We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4 to 8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 ×10 6 Mpc 3 over this epoch, allowing us to perform a robust search for faint (M UV = −18) and bright (M UV < −21) highredshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 < z < 8.5, with >1000 galaxies at z ≈ 6 -8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, as well as a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z ≥ 6. Our best-fit value of the characteristic magnitude M * UV is consistent with −21 at z ≥ 5, different than that inferred based on previous trends at lower redshift, and brighter at ∼2σ significance than previous measures at z = 6 and 7 (Bouwens et al. 2007(Bouwens et al. , 2011b. At z = 8, a single powerlaw provides an equally good fit to the UV luminosity function, while at z = 6 and 7, an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M * UV is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, though a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M UV = −17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1+z) −4.3±0.5 at z > 4, consistent with observations at z ≥ 9. Our observed luminosity functions are consistent with a reionization history that starts at z 10, completes at z > 6, and reaches a midpoint (x HII = 0.5) at 6.7 < z < 9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, though consistent with recent estimates of bright z ∼ 10 galaxies.

We present galaxy stellar mass functions (GSMFs) at z = 4-8 from a rest-frame ultraviolet (UV) selected sample of ∼4500 galaxies, found via photometric redshifts over an area of ∼280 arcmin 2 in the CANDELS/GOODS fields and the Hubble Ultra Deep Field. The deepest Spitzer/IRAC data yet-to-date and the relatively large volume allow us to place a better constraint at both the lowand high-mass ends of the GSMFs compared to previous space-based studies from pre-CANDELS observations. Supplemented by a stacking analysis, we find a linear correlation between the restframe UV absolute magnitude at 1500Å (M UV ) and logarithmic stellar mass (log M * ) that holds for galaxies with log(M * /M ) 10. We use simulations to validate our method of measuring the slope of the log M * -M UV relation, finding that the bias is minimized with a hybrid technique combining photometry of individual bright galaxies with stacked photometry for faint galaxies. The resultant measured slopes do not significantly evolve over z = 4-8, while the normalization of the trend exhibits a weak evolution toward lower masses at higher redshift. We combine the log M * -M UV distribution with observed rest-frame UV luminosity functions at each redshift to derive the GSMFs, finding that the low-mass-end slope becomes steeper with increasing redshift from α = −1.55 +0.08 −0.07 at z = 4 to α = −2.25 +0.72 −0.35 at z = 8. The inferred stellar mass density, when integrated over M * = 10 8 -10 13 M , increases by a factor of 10 +30 −2 between z = 7 and z = 4 and is in good agreement with the time integral of the cosmic star formation rate density. HST Data and Sample SelectionThe galaxy sample employed in this study is from Finkelstein et al. (2015), to which we refer the reader for full details of the HST data used and the galaxy sample selection. This sample consists of ∼7000 galaxies selected via photometric redshifts over a redshift range of z = 3.5-8.5. These galaxies were selected using HST

Out of several dozen z > 7 candidate galaxies observed spectroscopically, only five have been confirmed via Lymanα emission, at z=7.008, 7.045, 7.109, 7.213 and 7.215. [1][2][3][4] The small fraction of confirmed galaxies may indicate that the neutral fraction in the intergalactic medium (IGM) rises quickly at z > 6.5, as Lymanα is resonantly scattered by neutral gas.3, 5-8 However, the small samples and limited depth of previous observations makes these conclusions tentative. Here we report the results of a deep near-infrared spectroscopic survey of 43 z > 6.5 galaxies. We detect only a single galaxy, confirming that some process is making Lymanα difficult to detect. The detected emission line at 1.0343 µm is likely to be Lymanα emission, placing this galaxy at a redshift z = 7.51, an epoch 700 million years after the Big Bang. This galaxy's colors are consistent with significant metal content, implying that galaxies become enriched rapidly. We measure a surprisingly high star formation rate of 330 M⊙ yr −1 , more than a factor of 100 greater than seen in the Milky Way. Such a galaxy is unexpected in a survey of our size 9 , suggesting that the early universe may harbor more intense sites of star-formation than expected.We obtained near-infrared (near-IR) spectroscopy of galaxies originally discovered in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) 10, 11 with the newlycommissioned near-infrared spectrograph MOSFIRE 12 on the Keck I 10 meter telescope. From a parent sample of over 100 galaxy candidates at z > 7 selected via their HST colors through the photometric redshift technique [13][14][15][16] , we observed 43 candidate high-redshift galaxies over two MOSFIRE pointings with exposure times of 5.6 and 4.5 hr, respectively. Our observations covered Lyα emission at redshifts of 7.0 -8.2. We visually inspected the reduced data at the expected slit positions for our 43 observed sources and found plausible emission lines in eight objects, with only one line detected at >5σ significance. The detected emission line is at a wavelength of 1.0343µm with an integrated signal-to-noise (S/N) of 7.8 ( Figure 1) and comes from the object designated z8 GND 5296 in our sample (RA=12:36:37.90; Dec=62:18:08.5 J2000). Based on the arguments outlined below (and discussed extensively in the supplementary material), we identify this line as the Lyα transition of hydrogen at a line-peak redshift of z = 7.5078 ± 0.0004; consistent with our photometric redshift 68% confidence range of 7.5 < z < 7.9 for z8 GND 5296. As expected for a galaxy at z = 7.51, z8 GND 5296 is completely undetected in the HST optical bands, including an extremely deep 0.8 µm image (Figure 2). The galaxy is bright in the HST near-IR bands, becoming brighter with increasing wavelength, implying that the Lyman break lies near 1 µm and that the galaxy has a moderately red rest-frame UV color. The galaxy is well-detected in both Spitzer/IRAC bands and is much brighter at 4.5 µm than at 3.6 µm. The strong break at observed 1 µm res...

