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 ($M_{\mathrm{UV}}$-$\beta$). 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-$\beta$ relationships derived from $z \sim 3$ galaxy samples. Finally, assuming that high $1600 \r{A}$ 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 $M_{\mathrm{UV}}$-$\beta$ and LAE fractions. These predictions are readily testable by deep surveys with the James Webb Space Telescope.Comment: 14+4 pages, 11+5 figures, accepted for publication in MNRA

Keck Spectroscopy of Faint 3 < z < 7 Lyman Break Galaxies: III. The Mean Ultraviolet Spectrum at z=4

We present and discuss the mean rest-frame ultraviolet spectrum for a sample of 81 Lyman Break Galaxies (LBGs) selected to be B-band dropouts with a mean redshift of z=3.9 and apparent magnitudes z_AB<26. Most of the individual spectra are drawn from our ongoing survey in the GOODS fields with the Keck DEIMOS spectrograph, and we have augmented our sample with published data taken with FORS2 on the VLT. In general we find similar trends in the spectral diagnostics to those found in the earlier, more extensive survey of LBGs at z=3 undertaken by Shapley et al (2003). Specifically, we find low-ionization absorption lines which trace the presence of neutral outflowing gas are weaker in galaxies with stronger Lyman-alpha emission, bluer UV spectral slopes, lower stellar masses, lower UV luminosities and smaller half-light radii. This is consistent with a physical picture whereby star formation drives outflows of neutral gas which scatters Lyman-alpha and gives rise to strong low-ionization absorption lines, while increasing the stellar mass, size, metallicity, and dust content of galaxies. Typical galaxies are thus expected to have stronger Lyman-alpha emission and weaker low-ionization absorption at earlier times (higher redshifts). Indeed, our mean spectrum at z=4 shows somewhat weaker low-ionization absorption lines than at z=3 and available data at higher redshift indicates a rapid decrease in low-ionization absorption strength with redshift. We argue that the reduced low-ionization absorption is likely caused by a decrease in the covering fraction and/or velocity range of outflowing neutral gas at earlier epochs. Our continuing survey will enable us to extend these diagnostics more reliably to higher redshift and determine the implications for the escape fraction of ionizing photons which governs the role of early galaxies in cosmic reionization. [Abridged]Comment: 17 pages, 12 figures, submitted to ApJ. Comments welcom

Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies:- II. A High Fraction of Line Emitters at Redshift Six

As Lyman-alpha photons are scattered by neutral hydrogen, a change with redshift in the Lyman-alpha equivalent width distribution of distant galaxies offers a promising probe of the degree of ionization in the intergalactic medium and hence when cosmic reionization ended. This simple test is complicated by the fact that Lyman-alpha emission can also be affected by the evolving astrophysical details of the host galaxies. In the first paper in this series, we demonstrated both a luminosity and redshift dependent trend in the fraction of Lyman-alpha emitters seen within color-selected Lyman-break galaxies (LBGs) over the range 3<z<6; lower luminosity galaxies and those at higher redshift show an increased likelihood of strong emission. Here we present the results from much deeper 12.5 hour exposures with the Keck DEIMOS spectrograph focused primarily on LBGs at z~6 which enable us to confirm the redshift dependence of line emission more robustly and to higher redshift than was hitherto possible. We find 54+/-11% of faint z~6 LBGs show strong (W_0>25 A) emission, an increase of 1.6x from a similar sample observed at z~4. With a total sample of 74 z~6 LBGs, we determine the luminosity-dependent Lyman-alpha equivalent width distribution. Assuming continuity in these trends to the new population of z~7 sources located with the Hubble WFC3/IR camera, we predict that unless the neutral fraction rises in the intervening 200 Myr, the success rate for spectroscopic confirmation using Lyman-alpha emission should be high.Comment: 6 pages, 3 figures, submitted to ApJ

The [OIII]++Hβ\beta Equivalent Width Distribution at z≃\simeq7: Implications for the Contribution of Galaxies to Reionization

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.Comment: 20 pages, 11 figures. Accepted in MNRAS with minor revision

Line Emitting Galaxies Beyond a Redshift of 7: An Improved Method for Estimating the Evolving Neutrality of the Intergalactic Medium

The redshift-dependent fraction of color-selected galaxies revealing Lyman alpha emission has become the most valuable constraint on the evolving neutrality of the early intergalactic medium. However, in addition to resonant scattering by neutral gas, the visibility of Lyman alpha is also dependent on the intrinsic properties of the host galaxy, including its stellar population, dust content and the nature of outflowing gas. Taking advantage of significant progress we have made in determining the line emitting properties of $z \simeq 4-6$ galaxies, we propose an improved method, based on using the measured slopes of the rest-frame ultraviolet continua of galaxies, to interpret the growing body of near-infrared spectra of $z>7$ galaxies in order to take into account these host galaxy dependencies. In a first application of our new method, we demonstrate its potential via a new spectroscopic survey of $7<z<8$ galaxies undertaken with the Keck MOSFIRE spectrograph. Together with earlier published data our data provides improved estimates of the evolving visibility of Lyman alpha, particularly at redshift $z\simeq 8$. As a byproduct, we also present a new line emitting galaxy at a redshift $z=7.62$ which supersedes an earlier redshift record. We discuss the improving constraints on the evolving neutral fraction over $6<z<8$ and the implications for cosmic reionization.Comment: To be submitted to Ap

