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Strong [OIII]{\lambda}5007 emission line compact galaxies in LAMOST DR9: Blueberries, Green Peas and Purple Grapes

Green Pea and Blueberry galaxies are well-known for their compact size, low mass, strong emission lines and analogs to high-z Ly{\alpha} emitting galaxies. In this study, 1547 strong [OIII]{\lambda}5007 emission line compact galaxies with 1694 spectra are selected from LAMOST DR9 at the redshift range from 0.0 to 0.59. According to the redshift distribution, these samples can be separated into three groups: Blueberries, Green Peas and Purple Grapes. Optical [MgII]{\lambda}2800 line feature, BPT diagram, multi-wavelength SED fitting, MIR color, and MIR variability are deployed to identify 23 AGN candidates from these samples, which are excluded for the following SFR discussions. We perform the multi-wavelength SED fitting with GALEX UV and WISE MIR data. Color excess from Balmer decrement shows these strong [OIII]{\lambda}5007 emission line compact galaxies are not highly reddened. The stellar mass of the galaxies is obtained by fitting LAMOST calibrated spectra with the emission lines masked. We find that the SFR is increasing with the increase of redshift, while for the sources within the same redshift bin, the SFR increases with mass with a similar slope as the SFMS. These samples have a median metallicity of 12+log(O/H) of 8.10. The metallicity increases with mass, and all the sources are below the mass-metallicity relation. The direct-derived Te-based metallicity from the [OIII]{\lambda}4363 line agrees with the empirical N2-based empirical gas-phase metallicity. Moreover, these compact strong [OIII]{\lambda}5007 are mostly in a less dense environment.


INTRODUCTION
The Large sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) (Wang et al. 1996;Su & Cui 2004), located at the Xinglong Observatory in Hebei Province northeast of Beijing, has an effective aperture of 4 m. Ensembling 4000 fibers, the LAMOST facility found 251 Green Pea galaxies from SDSS DR 7 by photometric color criteria, where 80 of the sources have spectroscopic data. Following studies have enlarged the sample size for the Green Pea galaxies. Izotov et al. (2011) contains 803 star-forming luminous compact galaxies (LCGs) with high Hβ luminosities in the redshift range z = 0.02 − 0.63 from SDSS DR7 spectroscopic data. Jiang et al. (2019) contains 800+ Green Pea galaxies from the spectroscopic database of SDSS DR13 and use these samples to to do direct T e -based metallicity calibration. Blueberry galaxies at z < 0.05 are the lowest mass young starburst galaxies (Yang et al. 2017) where the [OIII]λ5007 emission line is located within the g−band and the Hα emission line is located within the r−band. Purple Grape galaxies (Izotov et al. 2011;Brunker et al. 2020) where either that the [OIII]λ5007 emission line is located in the i−band at z > 0.36 and the UV continuum is redshifted to the g−band, or the [OIII]λ5007 is located in the g-band and the Hα emission line is located in the i−band (0.112 < z < 0.36).
In this work, we compile a large catalog consisting of strong [OIII]λ5007 emission line compact galaxies with 1694 spectra from LAMOST DR9 at the redshift range from 0.0 to 0.722. Among the sources 219 have SDSS spectra detections from SDSS DR16 (Blanton et al. 2017;Ahumada et al. 2020). Joint with multi-band photometry, we can systematically learn about the properties for these high SFR, compact galaxies spanning a wide range of redshift coverage.
In Section 2, we describe sample selection criteria, how we do the flux calibration of the LAMOST spectra and the mulltiwavelength dataset we use for following discus-sions. In Section 3, we describe multi-wavelength SED fitting result, and how we identify the AGN candidates from the samples. In Section 4, we discuss the physical properties for these sources, including the stellar mass of the galaxies from spectral fitting, the SFR, the gasphase metallicity, and the environment. In Section 5, we summarize the results addressed in this work.
2. SAMPLE AND DATA

