High-redshift galaxies and low-mass stars

The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at z ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm – a capability that will be further extended with the launch and commissioning of the James Webb Space Telescope (JWST). As new regions of parameter space in both colour and depth are probed, new challenges for distant galaxy surveys are identified. In this paper, we present an analysis of the colours of L- and T-dwarf stars in widely used photometric systems. We also consider the implications of the newly identified Y-dwarf population – stars that are still cooler and less massive than T-dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ∼ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ∼ 7 galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low-signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population

New Star Forming Galaxies at z\approx 7 from WFC3 Imaging

The addition of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) has led to a dramatic increase in our ability to study the z>6 Universe. The increase in the near-infrared (NIR) sensitivity of WFC3 over previous instruments has enabled us to reach apparent magnitudes approaching 29 (AB). This allows us to probe the rest-frame ultraviolet (UV) continuum, redshifted into the NIR at $z>6$. Taking advantage of the large optical depths at this redshift, resulting in the Lyman-alpha break, we use a combination of WFC3 imaging and pre-existing Advanced Camera for Surveys (ACS) imaging to search for z approx 7 over 4 fields. Our analysis reveals 29 new z approx 7 star forming galaxy candidates in addition to 16 pre-existing candidates already discovered in these fields. The improved statistics from our doubling of the robust sample of z-drop candidates confirms the previously observed evolution of the bright end of the luminosity function.Comment: 15 pages, accepted in MNRA

The ultraviolet properties of star-forming galaxies - I. HST WFC3 observations of very high redshift galaxies

The acquisition of deep near-IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very high redshift Universe. For galaxies up to z≈ 7.7 sufficient wavelength coverage exists to probe the rest-frame ultraviolet (UV) continuum without contamination from either Lyman α emission or the Lyman α break. In this work we use near-infrared (near-IR) imaging to measure the rest-frame UV continuum colours of galaxies at 4.7 < z < 7.7. We have carefully defined a colour–colour selection to minimize any inherent bias in the measured UV continuum slope for the drop-out samples. For the highest redshift sample (6.7 < z < 7.7), selected as zf850lp-band drop-outs, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust-free, solar metallicity, star-forming population (or a moderately dusty population of low metallicity). At lower redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower redshift and more luminous galaxies are dustier; however, this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours

Interpreting the observed UV continuum slopes of high-redshift galaxies

The observed UV continuum slope of star-forming galaxies is strongly affected by the presence of dust. Its observation is then a potentially valuable diagnostic of dust attenuation, particularly at high redshift where other diagnostics are currently inaccessible. Interpreting the observed UV continuum slope in the context of dust attenuation is often achieved assuming the empirically calibrated Meurer et al. relation. Implicit in this relation is the assumption of an intrinsic UV continuum slope (β = −2.23). However, results from numerical simulations suggest that the intrinsic UV continuum slopes of high-redshift star-forming galaxies are bluer than this, and moreover vary with redshift. Using values of the intrinsic slope predicted by numerical models of galaxy formation combined with a Calzetti et al. reddening law we infer UV attenuations (A1500) 0.35–0.5 mag (AV: 0.14 − 0.2 mag assuming Calzetti et al. reddening law) greater than simply assuming the Meurer relation. This has significant implications for the inferred amount of dust attenuation at very high (z ≈ 7) redshift given current observational constraints on β, combined with the Meurer relation, suggesting dust attenuation to be virtually zero in all but the most luminous systems

Recalibrating the cosmic star formation history

The calibrations linking observed luminosities to the star formation rate depend on the assumed stellar population synthesis model, initial mass function, star formation and metal enrichment history, and whether reprocessing by dust and gas is included. Consequently the shape and normalisation of the inferred cosmic star formation history is sensitive to these assumptions. Using v2.2.1 of the Binary Population and Spectral Synthesis (\bpass) model we determine a new set of calibration coefficients for the ultraviolet, thermal-infrared, and, hydrogen recombination lines. These ultraviolet and thermal infrared coefficients are 0.15-0.2 dex higher than those widely utilised in the literature while the H$\alpha$ coefficient is $\sim 0.35$ dex larger. These differences arise in part due to the inclusion binary evolution pathways but predominantly reflect an extension in the IMF to 300 $M_{\odot}$ and a change in the choice of reference metallicity. We use these new coefficients to recalibrate the cosmic star formation history, and find improved agreement between the integrated cosmic star formation history and the in-situ measured stellar mass density as a function of redshift. However, these coefficients produce new tension between star formation rate densities inferred from the ultraviolet and thermal-infrared and those from H$\alpha$.Comment: 7 pages, 8 figures, accepted for publication in MNRA

