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Register a new userDetection of quiescent galaxies in a bicolor sequence from z=0-2
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Rik J. Williams
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We investigate the properties of quiescent and star-forming galaxy populations to z~2 with purely photometric data, employing a novel rest-frame color selection technique. From the UKIDSS Ultra-Deep Survey Data Release 1, with matched optical and mid-IR photometry taken from the Subaru XMM Deep Survey and Spitzer Wide-Area Infrared Extragalactic Survey respectively, we construct a K-selected galaxy catalog and calculate photometric redshifts. Excluding stars, objects with uncertain z_phot solutions, those that fall in bad or incomplete survey regions, and those for which reliable rest-frame colors could not be derived, 30108 galaxies with K<22.4 (AB) and z<2.5 remain. The galaxies in this sample are found to occupy two distinct populations in the rest-frame U-V vs. V-J color space: a clump of red, quiescent galaxies (analogous to the red sequence) and a track of star-forming galaxies extending from blue to red U-V colors. This bimodal behavior is seen up to z~2. Due to a combination of measurement errors and passive evolution, the color-color diagram is not suitable to distinguish the galaxy bimodality at z>2 for this sample, but we show that MIPS 24um data suggest that a significant population of quiescent galaxies exists even at these higher redshifts. At z=1-2, the most luminous objects in the sample are divided roughly equally between star-forming and quiescent galaxies, while at lower redshifts most of the brightest galaxies are quiescent. Moreover, quiescent galaxies at these redshifts are clustered more strongly than those actively forming stars, indicating that galaxies with early-quenched star formation may occupy more massive host dark matter halos. This suggests that the end of star formation is associated with, and perhaps brought about by, a mechanism related to halo mass.
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At $z=1-3$, the formation of new stars is dominated by dusty galaxies whose far-IR emission indicates they contain colder dust than local galaxies of a similar luminosity. We explore the reasons for the evolving IR emission of similar galaxies over cosmic time using: 1) Local galaxies from GOALS $(L_{rm IR}=10^{11}-10^{12},L_odot)$; 2) Galaxies at $zsim0.1-0.5$ from the 5MUSES ($L_{rm IR}=10^{10}-10^{12},L_odot$); 3) IR luminous galaxies spanning $z=0.5-3$ from GOODS and Spitzer xFLS ($L_{rm IR}>10^{11},L_odot$). All samples have Spitzer mid-IR spectra, and Herschel and ground-based submillimeter imaging covering the full IR spectral energy distribution, allowing us to robustly measure $L_{rm IR}^{rmscriptscriptstyle SF}$, $T_{rm dust}$, and $M_{rm dust}$ for every galaxy. Despite similar infrared luminosities, $z>0.5$ dusty star forming galaxies have a factor of 5 higher dust masses and 5K colder temperatures. The increase in dust mass is linked with an increase in the gas fractions with redshift, and we do not observe a similar increase in stellar mass or star formation efficiency. $L_{160}^{rmscriptscriptstyle SF}/L_{70}^{rmscriptscriptstyle SF}$, a proxy for $T_{rm dust}$, is strongly correlated with $L_{rm IR}^{rmscriptscriptstyle SF}/M_{rm dust}$ independently of redshift. We measure merger classification and galaxy size for a subsample, and there is no obvious correlation between these parameters and $L_{rm IR}^{rm scriptscriptstyle SF}/M_{rm dust}$ or $L_{160}^{rmscriptscriptstyle SF}/L_{70}^{rmscriptscriptstyle SF}$. In dusty star forming galaxies, the change in $L_{rm IR}^{rmscriptscriptstyle SF}/M_{rm dust}$ can fully account for the observed colder dust temperatures, suggesting that any change in the spatial extent of the interstellar medium is a second order effect.
The existence of massive galaxies with strongly suppressed star formation at z~2.3, identified in a previous paper, suggests that a red sequence may already be in place beyond z=2. In order to test this hypothesis, we study the rest-frame U-B color distribution of massive galaxies at 2<z<3. The sample is drawn from our near-infrared spectroscopic survey for massive galaxies. The color distribution shows a statistically significant (>3 sigma) red sequence, which hosts ~60% of the stellar mass at the high-mass end. The red-sequence galaxies have little or no ongoing star formation, as inferred from both emission-line diagnostics and stellar continuum shapes. Their strong Balmer breaks and their location in the rest-frame U-B, B-V plane indicate that they are in a post-starburst phase, with typical ages of ~0.5-1.0 Gyr. In order to study the evolution of the red sequence, we compare our sample with spectroscopic massive galaxy samples at 0.02<z<0.045 and 0.6<z<1.0. The rest-frame U-B color reddens by ~0.25 mag from z~2.3 to the present at a given mass. Over the same redshift interval, the number and stellar mass density on the high-mass end (>10^11 Msol) of the red sequence grow by factors of ~8 and ~6, respectively. We explore simple models to explain the observed evolution. Passive evolution models predict too strong d(U-B), and produce z~0 galaxies that are too red. More complicated models that include aging, galaxy transformations, and red mergers can explain both the number density and color evolution of the massive end of the red sequence between z~2.3 and the present.
Recent studies have identified a population of compact quiescent galaxies at zsim2. These galaxies are very rare today and establishing the existence of a nearby analog could allow us to study its structure in greater detail than is possible at high redshift. Here we present such a local analog, NGC 5845, which has a dynamical mass of M_dyn = 4.3pm0.6times10^10 M_sun and an effective radius of only r_e = 0.45pm0.05kpc. We study the structure and kinematics with HST/WFPC2 data and previously published spatially resolved kinematics. We find that NGC 5845 is similar to compact quiescent galaxies at zsim2 in terms of size versus dynamical mass (r_e-M_dyn), effective velocity dispersion versus size (sigma_e-r_e), and effective velocity dispersion versus dynamical mass (sigma_e-M_dyn). The galaxy has a prominent rotating disk evident in both the photometry and the kinematics: it extends to well beyond geq1/3 effective radius and contribute to geq1/4 of the total light of the galaxy. Our results lend support to the idea that a fraction of zsim2 compact galaxies have prominent disks and positive mass-to-light ratio gradients, although we caution that NGC 5845 may have had a different formation history than the more massive compact quiescent galaxies at zsim2.
We combine high-resolution HST/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (log(M*) > 10) galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10^8 yr). The cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary scenarios of QG formation: an early (z > 2), fast-formation path of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs without passing through a compact state.
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