arXiv subjects

Daniel P. Stark

Publications and source records attributed to Daniel P. Stark.

At least 109 records · Page 6Linked to original sources

arXiv2021

Spectroscopy of an extreme [OIII] emitting active galactic nucleus at $z=3.212$: implications for the reionisation era

Reionisation-era galaxies often display intense nebular emission lines, both in rest-frame optical ([OIII]+H$β$) 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 ($β<-2$), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGN at $z\sim2-3$, whose UV slopes are significantly redder ($β>-1$) 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 ($z\sim2-3$). 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$β$ line emission (EW $=1300$ Å) and a blue UV continuum slope ($β=-2.34$). 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.

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arXiv2021

The [OIII]$\lambda5007$ equivalent width distribution at z $\sim2$: The redshift evolution of the extreme emission line galaxies

We determine the [OIII]$\lambda5007$ equivalent width (EW) distribution of $1.700<\rm{z}<2.274$ rest-frame UV-selected (M$_{\rm{UV}}<-19$) star-forming galaxies in the GOODS North and South fields. We make use of deep HDUV broadband photometry catalogues for selection and 3D-HST WFC3/IR grism spectra for measurement of line properties. The [OIII]$\lambda5007$ EW distribution allows us to measure the abundance of extreme emission line galaxies (EELGs) within this population. We model a log-normal distribution to the [OIII]$\lambda5007$ rest-frame equivalent widths of galaxies in our sample, with location parameter $μ=4.24\pm0.07$ and variance parameter $σ= 1.33\pm0.06$. This EW distribution has a mean [OIII]$\lambda5007$ EW of 168$\pm1Å$. The fractions of $\rm{z}\sim2$ rest-UV-selected galaxies with [OIII]$\lambda5007$ EWs greater than $500, 750$ and $1000Å$ are measured to be $6.8^{+1.0}_{-0.9}\%$, $3.6^{+0.7}_{-0.6}\%$, and $2.2^{+0.5}_{-0.4}\%$ respectively. The EELG fractions do not vary strongly with UV luminosity in the range ($-21.6<M_{\rm{UV}}<-19.0$) considered in this paper, consistent with findings at higher redshifts. We compare our results to $\rm{z}\sim5$ and $\rm{z}\sim7$ studies where candidate EELGs have been discovered through Spitzer/IRAC colours, and we identify rapid evolution with redshift in the fraction of star-forming galaxies observed in an extreme emission line phase (a rise by a factor $\sim10$ between $\rm{z}\sim2$ and $\rm{z}\sim7$). This evolution is consistent with an increased incidence of strong bursts in the galaxy population of the reionisation era. While this population makes a sub-dominant contribution of the ionising emissivity at $\rm{z}\simeq2$, EELGs are likely to dominate the ionising output in the reionisation era.

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arXiv2021

Deep Realistic Extragalactic Model (DREaM) Galaxy Catalogs: Predictions for a Roman Ultra-Deep Field

In the next decade, deep galaxy surveys from telescopes such as the James Webb Space Telescope and Roman Space Telescope will provide transformational data sets that will greatly enhance the understanding of galaxy formation during the epoch of reionization (EoR). In this work, we present the Deep Realistic Extragalactic Model (DREaM) for creating synthetic galaxy catalogs. Our model combines dark matter simulations, subhalo abundance matching and empirical models, and includes galaxy positions, morphologies, and spectral energy distributions (SEDs). The resulting synthetic catalog extends to redshifts $z \sim 12 $, and galaxy masses $\log_{10}(M/M_{\odot}) = 5 $ covering an area of $1\, {\rm deg}^2$ on the sky. We use DREaM to explore the science returns of a $1\, {\rm deg}^2$ \emph{Roman} UDF, and to provide a resource for optimizing ultra-deep survey designs. We find that a \emph{Roman} UDF to $\sim 30\,m_{\rm AB}$ will potentially detect more than $10^6$ $M_{\rm UV}<-17$ galaxies, with more than $10^4$ at redshifts $z>7$, offering an unparalleled dataset for constraining galaxy properties during the EoR. Our synthetic catalogs and simulated images are made publicly available to provide the community with a tool to prepare for upcoming data.

