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Register a new userDiscovery of Extreme [OIII]+H$beta$ Emitting Galaxies Tracing an Overdensity at z~3.5 in CDF-South
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Using deep multi-wavelength photometry of galaxies from ZFOURGE, we group galaxies at $2.5<z<4.0$ by the shape of their spectral energy distributions (SEDs). We identify a population of galaxies with excess emission in the $K_s$-band, which corresponds to [OIII]+H$beta$ emission at $2.95<z<3.65$. This population includes 78% of the bluest galaxies with UV slopes steeper than $beta = -2$. We de-redshift and scale this photometry to build two composite SEDs, enabling us to measure equivalent widths of these Extreme [OIII]+H$beta$ Emission Line Galaxies (EELGs) at $zsim3.5$. We identify 60 galaxies that comprise a composite SED with [OIII]+H$beta$ rest-frame equivalent width of $803pm228$AA and another 218 galaxies in a composite SED with equivalent width of $230pm90$AA. These EELGs are analogous to the `green peas found in the SDSS, and are thought to be undergoing their first burst of star formation due to their blue colors ($beta < -1.6$), young ages ($log(rm{age}/yr)sim7.2$), and low dust attenuation values. Their strong nebular emission lines and compact sizes (typically $sim1.4$ kpc) are consistent with the properties of the star-forming galaxies possibly responsible for reionizing the universe at $z>6$. Many of the EELGs also exhibit Lyman-$alpha$ emission. Additionally, we find that many of these sources are clustered in an overdensity in the Chandra Deep Field South, with five spectroscopically confirmed members at $z=3.474 pm 0.004$. The spatial distribution and photometric redshifts of the ZFOURGE population further confirm the overdensity highlighted by the EELGs.
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Reionisation-era galaxies often display intense nebular emission lines, both in rest-frame optical ([OIII]+H$beta$) and ultraviolet (UV; CIII], CIV). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGN) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue ($beta<-2$), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGN at $zsim2-3$, whose UV slopes are significantly redder ($beta>-1$) than typical star-forming systems in the reionisation era. To investigate the properties of AGN in the reionisation era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift ($zsim2-3$). Our goals are to determine whether AGN with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper we report the detection of a X-ray luminous AGN at $z=3.21$ (UDS-24561) with extreme [OIII]+H$beta$ line emission (EW $=1300$ r{A}) and a blue UV continuum slope ($beta=-2.34$). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionisation models and rest-frame optical lines with both a narrow component (FWHM $=154$ km$/$s) and extended broad wings (FWHM $=977$ km$/$s), consistent with outflowing gas. We describe how such objects can be identified in future JWST emission line surveys in the reionisation era, thereby providing a valuable census of AGN activity at $z>6$ and understanding their contribution to cosmic reionisation.
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.
Bright quasars, observed when the Universe was less than one billion years old (z>5.5), are known to host massive black holes (~10$^{9}$ M$_{odot}$), and are thought to reside in the center of massive dark matter overdensities. In this picture, overdensities of galaxies are expected around high redshift quasars. However, observations based on the detection of Lyman Break Galaxies (LBGs) around these quasars do not offer a clear picture: this may be due to the uncertain redshift constraints of LBGs, which are selected through broad-band filters only. To circumvent such uncertainties, we here perform a search for Lyman Alpha Emitting galaxies (LAEs) in the field of the quasar PSO J215.1512-16.0417 at z~5.73, through narrow band, deep imaging with FORS2 at the VLT. We study an area of 37 arcmin$^{2}$, i.e. ~206 comoving Mpc$^{2}$ at the redshift of the quasar. We find no evidence for an overdensity of LAEs in the quasar field with respect to blank field studies. Possible explanations for these findings include that our survey volume is too small, or that the strong ionizing radiation from the quasar hinders galaxy formation in its immediate proximity. Another possibility is that these quasars are not situated in the dense environments predicted by some simulations.
Spectroscopic observations of massive $z>7$ galaxies selected to have extremely large [OIII]+H$beta$ equivalent width (EW $sim1500$ r{A}) have recently revealed large Ly$alpha$ detection rates, in contrast to the weak emission seen in the general population. Why these systems are uniquely visible in Ly$alpha$ at redshifts where the IGM is likely significantly neutral is not clear. With the goal of better understanding these results, we have begun a campaign with MMT and Magellan to measure Ly$alpha$ in galaxies with similar [OIII]+H$beta$ EWs at $zsimeq2-3$. At these redshifts, the IGM is highly ionized, allowing us to clearly disentangle how the Ly$alpha$ properties depend on the [OIII]+H$beta$ EW. Here we present Ly$alpha$ EWs of $49$ galaxies at $z=2.2-3.7$ with intense [OIII]+H$beta$ line emission (EW $=300-3000$ r{A}). Our results demonstrate that strong Ly$alpha$ emission (EW $>20$ r{A}) becomes more common in galaxies with larger [OIII]+H$beta$ EW, reflecting a combination of increasingly efficient ionizing photon production and enhanced transmission of Ly$alpha$. Among the galaxies with the most extreme [OIII]+H$beta$ emission (EW $sim1500$ r{A}), we find that strong Ly$alpha$ emission is not ubiquitous, with only $50$ per cent of our population showing Ly$alpha$ EW $>20$ r{A}. Our data suggest that the range of Ly$alpha$ strengths is related to the observed ellipticity, with those systems that appear edge-on or elongated having weaker Ly$alpha$ emission. We use these results to interpret the anomalous Ly$alpha$ properties seen in $z>7$ galaxies with extreme [OIII]+H$beta$ emission and discuss implications for the escape of ionizing radiation from these extreme line emitting galaxies.
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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