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Register a new userThe GREATS H$beta$+[OIII] Luminosity Function and Galaxy Properties at $mathbf{zsim8}$: Walking the Way of JWST
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The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [OIII] and H$beta$ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at $zsim8$ and study their rest-frame optical emission lines. Combining data from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and from HST, we perform spectral energy distribution (SED) fitting, using synthetic SEDs from a large grid of photoionization models. The deep Spitzer/IRAC data combined with our models exploring a large parameter space enables to constrain the [OIII]+H$beta$ fluxes and equivalent widths for our sample, as well as the average physical properties of $zsim8$ galaxies, such as the ionizing photon production efficiency with $log(xi_mathrm{ion}/mathrm{erg}^{-1}hspace{1mm}mathrm{Hz})geq25.77$. We find a relatively tight correlation between the [OIII]+H$beta$ and UV luminosity, which we use to derive for the first time the [OIII]+H$beta$ luminosity function (LF) at $zsim8$. The $zsim8$ [OIII]+H$beta$ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [OIII]+H$beta$ luminosity at a given UV luminosity from $zsim3$ to $zsim8$. Finally, using the [OIII]+H$beta$ LF, we make predictions for JWST/NIRSpec number counts of $zsim8$ galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for $zsim8$ spectroscopy even at 1hr depth and JWST pre-imaging to $gtrsim30$ mag will be required.
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We investigate the clustering properties of $sim 7000$ H$beta$+[OIII] and [OII] narrowband-selected emitters at $z sim 0.8 - 4.7$ from the High-$z$ Emission Line Survey. We find clustering lengths, $r_0$, of $1.5 - 4.0h^{-1}$ Mpc and minimum dark matter halo masses of $10^{10.7 - 12.1}rm{M}_odot$ for our $z = 0.8 - 3.2$ H$beta$+[OIII] emitters and $r_0 sim 2.0 - 8.3h^{-1}$ Mpc and halo masses of $10^{11.5 - 12.6}rm{M}_odot$ for our $z = 1.5 - 4.7$ [OII] emitters. We find $r_0$ to strongly increase both with increasing line luminosity and redshift. By taking into account the evolution of the characteristic line luminosity, $L^star(z)$, and using our model predictions of halo mass given $r_0$, we find a strong, redshift-independent increasing trend between $L/L^star(z)$ and minimum halo mass. The faintest H$beta$+[OIII] emitters are found to reside in $10^{9.5}rm{M}_odot$ halos and the brightest emitters in $10^{13.0}rm{M}_odot$ halos. For [OII] emitters, the faintest emitters are found in $10^{10.5} rm{M}_odot$ halos and the brightest emitters in $10^{12.6}rm{M}_odot$ halos. A redshift-independent stellar mass dependency is also observed where the halo mass increases from $10^{11}rm{M}_odot$ to $10^{12.5} rm{M}_odot$ for stellar masses of $10^{8.5}rm{M}_odot$ to $10^{11.5}rm{M}_odot$, respectively. We investigate the interdependencies of these trends by repeating our analysis in a $L_textrm{line} - rm{M}_textrm{star}$ grid space for our most populated samples (H$beta$+[OIII] $z = 0.84$ and [OII] $z = 1.47$) and find that the line luminosity dependency is stronger than the stellar mass dependency on halo mass. For $L > L^star$ emitters at all epochs, we find a relatively flat trend with halo masses of $10^{12.5 - 13}rm{M}_odot$ which may be due to quenching mechanisms in massive halos which is consistent with a transitional halo mass predicted by models.
Recent surveys have identified a seemingly ubiquitous population of galaxies with elevated [OIII]/H$beta$ emission line ratios at $z > 1$, though the nature of this phenomenon continues to be debated. The [OIII]/H$beta$ line ratio is of interest because it is a main component of the standard diagnostic tools used to differentiate between active galactic nuclei (AGN) and star-forming galaxies, as well as the gas-phase metallicity indicators $O_{23}$ and $R_{23}$. Here, we investigate the primary driver of increased [OIII]/H$beta$ ratios by median-stacking rest-frame optical spectra for a sample of star-forming galaxies in the 3D-HST survey in the redshift range $zsim1.4-2.2$. Using $N = 4220$ star-forming galaxies, we stack the data in bins of mass and specific star formation rates (sSFR) respectively. After accounting for stellar Balmer absorption, we measure [OIII]$lambda5007$AA/H$beta$ down to $mathrm{M} sim 10^{9.2} mathrm{M_odot}$ and sSFR $sim 10^{-9.6} mathrm{yr}^{-1}$, more than an order of magnitude lower than previous work at similar redshifts. We find an offset of $0.59pm0.05$ dex between the median ratios at $zsim2$ and $zsim0$ at fixed stellar mass, in agreement with existing studies. However, with respect to sSFR, the $z sim 2$ stacks all lie within 1$sigma$ of the median SDSS ratios, with an average offset of only $-0.06pm 0.05$. We find that the excitation properties of galaxies are tightly correlated with their sSFR at both $zsim2$ and $zsim0$, with a relation that appears to be roughly constant over the last 10 Gyr of cosmic time.