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arXiv2021

MMT Spectroscopy of Lyman-alpha at z$\simeq$7: Evidence for Accelerated Reionization Around Massive Galaxies

Reionization-era galaxies tend to exhibit weak Ly$α$ emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Ly$α$ emission now known in four of the most massive z=7$-$9 galaxies in the CANDELS fields, all of which also exhibit intense [OIII]$+$H$β$ emission (EW$>$800 $\mathrm{\mathring{A}}$). To better understand why Ly$α$ is anonymously strong in a subset of massive z$\simeq$7$-$9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N=22) of similarly luminous ($\simeq$1$-$6 L$^{\ast}_{\mathrm{UV}}$) z$\simeq$7 galaxies selected over very wide-area fields ($\sim$3 deg$^2$). We confidently ($>$7$σ$) detect Ly$α$ in 78% (7/9) of galaxies with strong [OIII]$+$H$β$ emission (EW$>$800 $\mathrm{\mathring{A}}$) as opposed to only 8% (1/12) of galaxies with more moderate (EW=200$-$800 $\mathrm{\mathring{A}}$) [OIII]$+$H$β$. We argue that the higher Ly$α$ EWs of the strong [OIII]$+$H$β$ population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs ($\gtrsim$30 Gyr$^{-1}$). We also find evidence that Ly$α$ transmission from massive galaxies declines less rapidly over $6<z<7$ than in low-mass lensed systems. In particular, our data suggest no strong evolution in Ly$α$ transmission, consistent with a picture wherein massive z$\simeq$7 galaxies often reside in large ionized regions. We detect three closely-separated ($R$ = 1.7 physical Mpc) z$\simeq$7 Ly$α$ emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume.

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arXiv2021

The Detection of Ionized Carbon Emission at z~8

We present deep Keck/MOSFIRE $H$-band spectroscopic observations covering the [CIII],CIII]$λ\lambda1907,1909$ doublet for three $z\sim8$ galaxy candidates in the AEGIS field. Along with non-detections in two galaxies, we obtain one of the highest-redshift detections to-date of [CIII]$λ1907$ for the galaxy AEGIS-33376, finding $z_{\rm spec}=7.945\pm0.001$. We measure a [CIII]$λ$1907 flux of $2.24\pm0.71\times10^{-18} \mbox{ erg}\mbox{ s}^{-1} \mbox{ cm}^{-2}$, corresponding to a rest-frame equivalent width of $20.3\pm6.5 \unicode{x212B}$ for the single line. Given the not very constraining upper limit for CIII]$λ1909$ based on strong sky-line contamination, we assume a [CIII]$λ$1907/CIII]$λ1909$ doublet ratio of 1.5 and infer a total [CIII],CIII]$λ\lambda1907,1909$ equivalent width of $33.7\pm 10.8 \unicode{x212B}$. We repeat the same reductions and analysis on multiple subsets of our raw data divided on the basis of time and observing conditions, verifying that the [CIII]$λ1907$ emission is present for AEGIS-33376 throughout our observations. We also confirm that the significance of the [CIII]$λ1907$ detection in different subsets of our data tracks that of brighter emission features detected on the same multi-slit mask. These multiple tests suggest that the observed emission line is real and associated with the $z\sim 8$ target. The strong observed [CIII],CIII]$λ\lambda1907,1909$ in AEGIS-33376 likely indicates ISM conditions of low metallicity, high ionization parameter, and a hard ionizing spectrum, although AGN contributions are possible. This single detection represents a sizable increase in the current sample [CIII],CIII]$λ\lambda1907,1909$ detections at $z>7$, while $\textit{JWST}$ will provide the first statistical samples of such measurements at these redshifts.

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arXiv2021

Kinematics of the Circumgalactic Medium of a $z = 0.77$ Galaxy from MgII Tomography

Galaxy evolution is thought to be driven in large part by the flow of gas between galaxies and the circumgalactic medium (CGM), a halo of metal-enriched gas extending out to $\gtrsim100$ kpc from each galaxy. Studying the spatial structure of the CGM holds promise for understanding these gas flow mechanisms; however, the common method using background quasar sightlines provides minimal spatial information. Recent works have shown the utility of extended background sources such as giant gravitationally lensed arcs. Using background lensed arcs from the CSWA 38 lens system, we continuously probed, at a resolution element of about 15 kpc$^2$, the spatial and kinematic distribution of MgII absorption in a star-forming galaxy at $z=0.77$ (stellar mass $\approx 10^{9.7}$ M$_\odot$, star formation rate $\approx 10$ M$_\odot$ yr$^{-1}$) at impact parameters $D \simeq 5-30$ kpc. Our results present an anisotropic, optically thick medium whose absorption strength decreases with increasing impact parameter, in agreement with the statistics towards quasars and other gravitational arcs. Furthermore, we find generally low line-of-sight velocities in comparison to the relatively high velocity dispersion in the MgII gas (with typical $σ\approx 50$ km s$^{-1}$). While the galaxy itself exhibits a clear outflow (with MgII velocities up to $\sim 500$ km s$^{-1}$) in the down-the-barrel spectrum, the outflow component is sub-dominant and only weakly detected at larger impact parameters probed by the background arcs. Our results provide evidence of mainly dispersion-supported, metal-enriched gas recycling through the CGM.