Sample
There are mainly two ranges that we select the strong [OIII]λ5007 samples from the LAMOST DR9 of the extragalactic survey based on the color selection: from both dedicated or non-dedicated "Green Pea" targets of the input-catalog.
The dedicated "Green Pea" targets in the input catalog originate from the PI project of LAMOST extragalactic survey add-on program covering a large area of the North Galactic Cap and a strip in the South Galactic Cap. By now, 2309 spectra have been observed in LAMOST DR9 1 u − r ≤ 2.5, For the non-dedicated "Green Pea" targets in the input-catalog (by non-dedicated we mean that the initial purpose for observing this target was not intended for observing Green Pea galaxies, Blueberry galaxies or other compact emission line galaxies), we select extra 23 Blueberry galaxies (25 spectra), 19 Green Pea galaxies (19 spectra) and 2 Purple Grape galaxies (3 spectra) using the same color criteria in Equation 1, as displayed in Fig.2. 1 http://www.lamost.org/dr9/. The initial color criteria determined from SDSS 12 2 ugriz photometry are as follows. For the sources in z < 0.12, where the [OIII]λ5007 is located within the g−band, the criteria are two-fold: loose color criteria with u − g ≤ 0.3, r − g ≤ 0.1 and i − g ≤ −0.7 but require the type=3 to be galaxy; or no constraint on morphology but require strict color criteria with u − g ≤ 0.5, r − g ≤ 0.5. For the sources in 0.12 < z < 0.4, where the [OIII]λ5007 is located within the r−band, the criteria are displayed in Equation 1 (the same as Equations (1)-(5) in Cardamone et al. (2009)). Next, we remove the sources with a large radius in r−band whose petroRad r > 5 arcsec. We only keep the sources where the mode keyword is equal to 1 from the PhotoObjAll in SDSS DR13 which means primary object. Furthermore, we visually inspect the SDSS images and only keep the isolated sources. For the last step, we visually inspect each spectrum to ensure the strong [OIII]λ5007 emission lines lie within the spectra and only keep the sources where the flux of [OIII]λ5007 line is above 3 × 10 17 erg/s/cm 2 .
Finally, 1547 in total strong [OIII]λ5007 emission line compact galaxies with 1694 spectra are selected. LAM-OST pipeline have classified these sources into four types: 1646 GALAXY, 3 STAR, 35 QSO and 14 Unknown. or barely resolved, where the Petrosian radius of the source is close to the FWHM of the PSF as displayed in Fig.4. We display our sources in the u − r vs M r colormagnitude diagram in Fig.5. As we have expected the majority of the strong [OIII]λ5007 galaxies are located in the blue cloud region of the color-magnitude diagram under the division line defined in Baldry et al. (2004).  All the sources within the redshift range within 0.59 < z < 0.73 are AGN candidates, whose selection criteria will be discussed in Section 3.

LAMOST spectra flux calibration
The 16 LAMOST spectrographs are designed to have a theoretical resolution of R = 1800, covering the wavelength range from 3600 − 9000Å. The blue portion spectrograph covers the wavelength range of 3690 − 5900Å and the red portion spectrograph spans the wavelength coverage is 5700 − 9100Å with 200Å coverage overlap (Luo et al. 2012).
The response curves of the 16 spectrographs have been removed from LAMOST spectra, but the spectra flux is not physically calibrated (Du et al. 2016). We recalibrate the LAMOST spectra according to the SDSS gri photometry following Wang et al. (2018) as described below: LAMOST spectra are convolved with the SDSS gri filters (Fukugita et al. 1996) to obtain the synthetic magnitude for these three bands, then compared with the SDSS photometric magnitude. A zeroth-order or first-order polynomial is used to fit the magnitude difference array and apply this correction to the LAM-  Chilingarian & Zolotukhin (2012). The dark blue dots mark the Blueberry galaxies, the dark green dots mark the Green Pea galaxies, the indigo stars mark the Purple Grape galaxies, and the orange dots mark the spectroscopically-confirmed AGNs. The majority of our samples locate in the blue could region of the color-magnitude diagram, below the dividing line defined by Baldry et al. (2004) between the red sequence and the blue cloud.
OST spectra. An example of the flux calibration result is shown in Fig.6.
For some sources that the LAMOST data reduction pipeline did not calculate their redshift, we need to determine the redshift by fitting the emission lines.

