Finding massive galaxies that stopped forming stars in the early Universe presents an observational challenge because their rest-frame ultraviolet emission is negligible and they can only be reliably ...identified by extremely deep near-infrared surveys. These surveys have revealed the presence of massive, quiescent early-type galaxies appearing as early as redshift z ≈ 2, an epoch three billion years after the Big Bang. Their age and formation processes have now been explained by an improved generation of galaxy-formation models, in which they form rapidly at z ≈ 3-4, consistent with the typical masses and ages derived from their observations. Deeper surveys have reported evidence for populations of massive, quiescent galaxies at even higher redshifts and earlier times, using coarsely sampled photometry. However, these early, massive, quiescent galaxies are not predicted by the latest generation of theoretical models. Here we report the spectroscopic confirmation of one such galaxy at redshift z = 3.717, with a stellar mass of 1.7 × 10
solar masses. We derive its age to be nearly half the age of the Universe at this redshift and the absorption line spectrum shows no current star formation. These observations demonstrate that the galaxy must have formed the majority of its stars quickly, within the first billion years of cosmic history in a short, extreme starburst. This ancestral starburst appears similar to those being found by submillimetre-wavelength surveys. The early formation of such massive systems implies that our picture of early galaxy assembly requires substantial revision.
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IJS, KISLJ, NUK, SBMB, UL, UM, UPUK
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 <. Similar to recent works we find that the average infrared spectral energy distributions 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 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 10 x higher than inferred from the SMF. However, at later times the SFHs under-predict the inferred evolution, as is expected in the case of additional growth due to mergers.
We present the second data release of the Large Early Galaxy Astrophysics Census (LEGA-C), an ESO 130−night public spectroscopic survey conducted with VIMOS on the Very Large Telescope. We release ...1988 spectra with typical continuum S/N 20 −1 of galaxies at 0.6 z 1.0, each observed for ∼20 hr and fully reduced with a custom-built pipeline. We also release a catalog with spectroscopic redshifts, emission-line fluxes, Lick/IDS indices, and observed stellar and gas velocity dispersions that are spatially integrated quantities, including both rotational motions and genuine dispersion. To illustrate the new parameter space in the intermediate-redshift regime probed by LEGA-C, we explore relationships between dynamical and stellar population properties. The star-forming galaxies typically have observed stellar velocity dispersions of ∼150 km s−1 and strong Hδ absorption (HδA ∼ 5 ), while passive galaxies have higher observed stellar velocity dispersions (∼200 km s−1) and weak Hδ absorption (HδA ∼ 0 ). Strong O III5007/Hβ ratios tend to occur mostly for galaxies with weak HδA or galaxies with higher observed velocity dispersion. Beyond these broad trends, we find a diversity of possible combinations of rest-frame colors, absorption-line strengths, and emission-line detections, illustrating the utility of spectroscopic measurements to more accurately understand galaxy evolution. By making the spectra and value-added catalogs publicly available we encourage the community to take advantage of this very substantial investment in telescope time provided by ESO.
Abstract
We study galactic star formation activity as a function of environment and stellar mass over 0.5 <
z
< 2.0 using the FourStar Galaxy Evolution (ZFOURGE) survey. We estimate the galaxy ...environment using a Bayesian-motivated measure of the distance to the third nearest neighbor for galaxies to the stellar mass completeness of our survey,
at
z
= 1.3 (2.0). This method, when applied to a mock catalog with the photometric-redshift precision (
) of ZFOURGE, accurately recovers galaxies in low- and high-density environments. We quantify the environmental quenching efficiency and show that at
, it depends on galaxy stellar mass, demonstrating that the effects of quenching related to (stellar) mass and environment are not separable. In high-density environments, the mass and environmental quenching efficiencies are comparable for massive galaxies (
) at all redshifts. For lower-mass galaxies (
), the environmental quenching efficiency is very low at
, but increases rapidly with decreasing redshift. Environmental quenching can account for nearly all quiescent lower-mass galaxies (
), which appear primarily at
. The morphologies of lower-mass quiescent galaxies are inconsistent with those expected of recently quenched star-forming galaxies. Some environmental process must transform the morphologies on similar timescales as the environmental quenching itself. The evolution of the environmental quenching favors models that combine gas starvation (as galaxies become satellites) with gas exhaustion through star formation and outflows (“overconsumption”), and additional processes such as galaxy interactions, tidal stripping, and disk fading to account for the morphological differences between the quiescent and star-forming galaxy populations.