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arXiv2021

RELICS: Properties of z>5.5 Galaxies Inferred from Spitzer and Hubble Imaging Including A Candidate z~6.8 Strong [OIII] Emitter

We present constraints on the physical properties (including stellar mass, age, and star formation rate) of 207 $6\lesssim z \lesssim8$ galaxy candidates from the Reionization Lensing Cluster Survey (RELICS) and companion Spitzer-RELICS surveys. We measure photometry using T-PHOT and perform spectral energy distribution fitting using EA$z$Y and BAGPIPES. Of the 207 candidates for which we could successfully measure Spitzer fluxes, 23 were demoted to likely low redshift ($z<4$). Among the remaining high redshift candidates, we find intrinsic stellar masses between $1\times10^6\rm{M_{\odot}}$ and $4\times10^9\rm{M_\odot}$, and rest-frame UV absolute magnitudes between $-22.6$ and $-14.5$ mag. While our sample is mostly comprised of $L_{UV}/L^*_{UV}<1$ galaxies, there are a number of brighter objects in the sample, extending to $L_{UV}/L^*_{UV}\sim2$. The galaxies in our sample span approximately four orders of magnitude in stellar mass and star-formation rates, and exhibit ages ranging from maximally young to maximally old. We highlight 11 galaxies which have detections in Spitzer/IRAC imaging and redshift estimates $z\geq6.5$, several of which show evidence for some combination of evolved stellar populations, large contributions of nebular emission lines, and/or dust. Among these is PLCKG287+32-2013, one of the brightest $z\sim7$ candidates known (AB mag 24.9) with a Spitzer 3.6$μ$m flux excess suggesting strong [OIII] + H-$β$ emission ($\sim$1000Å rest-frame equivalent width). We discuss the possible uses and limits of our sample and present a public catalog of Hubble 0.4--1.6$μ$m + Spitzer 3.6$μ$m and 4.5$μ$m photometry along with physical property estimates for all 207 objects in the sample. Because of their apparent brightnesses, high redshifts, and variety of stellar populations, these objects are excellent targets for follow-up with James Webb Space Telescope.

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arXiv2020

Evidence for GN-z11 as a luminous galaxy at redshift 10.957

GN-z11 was photometrically selected as a luminous star-forming galaxy candidate at redshift z > 10 based on Hubble Space Telescope (HST) imaging data. Follow-up HST near-infrared grism observations detected a continuum break that was explained as the Ly-alpha break corresponding to z = 11.09 (+0.08-0.12). However, its accurate redshift remained unclear. Here we report a probable detection of three ultraviolet (UV) emission lines from GN-z11, which can be interpreted as the [C III] 1907, C III] 1909 doublet and O III] 1666 at z = 10.957+/-0.001 (when the Universe was only ~420 Myr old, or ~3% of its current age). This is consistent with the redshift of the previous grism observations, supporting GN-z11 as the most distant galaxy known to date. Its UV lines likely originate from dense ionized gas that is rarely seen at low redshifts, and its strong [C III] and C III] emission is partly due to an active galactic nucleus (AGN) or enhanced carbon abundance. GN-z11 is luminous and young, yet moderately massive, implying a rapid build-up of stellar mass in the past. Future facilities will be able to find the progenitors of such galaxies at higher redshift and probe the cosmic epoch in the beginning of re-ionization.

astro-ph.HE↗

arXiv2020

The [OIII]$+$H$β$ Equivalent Width Distribution at z$\simeq$7: Implications for the Contribution of Galaxies to Reionization