UV and MIR photometry
We cross-match the sources with the GALAXY EVO-LUTION EXPLORER (GALEX) mission (Martin et al. 2005) the revised GALEX catalog of UV sources (GU-Vcat AIS) from GR6+7 (which contains 82,992,086 sources) ) using a radius of 3 arcsec and have obtained 1266 cross-matched results.
The Wide-field Infrared Survey Explorer (WISE) (Wright et al. 2010) mission has conducted a MIR survey with 3.4, 4.6, 12, and 22µm band-passes. We crossmatch our sources in 10 arcsec separation with the All-WISE source catalog, which contains photometry and astrometry of over 747 million objects. Although this separation is large, we have eye-checked every source to ensure there is no contamination source within the search radius. We perform the following criteria in SNR and χ 2 ν space to remove the fake objects from the crossmatched results, as Koenig & Leisawitz (2014) Section In the upper panel, we plot the original LAMOST spectra before flux calibration (transparent blue curve), the LAMOST spectra that are re-calibrated with SDSS gri photometric magnitudes (black curve), and the SDSS spectra (red curve). In the lower panel, we plot the relative residual = (LAMOST flux-SDSS flux)/SDSS flux. We also mark the 0 level with the horizontal line. We have smoothed both the LAMOST spectra and the SDSS spectra with a kernel of 3.
3.1.1 has performed. There are 138 sources after crossmatching with WISE.
We cross-match with Spitzer Space Telescope (Werner et al. 2004) data with Spitzer Enhanced Imaging Products (SEIP) using 3 arcsec radius separation and obtained 32 sources that have detections in Infrared Array Camera (IRAC) (Fazio et al. 2004) and 37 that have detections in Multiband Imaging Photometer for Spitzer (MIPS) (Rieke et al. 2004). We use Spitzer and WISE photometry for multi-band SED fitting as described in the next subsection.

CIGALE SED fitting
We fit the sources with multi-wavelength photometry with CIGALE (Burgarella et al. 2005;Noll et al. 2009;Boquien et al. 2019) trying to use the full photometric data for the 138 sources. As described in the above section, however, GALEX (97 sources) and Spitzer (5 sources) data are available for some of them in addition to SDSS and WISE photometry for the 138 sources. For the configuration of the SED creation modules, we use the delayed−τ star formation history, BC03  . CIGALE SED fitting with GALEX, SDSS ugriz and WISE W1 to W4 photometry. The dust temperature from the best-fitting result for this source is 325K and the AGN fraction is 20%.
For the optical spectra, we spectroscopically confirm 19 AGNs with 21 spectra from LAMOST DR9 with the existence of [MgII]λ2800 emission lines. We display the stacked spectra with three-epoch [MgII]λ2800 detections in Fig.8, and name the source by its LAMOST designation J162736.10+56225.8, where we have three spectra from LAMOST.
Using the BPT diagram (Baldwin et al. 1981;Veilleux & Osterbrock 1987) we classify the galaxies into different spectral types according to the definitions in Kewley et al. (2001); Kauffmann et al. (2003); Kewley et al. (2006). There are 1423 galaxies (1547 spectra) that are classified as star-forming galaxies, 16 galaxies (16 spectra) as composite, and 71 galaxies (73 spectra) as AGN. We plot the distribution for these galaxies in the BPT diagram in Fig.9. We do not consider all the sources classified as AGN through the BPT diagram are real AGN sources as the majority of the sources are located in the upper left region of the diagram, where the high ionization is caused by the high [OIII] lines excited from the star-forming region, not the nuclear region. On the other hand, there might be AGN contaminants located in the SF region especially in the low-metallicity regime as addressed in Harish et al. (2021).
We have confirmed the existence of AGNs from the CIGALE multi-band photometry fitting result, where we use Fritz et al. (2006) model. Among the 138 sources that have multi-wavelength photometry, we have 2 sources where the AGN fraction is over 20%. An example of the SED fitting result is shown in Fig.7 Fig.10 and Fig.11. There are 23 sources with the r 12 > 0.6 that meet the criteria in Harish et al. (2021) as MIR variables and cover the WISE and NEO-WISE detections of at least 9 epochs. Furthermore, we have identified a strong X-ray emission source from the strong [OIII]λ5007 emission line galaxies after cross-matching with 4XMM-Newton DR11 (Webb et al. 2020;Traulsen et al. 2020). This source has the EPIC (indicates parameters combined from those from the available cameras) broadband energy at 0.2 -12.0 keV of 1.033 × 10 43 erg/s, which is higher than the luminosity of the AGNs in nearby galaxies addressed in Maitra et al. (2019). The L 0.5−4.5keV = 9.27 × 10 42 erg/s higher than the AGN identification criteria for X-ray source in Section 4.4 of Xue et al. (2011) that a source with an intrinsic X-ray luminosity of L 0.5−8keV = 3 × 10 42 erg/s is identified as a luminous X-ray AGN. Located at z = 0.6022, this source has the EP 9 broad-band energy covering the energy range of 0.5 -4.5 keV in the observed frame with L 0.8−7.2keV = 9.26 × 10 42 erg/s, which is a lower limit for the L 0. We summarize the sources that have been identified as AGNs with over four of the methods mentioned above in Table 1. 12 galaxies that have been identified with three the methods (8 identified by [MgII]λ2008 emission lines, MIR color and MIR variability, and 4 galaxies identified with BPT diagram, MIR color, and MIR variability). For the galaxies that are identified with one or two methods, there are 11 spectra (9 galaxies) that have been identified with the existence of [MgII]λ2800 line. We consider all the sources that are identified as AGNs with more than 3 methods or have been identified with the existence of [MgII]λ2800 line as AGN candidates and exclude these 25 spectra (23 galaxies) from the star formation rate discussions in the following section.

