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Casey Papovich

Publications and source records attributed to Casey Papovich.

At least 235 records · Page 13Linked to original sources
arXiv2015

An Increasing Stellar Baryon Fraction in Bright Galaxies at High Redshift

Recent observations have shown that the characteristic luminosity of the rest-frame ultraviolet (UV) luminosity function does not significantly evolve at 4 < z < 7 and is approximately M*_UV ~ -21. We investigate this apparent non-evolution by examining a sample of 178 bright, M_UV < -21 galaxies at z=4 to 7, analyzing their stellar populations and host halo masses. Including deep Spitzer/IRAC imaging to constrain the rest-frame optical light, we find that M*_UV galaxies at z=4-7 have similar stellar masses of log(M/Msol)=9.6-9.9 and are thus relatively massive for these high redshifts. However, bright galaxies at z=4-7 are less massive and have younger inferred ages than similarly bright galaxies at z=2-3, even though the two populations have similar star formation rates and levels of dust attenuation. We match the abundances of these bright z=4-7 galaxies to halo mass functions from the Bolshoi Lambda-CDM simulation to estimate the halo masses. We find that the typical halo masses in ~M*_UV galaxies decrease from log(M_h/Msol)=11.9 at z=4 to log(M_h/Msol)=11.4 at z=7. Thus, although we are studying galaxies at a similar mass across multiple redshifts, these galaxies live in lower mass halos at higher redshift. The stellar baryon fraction in units of the cosmic mean Omega_b/Omega_m rises from 5.1% at z=4 to 11.7% at z=7; this evolution is significant at the ~3-sigma level. This rise does not agree with simple expectations of how galaxies grow, and implies that some effect, perhaps a diminishing efficiency of feedback, is allowing a higher fraction of available baryons to be converted into stars at high redshifts.

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arXiv2015

Satellite Quenching and Galactic Conformity at 0.3 < z < 2.5

We measure the evolution of the quiescent fraction and quenching efficiency of satellites around star-forming and quiescent central galaxies with stellar mass $\log(M_{\mathrm{cen}}/M_{\odot})>10.5$ at $0.3 9.3$. Satellites for both star-forming and quiescent central galaxies have higher quiescent fractions compared to field galaxies matched in stellar mass at all redshifts. We also observe "galactic conformity": satellites around quiescent centrals are more likely to be quenched compared to the satellites around star-forming centrals. In our sample, this conformity signal is significant at $\gtrsim3σ$ for $0.6<z<1.6$, whereas it is only weakly significant at $0.3<z<0.6$ and $1.6<z<2.5$. Therefore, conformity (and therefore satellite quenching) has been present for a significant fraction of the age of the universe. The satellite quenching efficiency increases with increasing stellar mass of the central, but does not appear to depend on the stellar mass of the satellite to the mass limit of our sample. When we compare the satellite quenching efficiency of star-forming centrals with stellar masses 0.2 dex higher than quiescent centrals (which should account for any difference in halo mass), the conformity signal decreases, but remains statistically significant at $0.6<z<0.9$. This is evidence that satellite quenching is connected to the star-formation properties of the central as well as to the mass of the halo. We discuss physical effects that may contribute to galactic conformity, and emphasize that they must allow for continued star-formation in the central galaxy even as the satellites are quenched.

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arXiv2015

The SFR-M* Relation and Empirical Star-Formation Histories from ZFOURGE at 0.5 < z < 4

We explore star-formation histories (SFHs) of galaxies based on the evolution of the star-formation rate stellar mass relation (SFR-M*). Using data from the FourStar Galaxy Evolution Survey (ZFOURGE) in combination with far-IR imaging from the Spitzer and Herschel observatories we measure the SFR-M* relation at 0.5 < z < 4. Similar to recent works we find that the average infrared SEDs of galaxies are roughly consistent with a single infrared template across a broad range of redshifts and stellar masses, with evidence for only weak deviations. We find that the SFR-M* relation is not consistent with a single power-law of the form SFR ~ M*^a at any redshift; it has a power-law slope of a~1 at low masses, and becomes shallower above a turnover mass (M_0) that ranges from 10^9.5 - 10^10.8 Msol, with evidence that M_0 increases with redshift. We compare our measurements to results from state-of-the-art cosmological simulations, and find general agreement in the slope of the SFR-M* relation albeit with systematic offsets. We use the evolving SFR-M* sequence to generate SFHs, finding that typical SFRs of individual galaxies rise at early times and decline after reaching a peak. This peak occurs earlier for more massive galaxies. We integrate these SFHs to generate mass-growth histories and compare to the implied mass-growth from the evolution of the stellar mass function. We find that these two estimates are in broad qualitative agreement, but that there is room for improvement at a more detailed level. At early times the SFHs suggest mass-growth rates that are as much as 10x higher than inferred from the stellar mass function. However, at later times the SFHs under-predict the inferred evolution, as is expected in the case of additional growth due to mergers.