We quantify the distribution of [OIII]+H$β$ 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$6.63$-$6.83, a redshift range where blue Spitzer/IRAC [3.6]$-$[4.5] colours unambiguously indicate strong [OIII]$+$H$β$ emission. These 20 galaxies suggest a log-normal [OIII]$+$H$β$ 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$β$ 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$). We also find evidence for the emergence of a population with yet more extreme nebular emission ([OIII]+H$β$ 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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arXiv2020

The Lyman Continuum Escape Survey: Connecting Time-Dependent [OIII] and [OII] Line Emission with Lyman Continuum Escape Fraction in Simulations of Galaxy Formation

Escaping Lyman continuum photons from galaxies likely reionized the intergalactic medium at redshifts $z\gtrsim6$. However, the Lyman continuum is not directly observable at these redshifts and secondary indicators of Lyman continuum escape must be used to estimate the budget of ionizing photons. Observationally, at redshifts $z\sim2-3$ where the Lyman continuum is observationally accessible, surveys have established that many objects that show appreciable Lyman continuum escape fractions $f_{esc}$ also show enhanced [OIII]/[OII] (O$_{32}$) emission line ratios. Here, we use radiative transfer analyses of cosmological zoom-in simulations of galaxy formation to study the physical connection between $f_{esc}$ and O$_{32}$. Like the observations, we find that the largest $f_{esc}$ values occur at elevated O$_{32}\sim3-10$ and that the combination of high $f_{esc}$ and low O$_{32}$ is extremely rare. While high $f_{esc}$ and O$_{32}$ often are observable concurrently, the timescales of the physical origin for the processes are very different. Large O$_{32}$ values fluctuate on short ($\sim$1 Myr) timescales during the Wolf-Rayet-powered phase after the formation of star clusters, while channels of low absorption are established over tens of megayears by collections of supernovae. We find that while there is no direct causal relation between $f_{esc}$ and O$_{32}$, high $f_{esc}$ most often occurs after continuous input from star formation-related feedback events that have corresponding excursions to large O$_{32}$ emission. These calculations are in agreement with interpretations of observations that large $f_{esc}$ tends to occur when O$_{32}$ is large, but large O$_{32}$ does not necessarily imply efficient Lyman continuum escape.

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arXiv2020

Ultraviolet spectra of extreme nearby star-forming regions: evidence for an overabundance of very massive stars

As deep spectroscopic campaigns extend to higher redshifts and lower stellar masses, the interpretation of galaxy spectra depends increasingly upon models for very young stellar populations. Here we present new HST/COS ultraviolet spectroscopy of seven nearby ($<120$ Mpc) star-forming regions hosting very young stellar populations ($\sim$ 4-20 Myr) with optical Wolf-Rayet stellar wind signatures, ideal laboratories in which to test these stellar models. We detect nebular C III] in all seven, but at equivalent widths uniformly $< 10$ Å. This suggests that even for very young stellar populations, the highest equivalent width C III] emission at $\geq 15$ Å is reserved for inefficiently-cooled gas at metallicities at or below that of the SMC. The spectra also reveal strong C IV P-Cygni profiles and broad He II emission formed in the winds of massive stars, including some of the most prominent He II stellar wind lines ever detected in integrated spectra. We find that the latest stellar population synthesis prescriptions with improved treatment of massive stars nearly reproduce the entire range of stellar He II wind strengths observed here. However, we find that these models cannot simultaneously match the strongest wind features alongside the optical nebular line constraints. This discrepancy can be naturally explained by an overabundance of very massive stars produced by a high incidence of binary mass transfer and mergers occurring on short $\lesssim 10$ Myr timescales, suggesting these processes may be crucial for understanding the highest-sSFR galaxies in the early Universe. Reproducing both the stellar and nebular light of young systems such as these will be a crucial benchmark for the next generation of stellar population synthesis models.

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arXiv2020

Effects of self-consistent rest-ultraviolet colours in semi-empirical galaxy formation models

