoch of reionization, and we assume that the IGM at z = 5.7 is completely ionized. Compared with L Lyα of SXDF-NB1006-2 and the Lyα LF at z = 7.3 constructed by the model of Kobayashi et al. (2010), the IGM transmission for Lyα photons T IGM Lyα can be estimated. Here, T IGM Lyα is calculated with T IGM Lyα = observed L Lyα / model L Lyα . As we use the faintest L Lyα in the lowest number density within their errors as the observed L Lyα , the derived T IGM Lyα is given as an lower limit, 0.27. Thus, the possible T IGM Lyα ranges 0.27 ≤ T IGM Lyα ≤ 1. If we apply the calculated T IGM Lyα to the Lyα flux attenuation model of (Santos 2004), the neutral fraction x HI at z = 7.3 is estimated to be 0.82. Note that the derived neutral fraction is strongly model dependent. However, it must be noticed that one remaining candidate, the SXDF-NB1006-1, has yet not been observed spectroscopically. If this candidate was a real LAE with similar Lyα luminosity with the SXDF-NB1006-2 at = 7.3, the number density would increase to that at z = 7, and not evolve between z = 7 and 7.3 as indicated by small points in Figure 9 and 10. On the other hand, if all the candidates were variable objects, which indicates that no LAE at z = 7.3 were detected in our survey volume, the derived x HI is only given as an upper • Compared with the number density and SFR density of LAEs at z ≤ 7, The number density and SFR density of LAEs monotonically decreases from z = 5.7 to 7.3 while they do not change from z = 3 to 6, and the number density of rest-UV continuum at z = 7.3 is comparable with the rest-UV cumulative LFs at z = 5.7 and 6.6. The derived decreases of the number density are n z=7.3 Lyα /n z=5.7 Lyα = 0.05 +0.11 −0.05 from z = 5.7 to 7.3 and n z=7.3 Lyα /n z=6.6 Lyα = 0.16 +0.38 −0.16 from z = 6.6 to 7.3 down to L Lyα = 1.0 × 10 43 erg s −1 . This trend is consistent with the results of the previous surveys for LAEs at z ≤ 7, and it might be caused by IGM attenuation.
• The magnitude of SDF-NB1006-2 in the NB1006 images varies and it might be a AGN/QSO at z = 7.3 or 1.7 rather than a LAE, in spite of the detected asymmetric emission line (S ω = 4.59 ± 1.25). The third object, SDF-NB1006-1, is likely a galaxy with temporal luminosity enhancement associated with a QSO, an AGN, or a supernova explosion, as the brightness of this object varies between the observed epochs. Its spectrum does not show any emission lines. The magnitude of the candidates in the SXDF, SXDF-NB1006-1 and 2, did not show any significant variation, though the time span to check the variability in SXDF was not long enough to completely trace the typical variability. Hence, it is important to the check whether the candidates are transient objects or not by dividing photometric images into several epochs.
In order to accurately measure the neutral fraction and reveal the relationship of LAEs and LBGs in the evolution of galaxy at z ≥ 7 in more detail, multi-epoch surveys using next generation facilities such as the Subaru/Hyper Suprime-Cam (HSC) that covers 1.5 deg FoV in diameter, equipped narrow band filters, are needed to establish larger LAE sample.