High-redshift galaxies and black holes in the eyes of JWST: a population synthesis model from infrared to X-rays

The first billion years of the Universe is a pivotal time: stars, black holes (BHs) and galaxies form and assemble, sowing the seeds of galaxies as we know them today. Detecting, identifying and understand- ing the first galaxies and BHs is one of the current observational and theoretical challenges in galaxy formation. In this paper we present a population synthesis model aimed at galaxies, BHs and Active Galactic Nuclei (AGNs) at high redshift. The model builds a population based on empirical relations. Galaxies are characterized by a spectral energy distribution determined by age and metallicity, and AGNs by a spectral energy distribution determined by BH mass and accretion rate. We validate the model against observational constraints, and then predict properties of galaxies and AGN in other wavelength and/or luminosity ranges, estimating the contamination of stellar populations (normal stars and high-mass X-ray binaries) for AGN searches from the infrared to X-rays, and vice-versa for galaxy searches. For high-redshift galaxies, with stellar ages < 1 Gyr, we find that disentangling stellar and AGN emission is challenging at restframe UV/optical wavelengths, while high-mass X-ray binaries become more important sources of confusion in X-rays. We propose a color-color selection in JWST bands to separate AGN vs star-dominated galaxies in photometric observations. We also esti- mate the AGN contribution, with respect to massive, hot, metal-poor stars, at driving high ionization lines, such as C IV and He II. Finally, we test the influence of the minimum BH mass and occupa- tion fraction of BHs in low mass galaxies on the restframe UV/near-IR and X-ray AGN luminosity function.Comment: Accepted for publication in the Astrophysical Journa

Photometric identification and MMT spectroscopy of new extremely metal-poor galaxies: towards a better understanding of young stellar populations at low metallicity

Extremely metal-poor star-forming galaxies (XMPs) represent one of our only laboratories for study of the low-metallicity stars we expect to encounter at early epochs. But as our understanding of the $z>6$ universe has improved, it has become clear that the majority of known XMPs within 100 Mpc host significantly less prominent massive star populations than their reionization-era counterparts, severely limiting their utility as testbeds for interpreting spectral features found at the highest redshifts. Here we present a new photometric selection technique designed to identify nearby XMPs dominated by young stellar populations comparable to those expected in the reionization era. We apply our technique to uncover candidate XMPs in SDSS imaging at magnitudes $16<i'<23$, extending significantly below the completeness limits of the SDSS spectroscopic survey. Spectroscopic observations with the MMT confirm that 32 of the 53 uniformly metal-poor and high specific star formation rate targets we observed have gas-phase oxygen abundances $12+\log\mathrm{O/H}<7.7$ ($Z/Z_\odot<0.1$), including two in the range of the lowest-metallicity galaxies known, $Z/Z_\odot<0.05$. Our observations shed new light onto the longstanding mystery of He II emission in star-forming galaxies: we find that the equivalent width of the He II $\lambda 4686$ high-ionization emission line does not scale with that of H$\beta$ in our sample, suggesting that binary evolution or other processes on $>10$ Myr timescales contribute substantially to the $\mathrm{He^+}$-ionizing photon budget in this metallicity regime. Applying such selection techniques coupled with deep spectroscopy to next-generation photometric surveys like LSST may eventually provide a basis for an empirical understanding of metal-poor massive stars.Comment: 16 pages, 10 figures, accepted for publication in MNRA

Searching for the Sources Responsible for Cosmic Reionization: Probing 7 < z < 10 and Beyond

(Abridged) We review recent observations that suggest the star formation rate density is declining over 3$<z<$7 and illustrate the challenges that this poses in explaining the assembled stellar mass in z$\sim$6 galaxies deduced from Spitzer data. A plausible conclusion is a vigorous period of earlier star formation. Prior to JWST and TMT, strong lensing offers a unique probe of the extent of this earlier activity. We discuss the first results of a blind spectroscopic survey of lensing clusters for 8.5$<z<$10 Ly$\alpha$ emitters using NIRSPEC which is achieving a limiting star formation rate of 0.1 $M_{\odot}$ yr$^{-1}$. A companion HST/IRAC survey is targeting lensed $z$ and $J$-band dropouts and probes a $\simeq$1 arcmin$^2$ region 1 magnitude deeper than the UDF/NICMOS observations. Both surveys will constrain the contribution of early, low luminosity, sources to cosmic reionization.Comment: 12 pages, 6 figures, to appear in "First Light & Reionization", eds. E. Barton & A. Cooray, New Astronomy Reviews, in pres

Evidence for a Hard Ionizing Spectrum from a z=6.11 Stellar Population

We present the Magellan/FIRE detection of highly-ionized CIV 1550 and OIII] 1666 in a deep infrared spectrum of the z=6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously-known Lyman-alpha. No corresponding emission is detected at the expected location of HeII 1640. The upper limit on HeII paired with detection of OIII] and CIV constrains possible ionization scenarios. Production of CIV and OIII] requires ionizing photons of 2.5-3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (~0.5 Ryd). As a pure recombination line, HeII emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher HeII flux. The conclusions based on ratios of metal line detections to Helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general z=2-3 population, in a way that suggests hard ionizing spectra that do not necessarily originate in AGN.Comment: 7 pages, 4 figures, 1 table. Accepted for publication to ApJ