The deep, wide-area (~800-900 arcmin**2) near-infrared/WFC3/IR + Spitzer/IRAC observations over the CANDELS fields have been a remarkable resource for constraining the bright end of high redshift UV luminosity functions (LFs). However, the lack of HS
T 1.05-micron observations over the CANDELS fields has made it difficult to identify z~9-10 sources robustly, since such data are needed to confirm the presence of an abrupt Lyman break at 1.2 microns. We report here on the successful identification of many such z~9-10 sources from a new HST program (z9-CANDELS) that targets the highest-probability z~9-10 galaxy candidates with observations at 1.05 microns, to search for a robust Lyman-break at 1.2 microns. The potential z~9-10 candidates are preselected from the full HST, Spitzer/IRAC S-CANDELS observations, and the deepest-available ground-based optical+near-infrared observations. We identified 15 credible z~9-10 galaxies over the CANDELS fields. Nine of these galaxies lie at z~9 and 5 are new identifications. Our targeted follow-up strategy has proven to be very efficient in making use of scarce HST time to secure a reliable sample of z~9-10 galaxies. Through extensive simulations, we replicate the selection process for our sample (both the preselection and follow-up) and use it to improve current estimates for the volume density of bright z~9 and z~10 galaxies. The volume densities we find are 5(-2)(+3)x and 8(-3)(+9)x lower, respectively, than found at z~8. When compared with the best-fit evolution (i.e., dlog_{10} rho(UV)/dz=-0.29+/-0.02) in the UV luminosities densities from z~8 to z~4 integrated to 0.3L*(z=3) (-20 mag), these luminosity densities are 2.6(-0.9)(+1.5)x and 2.2(-1.1)(+2.0)x lower, respectively, than the extrapolated trends. Our new results are broadly consistent with the accelerated evolution scenario at z>8, as seen in many theoretical models.
The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF from z~10 to z~4. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z~4, z~5, z~
6, z~7, z~8, and z~10, respectively from the ~1000 arcmin**2 area probed. The selection of z~4-8 galaxies over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag baseline (-22.5 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 3.4 sigma significance for a steeper faint-end slope to the UV LF at z>4, with alpha evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7 (and alpha = -2.02+/-0.23 at z~8), consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M* over the redshift range z~4 to z~7; the observed evolution in the LF is now largely represented by changes in phi*. 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 of halos ((1+z)**-1.5) provides a good representation of the observed evolution.
We utilise a two-color Lyman-Break selection criterion to search for z~9-10 galaxies over the first 19 clusters in the CLASH program. A systematic search yields three z~9-10 candidates. While we have already reported the most robust of these candidat
es, MACS1149-JD, two additional z~9 candidates are also found and have H_{160}-band magnitudes of ~26.2-26.9. A careful assessment of various sources of contamination suggests <~1 contaminants for our z~9-10 selection. To determine the implications of these search results for the LF and SFR density at z~9, we introduce a new differential approach to deriving these quantities in lensing fields. Our procedure is to derive the evolution by comparing the number of z~9-10 galaxy candidates found in CLASH with the number of galaxies in a slightly lower redshift sample (after correcting for the differences in selection volumes), here taken to be z~8. This procedure takes advantage of the fact that the relative volumes available for the z~8 and z~9-10 selections behind lensing clusters are not greatly dependent on the details of the lensing models. We find that the normalization of the UV LF at z~9 is just 0.28_{-0.20}^{+0.39}times that at z~8, ~1.4_{-0.8}^{+3.0}x lower than extrapolating z~4-8 LF results. While consistent with the evolution in the UV LF seen at z~4-8, these results marginally favor a more rapid evolution at z>8. Compared to similar evolutionary findings from the HUDF, our result is less insensitive to large-scale structure uncertainties, given our many independent sightlines on the high-redshift universe.
Ultra-deep ACS and WFC3/IR HUDF+HUDF09 data, along with the wide-area GOODS+ERS+CANDELS data over the CDF-S GOODS field, are used to measure UV colors, expressed as the UV-continuum slope beta, of star-forming galaxies over a wide range in luminosity
(0.1L*(z=3) to 2L*(z=3)) at high redshift (z~7 to z~4). Beta is measured using all ACS and WFC3/IR passbands uncontaminated by Ly_alpha and spectral breaks. Extensive tests show that our beta measurements are only subject to minimal biases. Using a different selection procedure, Dunlop et al. recently found large biases in their beta measurements. To reconcile these different results, we simulated both approaches and found that beta measurements for faint sources are subject to large biases if the same passbands are used both to select the sources and to measure beta. High-redshift galaxies show a well-defined rest-frame UV color-magnitude (CM) relationship that becomes systematically bluer towards fainter UV luminosities. No evolution is seen in the slope of the UV CM relationship in the first 1.5 Gyr, though there is a small evolution in the zero-point to redder colors from z~7 to z~4. This suggests that galaxies are evolving along a well-defined sequence in the L(UV)-color (beta) plane (a star-forming sequence?). Dust appears to be the principal factor driving changes in the UV color (beta) with luminosity. These new larger beta samples lead to improved dust extinction estimates at z~4-7 and confirm that the extinction is still essentially zero at low luminosities and high redshifts. Inclusion of the new dust extinction results leads to (i) excellent agreement between the SFR density at z~4-8 and that inferred from the stellar mass density, and (ii) to higher SSFRs at z>~4, suggesting the SSFR may evolve modestly (by factors of ~2) from z~4-7 to z~2.