Connecting the observed rest-ultraviolet (UV) luminosities of high-$z$ galaxies to their intrinsic luminosities (and thus star formation rates) requires correcting for the presence of dust. We bypass a common dust-correction approach that uses empirical relationships between infrared (IR) emission and UV colours, and instead augment a semi-empirical model for galaxy formation with a simple -- but self-consistent -- dust model and use it to jointly fit high-$z$ rest-UV luminosity functions (LFs) and colour-magnitude relations ($M_{\mathrm{UV}}$-$β$). In doing so, we find that UV colours evolve with redshift (at fixed UV magnitude), as suggested by observations, even in cases without underlying evolution in dust production, destruction, absorption, or geometry. The observed evolution in our model arises due to the reduction in the mean stellar age and rise in specific star formation rates with increasing $z$. The UV extinction, $A_{\mathrm{UV}}$, evolves similarly with redshift, though we find a systematically shallower relation between $A_{\mathrm{UV}}$ and $M_{\mathrm{UV}}$ than that predicted by IRX-$β$ relationships derived from $z \sim 3$ galaxy samples. Finally, assuming that high $1600 Å$ transmission ($\gtrsim 0.6$) is a reliable LAE indicator, modest scatter in the effective dust surface density of galaxies can explain the evolution both in $M_{\mathrm{UV}}$-$β$ and LAE fractions. These predictions are readily testable by deep surveys with the James Webb Space Telescope.

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arXiv2020

Exploring Chemical Homogeneity in Dwarf Galaxies: A VLT-MUSE study of JKB 18

Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies in particular can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12+log(O/H)=7.6$\pm$0.2 (~0.08 Zsol). Using high spatial-resolution integral field spectroscopy of the entire system, we calculate chemical abundances for individual HII regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H and N/O of ~0.5-0.6 dex and regions harbouring chemical abundances outside this 1$σ$ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing timescales, or self-enrichment from Wolf-Rayet stars are accountable. Using a galaxy-scale, multi-phase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star-formation activity. This study not only draws attention to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias.

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arXiv2020

High-mass X-ray binaries in nearby metal-poor galaxies: on the contribution to nebular HeII emission

Despite significant progress both observationally and theoretically, the origin of high-ionization nebular HeII emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing $\mathrm{He^+}$-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input SEDs containing the joint contribution of both stellar atmospheres and a multi-color disk model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power HeII, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of $10^{42}$ erg/s per unit star formation rate (SFR). To test this, we assemble a sample of eleven low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and HeII emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed HeII strengths, with typical X-ray luminosities or upper limits thereon of only $10^{40}$-$10^{41}$ erg/s per SFR. This indicates that HMXBs are not the dominant source of $\mathrm{He^+}$ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.

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arXiv2020

RELICS: A Very Large ($θ_{E}\sim40"$) Cluster Lens -- RXC J0032.1+1808

Extensive surveys with the \textit{Hubble Space Telescope} (HST) over the past decade, targeting some of the most massive clusters in the sky, have uncovered dozens of galaxy-cluster strong lenses. The massive cluster strong-lens scale is typically $θ_{E}\sim10\arcsec$ to $\sim30-35\arcsec$, with only a handful of clusters known with Einstein radii $θ_{E}\sim40\arcsec$ or above (for $z_{source}=2$, nominally). Here we report another very large cluster lens, RXC J0032.1+1808 ($z=0.3956$), the second richest cluster in the redMapper cluster catalog and the 85th most massive cluster in the Planck Sunyaev-Zel'dovich catalog. With our Light-Traces-Mass and fully parametric (dPIEeNFW) approaches, we construct strong lensing models based on 18 multiple images of 5 background galaxies newly identified in the \textit{Hubble} data mainly from the \textit{Reionization Lensing Cluster Survey} (RELICS), in addition to a known sextuply imaged system in this cluster. Furthermore, we compare these models to Lenstool and GLAFIC models that were produced independently as part of the RELICS program. All models reveal a large effective Einstein radius of $θ_{E}\simeq40\arcsec$ ($z_{source}=2$), owing to the obvious concentration of substructures near the cluster center. Although RXC J0032.1+1808 has a very large critical area and high lensing strength, only three magnified high-redshift candidates are found within the field targeted by RELICS. Nevertheless, we expect many more high-redshift candidates will be seen in wider and deeper observations with \textit{Hubble} or \emph{JWST}. Finally, the comparison between several algorithms demonstrates that the total error budget is largely dominated by systematic uncertainties.

astro-ph.CO↗

arXiv2020

The Ionizing Photon Production Efficiency ($ξ_{ion}$) Of Lensed Dwarf Galaxies At $z \sim 2 $

