UV LUMINOSITY FUNCTIONS AT REDSHIFTSz∼ 4 TOz∼ 10: 10,000 GALAXIES FROMHSTLEGACY FIELDS

Abstract

The remarkable Hubble Space Telescope(HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) fromz 10 toz 4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates atz 4,z 5,z 6,z 7,z 8, andz 10, respectively, from the ∼1000 arcmin 2 area covered by these data sets. This sample of >10,000 galaxy candidates at ⩾ z 4 is by far the largest assembled to date with HST. The selection ofz 4-8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at ⩾ z 7. Our new LF determinations atz 4 andz 5 span a 6 mag baseline and reach to -16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to s 3.4 significance for a steeper faint-end slope of the UV LF at > z 4, with α evolving from a = - 1.64 0.04 atz 4 to a = - 2.06 0.13 atz 7 (and a = - 2.02 0.23 atz 8), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M * fromz 7 toz 4; the observed evolution in the LF is now largely represented by changes in f*. No evidence for a non-Schechter-like form to the z ∼ 4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio µ +z ( ( 1) ) 1.5 of halos provides a good representation of the observed evolution.

The Galaxy UV Luminosity Function Before the Epoch of Reionization

“…We find the model marginally overpredicts the bright end of the LF at z ∼ 0.3 and z ∼ 2 by ∼ 0.1 dex due to the difficulty in modeling dust extinction at these redshifts, but the observations are still within 2σ of our model. In particular, our model predicts a steepening of the faint-end slope at higher redshifts, consistent with the observed trend (Bouwens et al 2015b).…”

“…The picture emerging from these observations is that the number density of galaxies decreases with increasing redshift, while the LF remains consistent with a Schechter (1976) form, Φ(L) = Φ * (L/L * ) α exp (−L/L * )/L * out to z ∼ 8, albeit the uncertainty and co-variance in the best fitting parameters grows at high redshift (Schmidt et al 2014;Bouwens et al 2015b). However, other studies find either an equally good fit with a double-power law at z ∼ 8 (Finkelstein et al 2015b), or a preference for the double power law at z ∼ 7 (Bowler et al 2014;Bowler et al 2015; but note that the z ∼ 7 investigations are based on ground, rather than space observations).…”

“…For comparison, we also plot in Figure 9 the model LFs LF has a lower value of M * than that of Bouwens et al (2015b), so it is not surprising that this calibration produces slightly lower number densities compared to our reference using the Bouwens et al (2015b) LF. However, Figure 9 clearly shows that the overall evolution trends are unchanged and that the two calibrations produce LFs which are consistent within one standard deviation both between themselves and the observed data over 0 ∼ < z ∼ < 10.…”

“…where we use the Sheth & Tormen (1999) HMF and calibrate at z ∼ 5 using the Bouwens et al (2015b) LF. We calibrate over the halo mass range 10 7 − 10 14 M , and therefore extrapolate the LF over M uv = −7.5 to M uv = −24.5 to perform the abundance matching.…”

“…lookback times greater than 13 Gyr (e.g. Bouwens et al 2015b). Transformational results on the rest-frame UV light from the epoch of reionization, when the universe was less than 0.8 Gyr old, have been made possible by large and dedicated community efforts that have identified a sample of more than 1000 galaxy candidates at z > 7, spanning a large range in luminosities.…”