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arXiv2015

Dusty galaxies and the degeneracy between their dust distributions and the attenuation formula

Do spatial distributions of dust grains in galaxies have typical forms, as do spatial distributions of stars? We investigate whether or not the distributions resemble uniform foreground screens, as commonly assumed by the high-redshift galaxy community. We use rest-frame infrared, ultraviolet, and H$α$ line luminosities of dust-poor and dusty galaxies at z ~ 0 and z ~ 1 to compare measured H$α$ escape fractions with those predicted by the Calzetti attenuation formula. The predictions, based on UV escape fractions, overestimate the measured H$α$ escape fractions for all samples. The interpretation of this result for dust-poor z ~ 0 galaxies is that regions with ionizing stars have more dust than regions with nonionizing UV-emitting stars. Dust distributions for these galaxies are nonuniform. The interpretation of the overestimates for dusty galaxies at both redshifts is less clear. If the attenuation formula is inapplicable to these galaxies, perhaps the disagreements are unphysical; perhaps dust distributions in these galaxies are uniform. If the attenuation formula does apply, then dusty galaxies have nonuniform dust distributions; the distributions are more uniform than they are in dust-poor galaxies. A broad range of H$α$ escape fractions at a given UV escape fraction for z ~ 1 dusty galaxies, if real, indicates diverse dust morphologies and the implausibility of the screen assumption.

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arXiv2015

The Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity function at z=4-8. We use deep Hubble Space Telescope imaging over the CANDELS/GOODS fields, the Hubble Ultra Deep Field and the Year 1 Hubble Frontier Field deep parallel observations. These surveys provides an effective volume of 0.6-1.2 x 10^6 Mpc^3 over this epoch, allowing us to perform a robust search for faint (M_UV=-18) and bright (M_UV < -21) galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 galaxies at 3.5 1000 galaxies at z~6-8. We measure the luminosity function using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end our results agree with previous studies, yet we find a higher abundance of UV-bright galaxies at z>6, with M* ~ -21 at z>5, different than that inferred based on previous trends at lower redshift. At z=8, a single power-law 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 to semi-analytical models, and find that the lack of evolution in M* is consistent with models where the impact of dust attenuation on the bright-end of the luminosity function decreases at higher redshift. We measure the evolution of the cosmic star-formation rate density, correcting for dust attenuation, and find that it declines as (1+z)^(-4.3 +/- 0.5) at z>4, consistent with observations at z>9. Our observations 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, 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. [abridged]

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arXiv2015

Quenching and Morphological Transformation in Semi-Analytic Models and CANDELS

We examine the spheroid growth and star formation quenching experienced by galaxies from z~3 to the present by studying the evolution with redshift of the quiescent and spheroid-dominated fractions of galaxies from the CANDELS and GAMA surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disk instabilities. We facilitate direct morphological comparison by converting our model bulge-to-total stellar mass ratios to Sersic indices. We then subdivide our population into the four quadrants of the sSFR-Sersic index plane and study the buildup of each of these subpopulations. We find that the fraction of star forming disks declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star forming spheroids and quiescent disks are both non-negligible, and stay nearly constant over the period we have studied, at about 10% and 15-20% respectively. Our model is qualitatively successful at reproducing the evolution of the two "main" populations (star forming disk-dominated galaxies and quiescent spheroid-dominated galaxies), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star forming spheroids, and increase in quiescent disks, than seen in the observations. A model with an additional channel for bulge growth via disk instabilities agrees better overall with the observations than a model in which bulges may grow only through mergers. We study evolutionary tracks of some individual galaxies as they experience morphological transformation and quenching, and examine the importance of different physical drivers of this transformation (major and minor mergers and disk instabilities). We find that complex histories with multiple transformative events are the norm.