We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption (EW(Hδ) ≥ 4 ) at z ∼ 0.8 drawn from DR2 of the LEGA-C survey to test the hypothesis that these ...galaxies experienced compact, central starbursts before quenching. These recently quenched galaxies, usually referred to as post-starburst galaxies, span a wide range of colors, and we find a clear correlation between color and half-light radius, such that bluer galaxies are smaller. We build simple toy models to explain this correlation: a normal star-forming disk plus a central, compact starburst component. Bursts with exponential decay timescale of ∼100 Myr that produce ∼10% to more than 100% of the preexisting masses can reproduce the observed correlation. More significant bursts also produce bluer and smaller descendants. Our findings imply that when galaxies shut down star formation rapidly, they generally had experienced compact, starburst events and that the large, observed spread in sizes and colors mostly reflects a variety of burst strengths. Recently quenched galaxies should have younger stellar ages in the centers; multiwavelength data with high spatial resolution are required to reveal the age gradient. Highly dissipative processes should be responsible for this type of formation history. While determining the mechanisms for individual galaxies is challenging, some recently quenched galaxies show signs of gravitational interactions, suggesting that mergers are likely an important mechanism in triggering the rapid shutdown of star formation activities at z ∼ 0.8.
We study the effects of galaxy environment on the evolution of the stellar mass function (SMF) over 0.2 < z < 2.0 using the FourStar Galaxy Evolution (ZFOURGE) Survey and NEWFIRM Medium-Band Survey ...(NMBS) down to the stellar mass completeness limit, (9.5) at z = 1.0 (2.0). We compare the SMFs for quiescent and star-forming galaxies in the highest and lowest environments using a density estimator based on the distance to the galaxies' third-nearest neighbors. For star-forming galaxies, at all redshifts there are only minor differences with environment in the shape of the SMF. For quiescent galaxies, the SMF in the lowest densities shows no evolution with redshift other than an overall increase in number density (φ*) with time. This suggests that the stellar mass dependence of quenching in relatively isolated galaxies both is universal and does not evolve strongly. While at , the SMF of quiescent galaxies is indistinguishable in the highest and lowest densities, at lower redshifts, it shows a rapidly increasing number density of lower-mass galaxies, , in the highest-density environments. We argue that this evolution can account for all the redshift evolution in the shape of the total quiescent galaxy SMF. This evolution in the quiescent galaxy SMF at higher redshift (z > 1) requires an environmental quenching efficiency that decreases with decreasing stellar mass at 0.5 < z < 1.5 or it would overproduce the number of lower-mass quiescent galaxies in denser environments. This requires a dominant environmental process such as starvation combined with rapid gas depletion and ejection at z > 0.5-1.0 for galaxies in our mass range. The efficiency of this process decreases with redshift, allowing other processes (such as galaxy interactions and ram-pressure stripping) to become more important at later times, z < 0.5.
Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function (SMF) at 0.2 < z < 3. We combine this with ...Hubble Space Telescope imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey, allowing for the efficient selection of both blue and red galaxies down to stellar masses of ~10 super(9.5) M sub(middot in circle) at z ~ 2.5. The total surveyed area is 316 arcmin super(2) distributed over three independent fields. We supplement these data with the wider and shallower NEWFIRM Medium-Band Survey to provide stronger constraints at high masses. Our data allow us, for the first time, to observe a rapid buildup at the low-mass end of the quiescent SMF. Since z = 2.5, the total stellar mass density of quiescent galaxies (down to 10 super(9) M sub(middot in circle)) has increased by a factor of ~12, whereas the mass density of star-forming galaxies only increases by a factor of ~2.2.