Color excess from the Balmer decrement
To ensure the quality of the flux ratio measurement, we select the sources at 0.173 ≤ z ≤ 0.385 where both the Hα and the Hβ emission line locate within the red portion of the spectrograph. To ensure the validness of the detection, we apply a SNR cut (median value of the flux over the flux error of the Hβ region) over 3 to select the sources. 62 spectra satisfy this criterion, and their flux ratio are displayed in Fig.12. We measure the color excess of the galaxies from the flux ratio of the Hα to Hβ assuming the Case B recombination with the intrinsic line ratio of 2.86. The color excess from the Flux ( where k(Hα) = 3.33 and k(Hβ) = 4.6 as Jiang et al. (2019). We assume that the nebular gas in these galaxies emit at T = 10 4 K and n e = 10 4 cm −3 . We show the distribution of the color excess in Fig.13. As is discussed in Section 4.1 in Cardamone et al. (2009), some of the sources have a flux ratio below 2.86, we contribute this to the uncertainties in flux calibration and low extinction and manually set their color excess to be 0.

Mass and SFR
The main goal for this subsection is to investigate the mass-metallicity relation and compare the SFR derived from Hα emission line and the FUV. We use 262 spectra (252 galaxies) with valid Starlight spectral fitting results for deriving the mass-metallicity relation, among which 4 are AGN candidates, 67 spectra (64 galaxies) are Blueberry galaxies, 123 spectra (116 galaxies) are Purple Grape galaxies (0.05 ≤ z < 0.112), and 68 spectra (68 galaxies) are Green Pea galaxies. We use 24 samples with positive color excess from the previous subsection and FUV detection from GALEX to derive the extinction corrected SFR.
To derive the physical properties for these sources (Cid Fernandes et al. 2005;Mateus et al. 2006), we fit the re-calibrated LAMOST spectra with the emission lines masked. The fitting procedure is based on a base set of 45 spectra of three metallicities and 15 ages, with the stellar population model based on the BC03 high resolution spectra, and the  extinction curve. The initial values of the velocity shift is set to be v 0 = 0.0 km/s, and the velocity dispersion is of v d = 150.0 km/s. We only consider the Starlight fitting of the LAMOST spectra a valid fitting result, where χ 2 N(λ eff ) < 0.2, adev < 30 and SNR (6000Å < λ rest−frame < 6500Å) ≥ 2.0 and have obtained 262 sources. Fig.14 displays an example of Starlight SED fitting result. Fig.15 demonstrates the relation of the stellar mass of the galaxies derived from Starlight with SFR (measured from the Hα luminosity without extinction correction) (Dopita & Ryder 1994;Kennicutt 1998;Panuzzo et al. 2003;Dopita 2005;Kennicutt & Evans 2012). We separate the strong [OIII]λ4363 emission compact galaxies into Blueberry galaxies, Green Pea galaxies, and Purple Grape galaxies, and compare the mass-SFR relation for these three sets of samples with the SFMS Within each redshift bin, we separate the sources into several mass bins and calculate the median value and the standard deviation of the mass and metallicity for the sources within where t is the age of the universe in Gyr. Similar to the discussions in Section 4.4 in Cardamone et al. (2009), our strong [OIII]λ5007 samples have much higher sSFR at this redshift range, where typically for the galaxies at z ∼ 0.2 the sSFR is around 10 −9 yr −1 (Brinchmann et al. 2004;Bauer et al. 2005).
With the Hα emission line measurement result and the FUV flux, we intend to use this sample to check if the SFRs derived from these two indicators agree. The calculation of the SFR from the recombination emission lines (24 spectra which do not have Hα spectral coverage are dropped out) and FUV flux follow the relation defined in Hao et al. (2011);Murphy et al. (2011);Kennicutt & Evans (2012), where the corresponding parameters are in Table 2: (4) As discussed in Section 4.2, we convert from to the emission lines for the 62 high Hβ SNR galaxies, under the assumption of  extinction curve and R V = 3.1. Within these 62 samples, 36 samples are with positive color excess. 24 out of the 36 samples have GALEX FUV detections. Fig.16 illustrates the comparison of SFR without extinction correction (left panel) and after extinction correction (right panel) with these 24 samples.