The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LF) from these new HUDF09 data for 132 zsim7 and zsim8 galaxies are combined with new LFs for zsim5-6 galaxies and the earlier zsim4 LF to reach to
very faint limits (<0.05 L*(z=3)). The faint-end slopes alpha are steep: -1.79+/-0.12 (zsim5), -1.73+/-0.20 (zsim6), -2.01+/-0.21 (zsim7), and -1.91+/-0.32 (zsim8). Slopes alphalesssim-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below sim-10 AB mag. These results have important implications for reionization. The weighted mean slope at zsim6-8 is -1.87+/-0.13. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. While current results show that galaxies can reionize the universe by zsim6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z>8, taking alpha to be -1.87+/-0.13 (the mean value at zsim6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061_{-0.006}^{+0.009}. However, this result will change if the faint-end slope alpha is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dalpha/dzsim-0.05+/-0.04), that would increase the Thomson optical depths to 0.079_{-0.017}^{+0.063}, consistent with recent WMAP estimates (tau=0.088+/-0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the ionizing flux from galaxies.
We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to <-18 AB mag and the star formation rate density at z~7 and z~8. The galaxy sa
mple is derived using a sophisticated Lyman-Break technique on the full two-year WFC3/IR and ACS data available over the HUDF09 (~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and the wider area ERS (~27.5 AB mag) ~40 arcmin**2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, AGN, spurious sources, low mass stars, and transients (e.g., SNe). From careful modelling of the selection volumes for each of our search fields we derive luminosity functions for galaxies at z~7 and z~8 to <-18 AB mag. The faint-end slopes alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05. With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The star formation rate densities derived from these new z~7 and z~8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency, with the SFR densities implied from reported stellar mass densities, being only ~40% higher at z<7. This suggests that (1) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be very different from that at later times.
We use ~88 arcmin**2 of deep (>~26.5 mag at 5 sigma) NICMOS data over the two GOODS fields and the HDF South to conduct a search for bright z>~7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin**2 of NICMOS H
-band data where only plausible z>~7 candidates are followed up with NICMOS J-band observations. ~248 arcmin**2 of deep ground-based near-infrared data (>~25.5 mag, 5 sigma) is also considered in the search. In total, we report 15 z-dropout candidates over this area -- 7 of which are new to these search fields. Two possible z~9 J-dropout candidates are also found, but seem unlikely to correspond to z~9 galaxies. The present z~9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low mass stars, supernovae (SNe), and spurious sources. The estimated contamination rate of our z~7 selection is ~24%. Through careful simulations, the effective volume available to our z>~7 selections is estimated and used to establish constraints on the volume density of luminous (L*(z=3), or -21 mag) galaxies from these searches. We find that the volume density of luminous star-forming galaxies at z~7 is 13_{-5}^{+8}x lower than at z~4 and >25x lower (1 sigma) at z~9 than at z~4. This is the most stringent constraint yet available on the volume density of >~L* galaxies at z~9. The present wide-area, multi-field search limits cosmic variance to <20%. The evolution we find at the bright end of the UV LF is similar to that found from recent Subaru Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z~7 z-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters).
Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900-2,000 million years (Myr) after the Big Bang (redshifts 6 > z > 3; ref. 1). The Hubble Ultra Deep Field (HUDF09) d
ata have yielded the first reliable detections of z ~ 8 galaxies that, together with reports of a gamma-ray burst at z ~ 8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z ~ 7-8 galaxies suggest substantial star formation at z > 9-10. Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z ~ 10 galaxies in the heart of the reionization epoch, only 500 Myr after the Big Bang. Not only do we find one possible z ~ 10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (~10%) at this time than it is just ~200 Myr later at z ~ 8. This demonstrates how rapid galaxy build-up was at z ~ 10, as galaxies increased in both luminosity density and volume density from z ~ 8 to z ~ 10. The 100-200 Myr before z ~ 10 is clearly a crucial phase in the assembly of the earliest galaxies.
We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope be
ta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue betas (i.e., beta~-3), extremely low metallicities and mechanisms to reduce the red nebular emission are likely required. For example, a large escape fraction (i.e., f_{esc}>~0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z~7 galaxies is >~0.3, it may help to explain how galaxies reionize the universe.
We provide a systematic measurement of the rest-frame UV continuum slope beta over a wide range in redshift (z~2-6) and rest-frame UV luminosity (0.1-2L*) to improve estimates of the SFR density at high redshift. We utilize the deep optical and infra
red data (ACS/NICMOS) over the CDF-S and HDF-N GOODS fields, as well as the UDF for our primary UBVi dropout sample. We correct the observed distributions for selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z~2-4 than it is at z~5-6. Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z~2.5 and z~4. We do not find a large number of galaxies with betas as red as -1 in our dropout selections at z~4, and particularly at z>~5, even though such sources could be readily selected from our data. This suggests that star-forming galaxies at z>~5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z>~5 that are missed in dropout searches. Using the same relation between UV-continuum slope and dust extinction as found to be appropriate at z~0 and z~2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. We find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (<~2x) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV, the overall dust extinction correction remains modest at all redshifts. We include the contribution from ULIRGs in our SFR density estimates at z~2-6, but find that they contribute only ~20% of the total at z~2.5 and <~10% at z>~4.