We investigate the evolution of the H$beta$+[OIII] and [OII] luminosity functions from $z sim 0.8$ to $sim5$ in four redshift slices per emission line using data from the High-{it z} Emission Line Survey (HiZELS). This is the first time that the H$beta$+[OIII] and [OII] luminosity functions have been studied at these redshifts in a self-consistent analysis. This is also the largest sample of [OII] and H$beta$+[OIII] emitters (3475 and 3298 emitters, respectively) in this redshift range, with large co-moving volumes $sim 1 times 10^6$ Mpc$^{-3}$ in two independent volumes (COSMOS and UDS), greatly reducing the effects of cosmic variance. The emitters were selected by a combination of photometric redshift and color-color selections, as well as spectroscopic follow-up, including recent spectroscopic observations using DEIMOS and MOSFIRE on the Keck Telescopes and FMOS on Subaru. We find a strong increase in $L_star$ and a decrease in $phi_star$ for both H$beta$+[OIII] and [OII] emitters. We derive the [OII] star-formation history of the Universe since $zsim5$ and find that the cosmic SFRD rises from $z sim 5$ to $sim 3$ and then drops towards $z sim 0$. We also find that our star-formation history is able to reproduce the evolution of the stellar mass density up to $zsim 5$ based only on a single tracer of star-formation. When comparing the H$beta$+[OIII] SFRDs to the [OII] and H$alpha$ SFRD measurements in the literature, we find that there is a remarkable agreement, suggesting that the H$beta$+[OIII] sample is dominated by star-forming galaxies at high-$z$ rather than AGNs.
We combined deep U-band imaging from the KPNO-4m/MOSAIC camera with very deep multi-waveband data from the optical to infrared, to select Lyman Break Galaxies (LBGs) at z~3 using U-V and V-R colors in the Subaru Deep Field. With the resulting sample of 5161 LBGs, we construct the UV luminosity function down to $M_{UV} = -18$ and find a steep faint-end slope of $alpha=-1.78 pm 0.05$. We analyze rest-frame UV-to-IR spectral energy distributions generated from the median optical photometry and photometry on median-stacked IR images. In the stacks of faint LBGs, we find a background depression centered on the galaxy. This deficit results from the systematic difficulty of SExtractor in finding faint galaxies in regions with higher-than-average surface densities of foreground galaxies. We corrected our stacked magnitudes for this. Best-fit stellar population templates for the stacked LBG SEDs indicate stellar masses and star-formation rates of log M*/Msun = 10 and 50 M$_odot$/yr at i = 24, down to log M*/Msun = 8 and = 3 M$_odot$/yr at i = 27. For the faint stacked LBGs there is a 1-mag excess over the expected stellar continuum in the K-band, which we attribute to redshifted [OIII]4959+5007 and H$beta$ lines. Their implied equivalent widths increase with decreasing mass, reaching $rm{EW_0([O III]4959,5007+Hbeta)}$ =1500A in the faintest bin. Such strong [OIII] emission is seen only in a miniscule fraction of the most extreme local emission-line galaxies, but it probably universal in the faint galaxies that reionized the universe. Finally, we analyze clustering by computing the angular correlation function and performing halo occupation distribution (HOD) analysis. We find a mean dark halo mass of log(Mhalo/h) Msun = 11.29$pm 0.12$ for the full sample of LBGs, and log(Mhalo/h) Msun = 11.49$pm 0.1$ for the brightest half.
We quantify the distribution of [OIII]+H$beta$ line strengths at z$simeq$7 using a sample of 20 bright (M$_{mathrm{UV}}$ $lesssim$ $-$21) galaxies. We select these systems over wide-area fields (2.3 deg$^2$ total) using a new colour-selection which precisely selects galaxies at z$simeq$6.63$-$6.83, a redshift range where blue Spitzer/IRAC [3.6]$-$[4.5] colours unambiguously indicate strong [OIII]$+$H$beta$ emission. These 20 galaxies suggest a log-normal [OIII]$+$H$beta$ EW distribution with median EW = 759$^{+112}_{-113}$ $mathrm{mathring{A}}$ and standard deviation = 0.26$^{+0.06}_{-0.05}$ dex. We find no evidence for strong variation in this EW distribution with UV luminosity. The typical [OIII]+H$beta$ EW at z$simeq$7 implied by our sample is considerably larger than that in massive star forming galaxies at z$simeq$2, consistent with a shift toward larger average sSFR (4.4 Gyr$^{-1}$) and lower metallicities (0.16 Z$_odot$). We also find evidence for the emergence of a population with yet more extreme nebular emission ([OIII]+H$beta$ EW$>$1200 $mathrm{mathring{A}}$) that is rarely seen at lower redshifts. These objects have extremely large sSFR ($>$30 Gyr$^{-1}$), as would be expected for systems undergoing a burst or upturn in star formation. While this may be a short-lived phase, our results suggest that 20% of the z$simeq$7 population has such extreme nebular emission, implying that galaxies likely undergo intense star formation episodes regularly at z$>$6. We argue that this population may be among the most effective ionizing agents in the reionization era, both in terms of photon production efficiency and escape fraction. We furthermore suggest that galaxies passing through this large sSFR phase are likely to be very efficient in forming bound star clusters.
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