We analyze the stellar age indicators (Dn4000 and EW(Hδ)) and sizes of 467 quiescent galaxies with M* ≥ 1010 M at z ∼ 0.7 drawn from DR2 of the LEGA-C survey. Interpreting index variations in terms ...of equivalent single stellar population age, we find that the median stellar population is younger for larger galaxies at fixed stellar mass. The effect is significant, yet small; the ages of the larger and smaller subsets differ by only <500 Myr, much less than the age variation among individual galaxies (∼1.5 Gyr). At the same time, post-starburst galaxies-those that experienced recent and rapid quenching events-are much smaller than expected based on the global correlation between age and size of normal quiescent galaxies. These coexisting trends unify seemingly contradictory results in the literature; the complex correlations between size and age indicators revealed by our large sample of galaxies with high-quality spectra suggest that there are multiple evolutionary pathways to quiescence. Regardless of the specific physical mechanisms responsible for the cessation of star formation in massive galaxies, the large scatter in Dn4000 and EW(Hδ) immediately implies that galaxies follow a large variety of evolutionary pathways. On the one hand, we see evidence for a process that slowly shuts off star formation and transforms star-forming galaxies to quiescent galaxies without necessarily changing their structures. On the other hand, there is likely a mechanism that rapidly quenches galaxies, an event that coincides with dramatic structural changes, producing post-starburst galaxies that can be smaller than their progenitors.
For the first time, we present the size evolution of a mass-complete (log(M*/M ) > 10) sample of star-forming galaxies over redshifts z = 1-7, selected from the FourStar Galaxy Evolution Survey. ...Observed H-band sizes are measured from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) Hubble Space Telescope (HST)/F160W imaging. Distributions of individual galaxy masses and sizes illustrate that a clear mass−size relation exists up to z ∼ 7. At z ∼ 7, we find that the average galaxy size from the mass−size relation is more compact at a fixed mass of log(M*/M ) = 10.1, with kpc, than at lower redshifts. This is consistent with our results from stacking the same CANDELS HST/F160W imaging, when we correct for galaxy position angle alignment. We find that the size evolution of star-forming galaxies is well fit by a power law of the form kpc, which is consistent with previous works for normal star-formers at 1 < z < 4. In order to compare our slope with those derived Lyman break galaxy studies, we correct for different IMFs and methodology and find a slope of −0.97 0.02, which is shallower than that reported for the evolution of Lyman break galaxies (LBGs) at z > 4 ( ). Therefore, we conclude the LBGs likely represent a subset of highly star-forming galaxies that exhibit rapid size growth at z > 4.
ABSTRACT
The
FourStar
galaxy evolution survey (ZFOURGE) is a 45 night legacy program with the
FourStar
near-infrared camera on Magellan and one of the most sensitive surveys to date. ZFOURGE covers a ...total of 400 arcmin
2
in cosmic fields CDFS, COSMOS and UDS, overlapping CANDELS. We present photometric catalogs comprising >70,000 galaxies, selected from ultradeep
K
s
-band detection images (25.5–26.5 AB mag, 5
σ
, total), and >80% complete to
K
s
< 25.3–25.9 AB. We use 5 near-IR medium-bandwidth filters (
J
1
,
J
2
,
J
3
,
H
s
,
H
l
) as well as broad-band
K
s
at 1.05–2.16
μ
m to 25–26 AB at a seeing of ∼0.″5. Each field has ancillary imaging in 26–40 filters at 0.3–8
μ
m. We derive photometric redshifts and stellar population properties. Comparing with spectroscopic redshifts indicates a photometric redshift uncertainty
σ
z
= 0.010, 0.009, and 0.011 in CDFS, COSMOS, and UDS. As spectroscopic samples are often biased toward bright and blue sources, we also inspect the photometric redshift differences between close pairs of galaxies, finding
σ
z
,pairs
= 0.01–0.02 at 1 <
z
< 2.5. We quantify how
σ
z
,pairs
depends on redshift, magnitude, spectral energy distribution type, and the inclusion of
FourStar
medium bands.
σ
z
,pairs
is smallest for bright, blue star-forming samples, while red star-forming galaxies have the worst
σ
z
,pairs
. Including
FourStar
medium bands reduces
σ
z
,pairs
by 50% at 1.5 <
z
< 2.5. We calculate star formation rates (SFRs) based on ultraviolet and ultradeep far-IR
Spitzer
/MIPS and
Herschel
/PACS data. We derive rest-frame
U
−
V
and
V
−
J
colors, and illustrate how these correlate with specific SFR and dust emission to
z
= 3.5. We confirm the existence of quiescent galaxies at
z
∼ 3, demonstrating their SFRs are suppressed by > ×15.
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