Galaxies with stellar masses near M* contain the majority of stellar mass in the universe, and are therefore of special interest in the study of galaxy evolution. The Milky Way (MW) and Andromeda (M31) have present day stellar masses near M*, at 5x10
^10 Msol (MW-mass) and 10^11 Msol (M31-mass). We study the typical progenitors of these galaxies using ZFOURGE, a deep medium-band near-IR imaging survey, which is sensitive to the progenitors of these galaxies out to z~3. We use abundance-matching techniques to identify the main progenitors of these galaxies at higher redshifts. We measure the evolution in the stellar mass, rest-frame colors, morphologies, far-IR luminosities, and star-formation rates combining our deep multiwavelength imaging with near-IR HST imaging from CANDELS, and far-IR imaging from GOODS-H and CANDELS-H. The typical MW-mass and M31-mass progenitors passed through the same evolution stages, evolving from blue, star-forming disk galaxies at the earliest stages, to redder dust-obscured IR-luminous galaxies in intermediate stages, and to red, more quiescent galaxies at their latest stages. The progenitors of the MW-mass galaxies reached each evolutionary stage at later times (lower redshifts) and with stellar masses that are a factor of 2-3 lower than the progenitors of the M31-mass galaxies. The process driving this evolution, including the suppression of star-formation in present-day M* galaxies requires an evolving stellar-mass/halo-mass ratio and/or evolving halo-mass threshold for quiescent galaxies. The effective size and star-formation rates imply that the baryonic cold-gas fractions drop as galaxies evolve from high redshift to z~0 and are strongly anticorrelated with an increase in the Sersic index. Therefore, the growth of galaxy bulges in M* galaxies corresponds to a rapid decline in the galaxy gas fractions and/or a decrease in the star-formation efficiency.
Distant star-forming galaxies show a correlation between their star formation rates (SFR) and stellar masses, and this has deep implications for galaxy formation. Here, we present a study on the evolution of the slope and scatter of the SFR-stellar m
ass relation for galaxies at $3.5leq zleq 6.5$ using multi-wavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We describe an updated, Bayesian spectral-energy distribution fitting method that incorporates effects of nebular line emission, star formation histories that are constant or rising with time, and different dust attenuation prescriptions (starburst and Small Magellanic Cloud). From $z$=6.5 to $z$=3.5 star-forming galaxies in CANDELS follow a nearly unevolving correlation between stellar mass and SFR that follows SFR $sim$ $M_star^a$ with $a = 0.54 pm 0.16$ at $zsim 6$ and $0.70 pm 0.21$ at $zsim 4$. This evolution requires a star formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The observed scatter in the SFR-stellar mass relation is tight, $sigma(log mathrm{SFR}/mathrm{M}_odot$ yr$^{-1})< 0.3 - $ 0.4 dex, for galaxies with $log M_star/mathrm{M}_odot > 9$ dex. Assuming that the SFR is tied to the net gas inflow rate (SFR $sim$ $dot{M}_mathrm{gas}$), then the scatter in the gas inflow rate is also smaller than 0.3$-$0.4 dex for star-forming galaxies in these stellar mass and redshift ranges, at least when averaged over the timescale of star formation. We further show that the implied star formation history of objects selected on the basis of their co-moving number densities is consistent with the evolution in the SFR-stellar mass relation.
The observed deficit of strongly Lyman-alpha 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 n
ear-IR spectroscopy of z>7 galaxies using MOSFIRE on the Keck-I Telescope. We measure the Lyman-alpha fraction at z~8 (combined photometric redshift peak at z=7.7) using two methods. First, we derived NLy{alpha}/Ntot 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-alpha equivalent width (WLy{alpha}) distribution at z~6. We explored two simple evolutionary scenarios: smooth evolution where Lyman-alpha is attenuated in all galaxies by a constant factor (perhaps owing to processes from galaxy evolution or a slowly increasing IGM opacity), and patchy evolution where Lyman-alpha is blocked in some fraction of galaxies (perhaps due to the IGM being opaque along only some fraction of sightlines). 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-alpha fraction has dropped by a factor >3(84% confidence interval) using both the smooth and patchy scenarios compared to the z~6 values. Furthermore, we find a tentative evidence that the data favor the patchy scenario over smooth (with positive Bayesian evidence), extending trends observed at z~7 to higher redshift. If this decrease is a result of reionization as predicted by theory, then our data imply the volume averaged neutral hydrogen fraction in the IGM to be >0.3 suggesting that the reionization of the universe is in progress at z~8.
