The presence of extremely compact galaxies at z ∼ 2 and their subsequent growth in physical size has been the cause of much puzzlement. We revisit the question using deep infrared Wide Field Camera 3 data to probe the rest-frame optical structure of 935 galaxies selected with 0.4 < z < 2.5 and stellar masses M * > 10 10.7 M in the UKIRT Ultra Deep Survey and GOODS-South fields of the CANDELS survey. At each redshift, the most compact sources are those with little or no star formation, and the mean size of these systems at fixed stellar mass grows by a factor of 3.5 ± 0.3 over this redshift interval. The data are sufficiently deep to identify companions to these hosts whose stellar masses are ten times smaller. By searching for these around 404 quiescent hosts within a physical annulus 10 h −1 kpc < R < 30 h −1 kpc, we estimate the minor merger rate over 0.4 < z < 2. We find that 13%-18% of quiescent hosts have likely physical companions with stellar mass ratios of 0.1 or greater. Mergers of these companions will typically increase the host mass by 6% ± 2% per merger timescale. We estimate the minimum growth rate necessary to explain the declining abundance of compact galaxies. Using a simple model motivated by recent numerical simulations, we then assess whether mergers of the faint companions with their hosts are sufficient to explain this minimal rate. We find that mergers may explain most of the size evolution observed at z 1 if a relatively short merger timescale is assumed, but the rapid growth seen at higher redshift likely requires additional physical processes.
Clusters of galaxies are excellent locations to probe the distribution of baryons and dark matter (DM) over a wide range of scales. We study a sample of seven massive (M 200 = 0.4 − 2 × 10 15 M ), relaxed galaxy clusters with centrally-located brightest cluster galaxies (BCGs) at z = 0.2 − 0.3. Using the observational tools of strong and weak gravitational lensing, combined with resolved stellar kinematics within the BCG, we measure the total radial density profile, comprising both dark and baryonic matter, over scales of 3−3000 kpc. We present Keck spectroscopy yielding seven new spectroscopic redshifts of multiply imaged sources and extended stellar velocity dispersion profiles of the BCGs. Lensingderived mass profiles typically agree with independent X-ray estimates within 15%, suggesting that departures from hydrostatic equilibrium are small and that the clusters in our sample (except A383) are not strongly elongated or compressed along the line of sight. The inner logarithmic slope γ tot of the total density profile measured over r/r 200 = 0.003 − 0.03, where ρ tot ∝ r −γtot , is found to be nearly universal, with a mean γ tot = 1.16 ± 0.05 (random) +0.05 −0.07 (systematic) and an intrinsic scatter σ γ < 0.13 (68% confidence). This is further supported by the very homogeneous shape of the observed velocity dispersion profiles, which are mutually consistent after a simple scaling. Remarkably, this slope agrees closely with high-resolution numerical simulations that contain only DM, despite the significant contribution of stellar mass on the scales we probe. The Navarro-Frenk-White profile characteristic of collisionless cold DM is a better description of the total mass density at radii 5 − 10 kpc than that of DM alone. Hydrodynamical simulations that include baryons, cooling, and feedback currently provide a poorer match. We discuss the significance of our findings for understanding the physical processes governing the assembly of BCGs and cluster cores, particularly the influence of baryons on the inner DM halo.
We present stellar and dark matter (DM) density profiles for a sample of seven massive, relaxed galaxy clusters derived from strong and weak gravitational lensing and resolved stellar kinematic observations within the centrally-located brightest cluster galaxies (BCGs). In Paper I of the series, we demonstrated that the total density profile derived from these data, which span three decades in radius, is consistent with numerical DM-only simulations at radii 5 − 10 kpc, despite the significant contribution of stellar material in the core. Here we decompose the inner mass profiles of these clusters into stellar and dark components. Parametrizing the DM density profile as a power law ρ DM ∝ r −β on small scales, we find a mean slope β = 0.50 ± 0.10 (random) +0.14 −0.13 (systematic). Alternatively, cored Navarro-Frenk-White (NFW) profiles with log r core /kpc = 1.14 ± 0.13 +0.14 −0.22 provide an equally good description. These density profiles are significantly shallower than canonical NFW models at radii 30 kpc, comparable to the effective radii of the BCGs. The inner DM profile is correlated with the distribution of stars in the BCG, suggesting a connection between the inner halo and the assembly of stars in the central galaxy. The stellar mass-to-light ratio inferred from lensing and stellar dynamics is consistent with that inferred using stellar population synthesis models if a Salpeter initial mass function is adopted. We compare these results to theories describing the interaction between baryons and DM in cluster cores, including adiabatic contraction models and the possible effects of galaxy mergers and active galactic nucleus feedback, and evaluate possible signatures of alternative DM candidates.
We investigate the stellar populations for a sample of 24 quiescent galaxies at 1.5 < z < 2.5 using deep rest-frame optical spectra obtained with Keck MOSFIRE. By fitting templates simultaneously to the spectroscopic and photometric data, and exploring a variety of star formation histories, we obtain robust measurements of median stellar ages and residual levels of star formation. After subtracting the stellar templates, the stacked spectrum reveals the Hα and [N II] emission lines, providing an upper limit on the ongoing star formation rate of 0.9 ± 0.1 M /yr. By combining the MOSFIRE data to our sample of Keck LRIS spectra at lower redshift, we analyze in a consistent manner the quiescent population at 1 < z < 2.5. We find a tight relation (with a scatter of 0.13 dex) between the stellar age and the rest-frame U − V and V − J colors, which can be used to estimate the age of quiescent galaxies given their colors. Applying this age-color relation to large, photometric samples, we are able to model the number density evolution for quiescent galaxies of various ages. We find evidence for two distinct quenching paths: a fast quenching that produces compact post-starburst systems, and a slow quenching of larger galaxies. Fast quenching accounts for about a fifth of the growth of the red sequence at z ∼ 1.4, and half at z ∼ 2.2. We conclude that fast quenching is triggered by dramatic events such as gas-rich mergers, while slow quenching is likely caused by a different physical mechanism.
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