We measure the ionizing photon production efficiency ($ξ_{ion}$) of low-mass galaxies ($10^{7.8}$-$10^{9.8}$ $M_{\odot}$) at $1.4<z<2.7$, allowing us to better understand the contribution of dwarf galaxies to the ionizing background and cosmic reionization. We target galaxies that are magnified by the strong lensing galaxy clusters Abell 1689, MACS J0717, and MACS J1149. We utilize Keck/MOSFIRE spectra to measure optical nebular emission line fluxes and HST imaging to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log(L$_{Hα}$ /L$_{UV}$) of galaxies in our sample to determine the typical log($ξ_{ion}$). Second, we take the logarithm of the total L$_{Hα}$ over the total L$_{UV}$. We prefer the latter as it provides the total ionizing UV luminosity density of galaxies when multiplied by the non-ionizing UV luminosity density from the UV luminosity function. log($ξ_{ion}$) calculated from the second method is $\sim$ 0.2 dex higher than the first method. We do not find any strong dependence between log($ξ_{ion}$) and stellar mass, M$_{UV}$ or UV spectral slope ($β$). We report a value of log($ξ_{ion}$) $\sim25.47\pm 0.09$ for our UV-complete sample ($-22<M_{UV}<-17.3$) and $\sim25.37\pm0.11$ for our mass-complete sample ($7.8<\log(M_*)<9.8)$. These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log($ξ_{ion}$) is $0.2-0.3$ dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity, age, or the prevalence of binary stars. We also find a correlation between log($ξ_{ion}$) and the equivalent widths of H$α$ and [OIII]$λ$5007 fluxes, confirming that these equivalent widths can be used to estimate $ξ_{ion}$.

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arXiv2020

Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation

Strong [OIII]$λλ$4959,5007+H$β$ emission appears to be typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic reionization, these galaxies and the physical conditions within them are of great interest. At z > 6.5, where Ly$α$ is greatly attenuated by the intergalactic medium, rest-UV metal emission lines provide an alternative measure of redshift and also constraints on the physical properties of star-forming regions and massive stars. We present the first statistical sample of rest-UV line measurements in z $\sim$ 2 galaxies selected as analogs of those in the reionization era based on [OIII]$λλ$4959,5007 EW or rest-frame U-B color. Our sample is drawn from the 3D-HST Survey and spans the redshift range 1.36 $\leqslant$ z $\leqslant$ 2.49. We find that the median Ly$α$ and CIII]$λλ$1907,1909 EWs of our sample are significantly greater than those of z $\sim$ 2 UV-continuum-selected star-forming galaxies. Measurements from both individual and composite spectra indicate a monotonic, positive correlation between CIII] and [OIII], while a lack of trend is observed between Ly$α$ and [OIII] at [OIII] EW < 1000$\unicode{x212B}$. At higher [OIII] EW, extreme Ly$α$ emission starts to emerge. Using stacked spectra, we find that Ly$α$ and CIII] are significantly enhanced in galaxies with lower metallicity. Two objects in our sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV metal line emission. These objects have significant CIV$λλ$1548,1550, HeII$λ$1640, and OIII]$λλ$1661,1665 emission in addition to intense Ly$α$ or CIII]. Detailed characterization of these lower-redshift analogs provides unique insights into the physical conditions in z > 6.5 star-forming regions, motivating future observations of reionization-era analogs at lower redshifts.

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arXiv2019

The Lyman Continuum Escape Survey -- II: Ionizing Radiation as a Function of the [OIII]/[OII] Line Ratio

We discuss the rest-frame optical emission line spectra of a large (~50) sample of z=3.1 Lyman alpha emitters (LAEs) whose physical properties suggest such sources are promising analogs of galaxies in the reionization era. Reliable Lyman continuum escape fractions have now been determined for a large sample of such LAEs from the Lyman Continuum Escape Survey (LACES) undertaken via deep HST imaging in the SSA22 survey area reported in Fletcher et al. (2019). Using new measures of [OII] emission secured from Keck MOSFIRE spectra we re-examine, for a larger sample, earlier claims that Lyman continuum leakages may correlate with the nebular emission line ratio [OIII]/[OII] as expected for density-bound HII regions. We find that a large [OIII]/[OII] line ratio is indeed a necessary condition for Lyman continuum leakage, strengthening earlier claims made using smaller samples at various redshifts. However, not all LAEs with large [OIII]/[OII] line ratios are leakers and leaking radiation appears not to be associated with differences in other spectral diagnostics. This suggests the detection of leaking radiation is modulated by an additional property, possibly the viewing angle for porous HII regions. We discuss our new results in the context of the striking bimodality of LAE leakers and non-leakers found in the LACES program and the implications for the sources of cosmic reionization.

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