Gas-phase metallicity
In this subsection, we first show the distribution of metallicity measured from the N2-based method with 1337 spectra, where the [NII]λ6585 emission line is covered within the spectral range. Then we demonstrate the mass-metallicity relation of 252 sources that have Starlight-derived mass and the N2-based metallicity. Furthermore, we discuss the gas-phase metallicity calculated from the empirical N2-based method and the direct T e -based method with 21 galaxies whose specific flux of the [OIII]λ4363 emission lines are above 8.85 × 10 38 erg/s/Å, where the detection of [OIII]λ4364 is rare (Gao et al. 2017).
We use the N2-based method (van Zee et al. 1998;Pettini & Pagel 2004;Marino et al. 2013) to empirically calculate the gas-phase metallicity. Because the two lines used in this method are close, thus less prone to the flux calibration error. The definition for this index is as follows: N2 = log 10 ([NII]λ6585/Hα), We show the distribution of the mass-metallicity relation in Fig. 18. We separate 248 galaxies (258 spectra), which consist of 64 Blueberry galaxies (67 spectra), 68 Green Pea galaxies (68 spectra), and 116 Purple Grape galaxies (123 spectra) into seven mass bins and calculate the median value and the standard deviation of the mass and metallicity for the sources within this mass bin. Mass-Metallicity relation from Tremonti et al (2004) this work Figure 18. The mass-metallicity relation of 258 spectra with valid Starlight mass measurement. The blue dots mark the measurements from the Blueberry galaxies, the indigo stars mark the measurements from the Purple Grape galaxies at 0.05 ≤ z < 0.112 and the green dots mark the measurements from the Green Pea galaxies. We separate the whole sample into seven mass bins and mark the median value and the standard deviation of the mass and metallicity with black error bars. The black solid curve on the top right is from Tremonti et al. (2004).
We demonstrate that the metallicity increases with the mass by comparing the points, with the smallest mass bin and the largest mass bin excluded where are fewer data points in these two mass bins. All of these strong [OIII]λ4363 emission line compact galaxies are below the Tremonti et al. (2004) mass-metallicity relation.
Besides the empirical method to calculate the gasphase metallicity, there is also the direct T e -based method. However, the auroral [OIII]λ4363 line is not easy to detect. In this work, 21 galaxies whose specific flux of the [OIII]λ4363 emission lines are above 8.85 × 10 38 erg/s/Å are used to test the direct T e -based method to calculate the electron temperature and the metallicity referring to Izotov et al. (2006) Section 3.1 and Jiang et al. (2019). The purpose of this discussion is to show that we have detected the auroral [OIII]λ4363 emission line, which increases the samples of detections in LAMOST (Gao et al. 2017), and to demonstrate the current N2-based empirical metallicity relation and direct T e -based metallicity relation in Jiang et al. (2019) are in agreement within 1−σ level.
This approach assumes two electron temperatures for O + and O ++ in a two-zone photo-ionization model. We calculate the O ++ electron temperature with the following equation, (as in Izotov et al. (2006) Equations (1) and (2)): where t = 10 −4 T e (OIII), and  (3) and (5) Fig.19 demonstrates the comparison of the gas-phase metallicity estimated from the N2-based method and the direct T e -based method. We calculate the error bars in Fig 19 by