Extragalactic Constraints on the Initial Mass Function

The local stellar mass density is observed to be significantly lower than the value obtained from integrating the cosmic star formation history (SFH), assuming that all the stars formed with a Salpeter initial mass function (IMF). Even other favoured IMFs, more successful in reconciling the observed $z=0$ stellar mass density with that inferred from the SFH, have difficulties in reproducing the stellar mass density observed at higher redshift. In this study we investigate to what extent this discrepancy can be alleviated for any universal power-law IMF. We find that an IMF with a high-mass slope shallower (2.15) than the Salpeter slope (2.35) reconciles the observed stellar mass density with the cosmic star formation history, but only at low redshifts. At higher redshifts $z>0.5$ we find that observed stellar mass densities are systematically lower than predicted from the cosmic star formation history, for any universal power-law IMF.Comment: 6 pages, 5 figures, accepted to MNRA

Spectroscopy of z ~ 7 candidate galaxies: using Lyman α to constrain the neutral fraction of hydrogen in the high-redshift universe

Following our previous spectroscopic observations of z > 7 galaxies with Gemini/Gemini Near Infra-Red Spectrograph (GNIRS) and Very Large Telescope (VLT)/XSHOOTER, which targeted a total of eight objects, we present here our results from a deeper and larger VLT/FOcal Reducer and Spectrograph (FORS2) spectroscopic sample of Wide Field Camera 3 selected z > 7 candidate galaxies. With our FORS2 setup we cover the 737–1070 nm wavelength range, enabling a search for Lyman α in the redshift range spanning 5.06–7.80. We target 22 z-band dropouts and find no evidence of Lyman α emission, with the exception of a tentative detection (<5σ, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3-6.5 is inconsistent with our non-detections at z ∼ 7.8 at the 96 per cent confidence level. This may indicate that a significant neutral H I fraction in the intergalactic medium suppresses Lyman α, with an estimated neutral fraction χHI∼0.5, in agreement with other estimates

Constraining the bright-end of the UV luminosity function for z 7-9 galaxies: results from CANDELS/GOODS-South

The recent Hubble Space Telescope near-infrared imaging with the Wide-Field Camera #3 (WFC 3) of the Great Observatories Origins Deep Survey South (GOODS-S) field in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) programme covering nearly 100 arcmin2, along with already existing Advanced Camera for Surveys optical data, makes possible the search for bright galaxy candidates at redshift z ≈ 7–9 using the Lyman break technique. We present the first analysis of z′-drop z ≈ 7 candidate galaxies in this area, finding 19 objects. We also analyse Y-drops at z ≈ 8, trebling the number of bright (HAB < 27 mag) Y-drops from our previous work, and compare our results with those of other groups based on the same data. The bright high-redshift galaxy candidates we find serve to better constrain the bright end of the luminosity function at those redshift, and may also be more amenable to spectroscopic confirmation than the fainter ones presented in various previous work on the smaller fields (the Hubble Ultra Deep Field and the WFC 3 Early Release Science observations). We also look at the agreement with previous luminosity functions derived from WFC 3 drop-out counts, finding a generally good agreement, except for the luminosity function of Yan et al. at z ≈ 8, which is strongly ruled out

Confronting predictions of the galaxy stellar mass function with observations at high redshift

We investigate the evolution of the galaxy stellar mass function at high redshift (z ≥ 5) using a pair of large cosmological hydrodynamical simulations: MassiveBlack and MassiveBlack-II. By combining these simulations, we can study the properties of galaxies with stellar masses greater than 108 M⊙ h−1 and (comoving) number densities of log10(ϕ [Mpc−3 dex−1 h3]) > −8. Observational determinations of the galaxy stellar mass function at very high redshift typically assume a relation between the observed ultraviolet (UV) luminosity and stellar mass-to-light ratio which is applied to high-redshift samples in order to estimate stellar masses. This relation can also be measured from the simulations. We do this, finding two significant differences with the usual observational assumption: it evolves strongly with redshift and has a different shape. Using this relation to make a consistent comparison between galaxy stellar mass functions, we find that at z = 6 and above the simulation predictions are in good agreement with observed data over the whole mass range. Without using the correct UV luminosity and stellar mass-to-light ratio, the discrepancy would be up to two orders of magnitude for large galaxies (>1010 M⊙ h−1). At z = 5, however, the stellar mass function for low-mass galaxies (<109 M⊙ h−1) is overpredicted by factors of a few, consistent with the behaviour of the UV luminosity function, and perhaps a sign that feedback in the simulation is not efficient enough for these galaxies