We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6<z<8. These galaxies were selected from new deep near-infrared HST imaging from the CANDELS, HUDF09 and ERS programs. In contrast to the majority
of previous analyses, which assume that the distribution of galaxy ultraviolet (UV) luminosities follows a Schechter distribution, and that the distribution continues to luminosities far below our observable limit, we investigate the contribution to reionization from galaxies which we can observe, free from these assumptions. We find that the observable population of galaxies can sustain a fully reionized IGM at z=6, if the average ionizing photon escape fraction (f_esc) is ~30%. A number of previous studies have measured UV luminosity densities at these redshifts that vary by 5X, with many concluding that galaxies could not complete reionization by z=6 unless a large population of galaxies fainter than the detection limit were invoked, or extremely high values of f_esc were present. The observed UV luminosity density from our observed galaxy samples at z=7-8 is not sufficient to maintain a fully reionized IGM unless f_esc>50%. Combining our observations with constraints on the emission rate of ionizing photons from Ly-alpha forest observations at z=6, we can constrain f_esc<34% (2-sigma) if the observed galaxies are the only contributors to reionization, or <13% (2-sigma) if the luminosity function extends to M_UV = -13. These escape fractions are sufficient to complete reionization by z=6. These constraints imply that the volume ionized fraction of the IGM becomes less than unity at z>7, consistent with a number of complementary reionization probes. If faint galaxies dominate reionization, future JWST observations will probe deep enough to see them, providing an indirect constraint on the ionizing photon escape fraction [abridged].
We study the red sequence in a cluster of galaxies at z=1.62 and follow its evolution over the intervening 9.5 Gyr to the present day. Using deep YJKs imaging with the HAWK-I instrument on the VLT we identify a tight red sequence and construct its re
st-frame i-band luminosity function (LF). There is a marked deficit of faint red galaxies in the cluster that causes a turnover in the LF. We compare the red sequence LF to that for clusters at z<0.8 correcting the luminosities for passive evolution. The shape of the cluster red sequence LF does not evolve between z=1.62 and z=0.6 but at z<0.6 the faint population builds up significantly. Meanwhile, between z=1.62 to 0.6 the inferred total light on the red sequence grows by a factor of about 2 and the bright end of the LF becomes more populated. We construct a simple model for red sequence evolution that grows the red sequence in total luminosity and matches the constant LF shape at z>0.6. In this model the cluster accretes blue galaxies from the field that are then quenched and subsequently allowed to merge. We find that 3--4 mergers among cluster galaxies during the 4 Gyr between z=1.62 and z=0.6 matches the observed luminosity function evolution between the two redshifts. The inferred merger rate is consistent with other studies of this cluster. Our result supports the picture that galaxy merging during the major growth phase of massive clusters is an important process in shaping the red sequence population at all luminosities.
We discuss the structural and morphological properties of galaxies in a z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). Th
e cluster galaxies exhibit a clear color-morphology relation: galaxies with colors of quiescent stellar populations generally have morphologies consistent with spheroids, and galaxies with colors consistent with ongoing star formation have disk-like and irregular morphologies. The size distribution of the quiescent cluster galaxies shows a deficit of compact (< 1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a result the cluster quiescent galaxies have larger average effective sizes compared to field galaxies at fixed mass at greater than 90% significance. Combined with data from the literature, the size evolution of quiescent cluster galaxies is relatively slow from z~1.6 to the present, growing as (1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that physical processes associated with the denser cluster region seems to have caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower subsequent growth at z<1.6 compared to galaxies in the lower density field. The quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower redshift samples at fixed mass, and their surface-brightness profiles suggest that they contain extended stellar disks. We argue the cluster galaxies require dissipationless (i.e., gas-poor or dry) mergers to reorganize the disk material and to match the relations for ellipticity, stellar mass, size, and color of early-type galaxies in z<1 clusters.