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arXiv2015

Star-forming blue ETGs in two newly discovered galaxy overdensities in the HUDF at z=1.84 and 1.9: unveiling the progenitors of passive ETGs in cluster cores

We present the discovery of two galaxy overdensities in the Hubble Space Telescope UDF: a proto-cluster, HUDFJ0332.4-2746.6 at $z = 1.84 \pm 0.01$, and a group, HUDFJ0332.5-2747.3 at $z =1.90 \pm 0.01$. Assuming viralization, the velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of $M_{200}= (2.2 \pm 1.8) \times 10^{14} M_{\odot}$, consistent with the lack of extended X-ray emission. Neither overdensity shows evidence of a red sequence. About $50\%$ of their members show interactions and/or disturbed morphologies, which are signatures of merger remnants or disk instability. Most of their ETGs have blue colors and show recent star-formation. These observations reveal for the first time large fractions of spectroscopically confirmed star-forming blue ETGs in proto-clusters at $z\approx 2$. These star-forming ETGs are most likely among the progenitors of the quiescent population in clusters at more recent epochs. Their mass-size relation is consistent with that of passive ETGs in clusters at $z\sim0.7-1.5$. If these galaxies are the progenitors of cluster ETGs at these lower redshifts, their size would evolve according to a similar mass-size relation. It is noteworthy that quiescent ETGs in clusters at $z=1.8-2$ also do not show any significant size evolution over this redshift range, contrary to field ETGs. The ETG fraction is $\lesssim 50\%$, compared to the typical quiescent ETG fraction of $\approx 80\%$ in cluster cores at $z< 1$. The fraction, masses, and colors of the newly discovered ETGs imply that other cluster ETGs will be formed/accreted at a later time.

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arXiv2015

The Differential Size Growth of Field and Cluster Galaxies at z=2.1 Using the ZFOURGE Survey

There is ongoing debate regarding the extent that environment affects galaxy size growth beyond z>1. To investigate the differences in star-forming and quiescent galaxy properties as a function of environment at z=2.1, we create a mass-complete sample of 59 cluster galaxies Spitler et al. (2012) and 478 field galaxies with log(M)>9 using photometric redshifts from the ZFOURGE survey. We compare the mass-size relation of field and cluster galaxies using measured galaxy semi-major axis half-light radii ($r_{1/2,maj}$) from CANDELS HST/F160W imaging. We find consistent mass normalized (log(M)=10.7) sizes for quiescent field galaxies ($r_{1/2,maj}=1.81\pm0.29$ kpc) and quiescent cluster galaxies ($r_{1/2,maj}=2.17\pm0.63$ kpc). The mass normalized size of star-forming cluster galaxies ($r_{1/2,maj}=4.00\pm0.26$ kpc ) is 12% larger (KS test $2.1σ$) than star-forming field galaxies ($r_{1/2,maj}=3.57\pm0.10$ kpc). From the mass-color relation we find that quiescent field galaxies with 9.7 10.4 have consistent colors. We find that star-forming cluster galaxies are on average 20% redder than star-forming field galaxies at all masses. Furthermore, we stack galaxy images to measure average radial color profiles as a function of mass. Negative color gradients are only present for massive star-forming field and cluster galaxies with log(M)>10.4, the remaining galaxy masses and types have flat profiles. Our results suggest given the observed differences in size and color of star-forming field and cluster galaxies, that the environment has begun to influence/accelerate their evolution. However, the lack of differences between field and cluster quiescent galaxies indicates that the environment has not begun to significantly influence their evolution at z~2.

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arXiv2015

The Relation Between SFR and Stellar Mass for Galaxies at 3.5 $\le z\le$ 6.5 in CANDELS

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 mass relation for galaxies at $3.5\leq z\leq 6.5$ 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 $z\sim 6$ and $0.70 \pm 0.21$ at $z\sim 4$. 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, $σ(\log \mathrm{SFR}/\mathrm{M}_\odot$ 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.

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arXiv2015

The Herschel view of the dominant mode of galaxy growth from z=4 to the present day

We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS-North, GOODS-South, UDS and COSMOS obtained within the GOODS-Herschel and CANDELS-Herschel key programs. The picture provided by 10497 individual far-infrared detections is supplemented by the stacking analysis of a mass-complete sample of 62361 star-forming galaxies from the CANDELS-HST H band-selected catalogs and from two deep ground-based Ks band-selected catalogs in the GOODS-North and the COSMOS-wide fields, in order to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called "scatter stacking" that may be easily generalized to other experiments. We demonstrate that galaxies of all masses from z=4 to 0 follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the logSFR-logM* relation with evidence for a flattening of the main sequence at high masses (log(M*/Msun) > 10.5) that becomes less prominent with increasing redshift and almost vanishes by z~2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies. Within the main sequence, we measure a non varying SFR dispersion of 0.3 dex. The specific SFR (sSFR=SFR/M*) of star-forming galaxies is found to continuously increase from z=0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and show that more than 2/3 of present-day stars must have formed in a regime dominated by the main sequence mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 Gyr.