Environment
We identify a parent sample of comparison star forming galaxies from the "emissionLinesPort" catalog (Thomas et al. 2013) whose spectroscopic classification is 'Galaxy', the BPT classification is 'star-forming' and cover the same redshift range as our Blueberry galaxies (z ≤ 0.05), Purple Grape galaxies (0.05 < z ≤ 0.112 and 0.36 ≤ z < 0.59), Green Peas (0.112 ≤ z < 0.36). We ensure the parent comparison sample covers the same mag-z space as our selected samples in the r−band as displayed in the following plot. The Neighbors table from SDSS16 provides the angular separation to the nearby objects within 0.5 arc minutes. We only consider the primary detections as the valid neighbors. We calculate the projected linear separation between our source and all the valid neighbors by multiplying the angular separation from the Neighbors table with the luminosity distance at the redshift of our strong [OIII]λ5007 emission line compact dwarf galaxies. We consider the smallest linear separation as the distance to the nearest neighbor. We display the distance for our samples to the nearest neighbor in different redshift bins in Fig.20. We also perform the Komogorov-Smirnov test for these four sets of comparisons, and the p-value between the distance to the nearest neighbor of the parent sample and our selected sample in four redshift bins are all smaller than 0.05. Therefore, it is apparent that the strong [OIII]λ5007 samples are further away from the nearest neighbor compared with typical star forming galaxies.

RESULT
We have selected 1547 unique strong [OIII]λ5007 emission line compact galaxies with 1694 spectra 4 from LAMOST DR9. 1342 galaxies are observed spectroscopically for the first time. Our samples enlarge the current samples of luminous compact galaxies, Green Pea galaxies, and Blueberry galaxies. The following LAMOST extragalactic survey will keep providing large amount of spectra for these targets and provide duplicate detections for the same source helpful for observing the line variability. We have confirmed 23 AGNs candidates (25 spectra) from this strong [OIII]λ5007 emission line compact galaxies by LAMOST optical spectra, BPT diagram, CIGALE SED fitting, MIR color, and MIR variability.
Our samples show that strong [OIII]λ5007 emissionline galaxies have higher SFR compared with the mainsequence star formation rate. The SFR from the FUV luminosity with the Hα luminosity agrees with the coefficients after extinction correction from Kennicutt & Evans (2012). From the mass-SFR plot, we show that with the increase of redshift, the SFR is increasing. For the sources within the same redshift bin, the SFR increases with mass with a similar slope as the SFMS.
For the gas-phase metallicity for these samples, these samples have a median metallicity of 12+log(O/H) of 8.10. In the mass-metallicity plot, all the sources are below the Tremonti et al. (2004) mass-metallicity relation. The metallicity increases with mass. 21 galaxies are found with [OIII]λ4363 emission lines. The direct T e -based metallicity measurement result is in agreement with the N2-based metallicity result.
Lastly, Strong [OIII]λ5007 emission-line galaxies are in a less dense environment.