We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 < z < 8. We use new wide-field near-infrared data in GOODS-S from the CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift measurements. Our sa
mple consists of 2812 candidate galaxies at z > 3.5, including 113 at z = 7 to 8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. The median value of beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37 (+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require exotic stellar populations to explain their colors. This evolution can be explained by an increase in dust extinction, with the timescale consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2-sigma) correlation between beta and M_UV when measuring M_UV at a consistent rest-frame wavelength of 1500 A. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z > 7. Thus the stellar-mass - metallicity relation, previously observed up to z ~ 3, may extend out to z = 7 - 8.
We present the analysis of Herschel SPIRE far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 micron observations with existing data, we make an improved fit to the FIR spectral energy
distribution (SED) of this galaxy. We find a total infrared (IR) luminosity of L(8--1000 micron) = 6.9 +/- 0.6x10^11 Lsol; a factor of 3 more precise over previous L_IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 Msol per yr, which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa-alpha emission line (a 2.5-sigma discrepancy). Both SFR indicators assume identical Salpeter initial mass functions (IMF) with slope Gamma=2.35 over a mass range of 0.1 - 100 Msol, thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa-alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing star formation rate (SFR), paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper-mass cutoff to ~ 200 Msol, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 Msol, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.
Distant galaxies show correlations between their current star-formation rates (SFRs) and stellar masses, implying that their star-formation histories (SFHs) are highly similar. Moreover, observations show that the UV luminosities and stellar masses g
row from z=8 to 3, implying that the SFRs increase with time. We compare the cosmologically averaged evolution in galaxies at 3 < z < 8 at constant comoving number density, n = 2 x 10^-4 Mpc^-3. This allows us to study the evolution of stellar mass and star formation in the galaxy predecessors and descendants in ways not possible using galaxies selected at constant stellar mass or SFR, quantities that evolve strongly in time. We show that the average SFH of these galaxies increase smoothly from z=8 to 3 as SFR ~ t^alpha with alpha = 1.7 +/- 0.2. This conflicts with assumptions that the SFR is either constant or declines exponentially in time. We show that the stellar mass growth in these galaxies is consistent with this derived SFH. This provides evidence that the slope of the high-mass end of the IMF is approximately Salpeter unless the duty cycle of star formation is much less than unity. We argue that these relations follow from gas accretion (either through accretion or delivered by mergers) coupled with galaxy disk growth under the assumption that the SFR depends on the local gas surface density. This predicts that gas fractions decrease from z=8 to 3 on average as f_gas ~ (1+z)^0.9 for galaxies with this number density. The implied galaxy gas accretion rates at z > 4 are as fast and may even exceed the SFR: this is the gas accretion epoch. At z < 4 the SFR overtakes the implied gas accretion rate, indicating a period where galaxies consume gas faster than it is acquired. At z < 3, galaxies with this number density depart from these relations implying that star formation and gas accretion are slowed at later times.
We report spectroscopic confirmation and high-resolution infrared imaging of a z=2.79 triply-imaged galaxy behind the Bullet Cluster. This source, a Spitzer-selected luminous infrared galaxy (LIRG), is confirmed via polycyclic aromatic hydrocarbon (P
AH) features using the Spitzer Infrared Spectrograph (IRS) and resolved with HST WFC3 imaging. In this galaxy, which with a stellar mass of M*=4e9 Msun is one of the two least massive ones studied with IRS at z>2, we also detect H_2 S(4) and H_2 S(5) pure rotational lines (at 3.1 sigma and 2.1 sigma) - the first detection of these molecular hydrogen lines in a high-redshift galaxy. From the molecular hydrogen lines we infer an excitation temperature T=377+68-84 K. The detection of these lines indicates that the warm molecular gas mass is 6(+36-4)% of the stellar mass and implies the likely existence of a substantial reservoir of cold molecular gas in the galaxy. Future spectral observations at longer wavelengths with facilities like the Herschel Space Observatory, the Large Millimeter Telescope, and the Atacama Pathfinder EXperiment (APEX) thus hold the promise of precisely determining the total molecular gas mass. Given the redshift, and using refined astrometric positions from the high resolution imaging, we also update the magnification estimate and derived fundamental physical properties of this system. The previously published values for total infrared luminosity, star formation rate, and dust temperature are confirmed modulo the revised magnification; however we find that PAH emission is roughly a factor of five stronger than would be predicted by the relations between the total infrared and PAH luminosity reported for SMGs and starbursts in Pope et al. (2008).