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arXiv2014

ZFOURGE/CANDELS: On the Evolution of M* Galaxy Progenitors from z=3 to 0.5

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 Sérsic 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.

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arXiv2014

Rapid Decline of Lyman-alpha Emission Toward the Reionization Era

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 near-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 NLyα/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 (WLyα) 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.

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arXiv2014

Kpc-scale Properties of Emission-line Galaxies

We perform a detailed study of the resolved properties of emission-line galaxies at kpc-scale to investigate how small-scale and global properties of galaxies are related. 119 galaxies with high-resolution Keck/DEIMOS spectra are selected to cover a wide range in morphologies over the redshift range 0.2<z<1.3. Using the HST/ACS and HST/WFC3 imaging data taken as a part of the CANDELS project, for each galaxy we perform SED fitting per resolution element, producing resolved rest-frame U-V color, stellar mass, star formation rate, age and extinction maps. We develop a technique to identify blue and red "regions" within individual galaxies, using their rest-frame color maps. As expected, for any given galaxy, the red regions are found to have higher stellar mass surface densities and older ages compared to the blue regions. Furthermore, we quantify the spatial distribution of red and blue regions with respect to both redshift and stellar mass, finding that the stronger concentration of red regions toward the centers of galaxies is not a significant function of either redshift or stellar mass. We find that the "main sequence" of star forming galaxies exists among both red and blue regions inside galaxies, with the median of blue regions forming a tighter relation with a slope of 1.1+/-0.1 and a scatter of ~0.2 dex compared to red regions with a slope of 1.3+/-0.1 and a scatter of ~0.6 dex. The blue regions show higher specific Star Formation Rates (sSFR) than their red counterparts with the sSFR decreasing since z~1, driver primarily by the stellar mass surface densities rather than the SFRs at a giver resolution element.

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arXiv2014

The distribution of satellites around massive galaxies at 1<z<3 in ZFOURGE/CANDELS: dependence on star formation activity

We study the statistical distribution of satellites around star-forming and quiescent central galaxies at 1 9$ at z<3. The radial satellite distribution around centrals is consistent with a projected NFW profile. Massive quiescent centrals, $\log(M/M_\odot)>10.78$, have $\sim$2 times the number of satellites compared to star-forming centrals with a significance of 2.7$σ$ even after accounting for differences in the centrals' stellar-mass distributions. We find no statistical difference in the satellite distributions of intermediate-mass quiescent and star-forming centrals, $10.48<\log(M/M_\odot)<10.78$. Comparing to the Guo2011 semi-analytic model, the excess number of satellites indicates that quiescent centrals have halo masses 0.3 dex larger than star-forming centrals, even when the stellar-mass distributions are fixed. We use a simple toy model that relates halo mass and quenching, which roughly reproduces the observed quenched fractions and the differences in halo mass between star-forming and quenched galaxies only if galaxies have a quenching probability that increases with halo mass from $\sim$0 for $\log(M_h/M_\odot)\sim$11 to $\sim$1 for $\log(M_h/M_\odot)\sim$13.5. A single halo-mass quenching threshold is unable to reproduce the quiescent fraction and satellite distribution of centrals. Therefore, while halo quenching may be an important mechanism, it is unlikely to be the only factor driving quenching. It remains unclear why a high fraction of centrals remain star-forming even in relatively massive halos.

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arXiv2014

Mid-Infrared Selected Quasars I: Virial Black Hole Mass and Eddington Ratios

We provide a catalog of 391 mid-infrared-selected (MIR, 24$μ$m) broad-emission-line (BEL, type 1) quasars in the 22 deg$^2$ SWIRE Lockman Hole field. This quasar sample is selected in the MIR from Spitzer MIPS with $S_{\rm 24} > 400μ$Jy, jointly with an optical magnitude limit of r (AB) $<$ 22.5 for broad line identification. The catalog is based on MMT and SDSS spectroscopy to select BEL quasars, extends the SDSS coverage to fainter magnitudes and lower redshifts, and recovers a more complete quasar population. The MIR-selected quasar sample peaks at $z\sim$1.4, and recovers a significant and constant (20\%) fraction of extended objects with SDSS photometry across magnitudes, which was not included in the SDSS quasar survey dominated by point sources. This sample also recovers a significant population of $z < 3$ quasars at $i > 19.1$. We then investigate the continuum luminosity and line profiles of these MIR quasars, and estimate their virial black hole masses and the Eddington ratios. The SMBH mass shows evidence of downsizing, though the Eddington ratios remain constant at $1 < z < 4$. Compared to point sources in the same redshift range, extended sources at $z < 1$ show systematically lower Eddington ratios. The catalog and spectra are publicly available online.