Context. The form and evolution of the galaxy stellar mass function (GSMF) at high redshifts provide crucial information on star formation history and mass assembly in the young Universe, close or even prior to the epoch of reionization. Aims. We used the unique combination of deep optical/near-infrared/mid-infrared imaging provided by HST, Spitzer, and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the GSMF over the redshift range 3.5 ≤ z ≤ 7.5. Methods. We used the HST WFC3/IR near-infrared imaging from CANDELS and HUDF09, reaching H 27−28.5 over a total area of 369 arcmin 2 , in combination with associated deep HST ACS optical data, deep Spitzer IRAC imaging from the SEDS programme, and deep Y and K-band VLT Hawk-I images from the HUGS programme, to select a galaxy sample with high-quality photometric redshifts. These have been calibrated with more than 150 spectroscopic redshifts in the range 3.5 ≤ z ≤ 7.5, resulting in an overall precision of σ z /(1 + z) ∼ 0.037. With this database we have determined the low-mass end of the high-redshift GSMF with unprecedented precision, reaching down to masses as low as M * ∼ 10 9 M at z = 4 and ∼6 × 10 9 M at z = 7. Results. We find that the GSMF at 3.5 ≤ z ≤ 7.5 depends only slightly on the recipes adopted to measure the stellar masses, namely the photometric redshifts, the star formation histories, the nebular contribution, or the presence of AGN in the parent sample. The low-mass end of the GSMF is steeper than has been found at lower redshifts, but appears to be unchanged over the redshift range probed here. Meanwhile the high-mass end of the GSMF appears to evolve primarily in density, although there is also some evidence of evolution in characteristic mass. Our results are very different from previous mass function estimates based on converting UV galaxy luminosity functions into mass functions via tight mass-to-light relations. Integrating our evolving GSMF over mass, we find that the growth of stellar mass density is barely consistent with the time-integral of the star formation rate density over cosmic time at z > 4. Conclusions. These results confirm the unique synergy of the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of moderate/low-mass galaxies at high redshifts, and reaffirm the importance of space-based infrared selection for the unbiased measurement of the evolving GSMF in the young Universe.

The observed deficit of strongly Lyman-α emitting galaxies at z > 6.5 is attributed to either increasing neutral hydrogen in the intergalactic medium (IGM) and/or to the evolving galaxy properties. To investigate this, we have performed very deep near-IR spectroscopy of z 7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyman-α fraction at z ∼ 8 using two methods. First, we derived N Lyα /N tot directly using extensive simulations to correct for incompleteness. Second, we used a Bayesian formalism (introduced by Treu et al. 2012) that compares the z > 7 galaxy spectra to models of the Lyman-α equivalent width (W Lyα ) distribution at z ∼ 6. We explored two simple evolutionary scenarios: pure number evolution where Lyman-α is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines) and uniform dimming evolution where Lyman-α is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity). The Bayesian formalism places stronger constraints compared with the direct method. Combining our data with that in the literature we find that at z ∼ 8 the Lyman-α fraction has dropped by a factor >3 (84% confidence interval) using both the dimming and number evolution scenarios, compared to the z ∼ 6 values. Furthermore, we find a tentative "positive" Bayesian evidence favoring the number evolution scenario over dimming evolution, extending trends observed at z 7 to higher redshift. Comparison of our results with theoretical models imply the IGM volume averaged neutral hydrogen fraction 0.3 suggesting that we are likely witnessing reionization in progress at z ∼ 8.