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arXiv2014

Exploring the z=3-4 massive galaxy population with ZFOURGE: the prevalence of dusty and quiescent galaxies

Our understanding of the redshift $z>3$ galaxy population relies largely on samples selected using the popular "dropout" technique, typically consisting of UV-bright galaxies with blue colors and prominent Lyman breaks. As it is currently unknown if these galaxies are representative of the massive galaxy population, we here use the FourStar Galaxy Evolution (ZFOURGE) Survey to create a stellar mass-limited sample at $z=3-4$. Uniquely, ZFOURGE uses deep near-infrared medium-bandwidth filters to derive accurate photometric redshifts and stellar population properties. The mass-complete sample consists of 57 galaxies with log M $>10.6$, reaching below $M^{\star}$ at $z=3-4$. On average, the massive $z=3-4$ galaxies are extremely faint in the observed optical with median $R_{tot}^{AB}=27.48\pm0.41$ (restframe $M_{1700}=-18.05\pm0.37$). They lie far below the UV luminosity-stellar mass relation for Lyman break galaxies and are about $\sim100\times$ fainter at the same mass. The massive galaxies are red ($R-Ks_{AB}=3.9\pm0.2$; restframe UV-slope $β=-0.2\pm0.3$) likely from dust or old stellar ages. We classify the galaxy SEDs by their restframe $U-V$ and $V-J$ colors and find a diverse population: $46^{+6+10}_{-6-17}$% of the massive galaxies are quiescent, $54^{+8+17}_{-8-10}$% are dusty star-forming galaxies, and only $14^{+3+10}_{-3-4}$% resemble luminous blue star forming Lyman break galaxies. This study clearly demonstrates an inherent diversity among massive galaxies at higher redshift than previously known. Furthermore,we uncover a reservoir of dusty star-forming galaxies with $4\times$ lower specific star-formation rates compared to submillimeter-selected starbursts at $z>3$. With $5\times$ higher numbers, the dusty galaxies may represent a more typical mode of star formation compared to submillimeter-bright starbursts.

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arXiv2014

A substantial population of massive quiescent galaxies at z~4 from ZFOURGE

We report the likely identification of a substantial population of massive M~10^11M_Sun galaxies at z~4 with suppressed star formation rates (SFRs), selected on rest-frame optical to near-IR colors from the FourStar Galaxy Evolution Survey. The observed spectral energy distributions show pronounced breaks, sampled by a set of near-IR medium-bandwidth filters, resulting in tightly constrained photometric redshifts. Fitting stellar population models suggests large Balmer/4000AA breaks, relatively old stellar populations, large stellar masses and low SFRs, with a median specific SFR of 2.9+/-1.8 x 10^-11/yr. Ultradeep Herschel/PACS 100micron, 160micron and Spitzer/MIPS 24micron data reveal no dust-obscured SFR activity for 15/19 (79%) galaxies. Two far-IR detected galaxies are obscured QSOs. Stacking the far-IR undetected galaxies yields no detection, consistent with the SED fit, indicating independently that the average specific SFR is at least 10x smaller than of typical star-forming galaxies at z~4. Assuming all far-IR undetected galaxies are indeed quiescent, the volume density is 1.8+/-0.7 x 10^-5Mpc^-3 to a limit of log10M/M_Sun>10.6, which is 10x and 80x lower than at z = 2 and z = 0.1. They comprise a remarkably high fraction (~35%) of z~4 massive galaxies, suggesting that suppression of star formation was efficient even at very high redshift. Given the average stellar age of 0.8Gyr and stellar mass of 0.8x10^11M_Sun, the galaxies likely started forming stars before z =5, with SFRs well in excess of 100M_Sun/yr, far exceeding that of similarly abundant UV-bright galaxies at z>4. This suggests that most of the star-formation in the progenitors of quiescent z~4 galaxies was obscured by dust.

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