Arjen van der Wel

Recent Structural Evolution of Early-Type Galaxies: Size Growth from z = 1 to z = 0*

Strong size and internal density evolution of early-type galaxies between -->z ~ 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant ( -->z ~ 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (σ) measurements are taken from the literature for 50 morphologically selected -->0.8 Mdyn = 2 × 1011 M☉. Sizes ( -->Reff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby ( -->0.04 σ − Reff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in -->Reff at fixed mass between -->z = 1 and the present is a factor of -->1.97 ± 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since -->z ~ 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations.

EVOLUTION OF THE COLOR-MAGNITUDE RELATION IN GALAXY CLUSTERS AT z ∼ 1 FROM THE ACS INTERMEDIATE REDSHIFT CLUSTER SURVEY

We apply detailed observations of the color-magnitude relation (CMR) with the Advanced Camera for Surveys on the Hubble Space Telescope to study galaxy evolution in eight clusters at z 1. The early-type red sequence is well defined and elliptical and lenticular galaxies lie on similar CMRs. We analyze CMR parameters—scatter, slope, and zero point—as a function of redshift, galaxy properties and cluster mass. For bright galaxies (MB – 21 mag). While the bright S0 population consistently shows larger scatter than the ellipticals, the scatter of the latter increases in the peripheral cluster regions. If we interpret these results as due to age differences, bright elliptical galaxies in cluster cores are, on average, older than S0 galaxies and peripheral elliptical galaxies (by about 0.5 Gyr, using a simple, single-burst solar metallicity stellar population model). The CMR zero point, slope, and scatter in the (U – B) z = 0 rest-frame show no significant evolution out to redshift z 1.3 or significant dependence on cluster mass. Two of our clusters display CMR zero points that are redder (by 2σ) than the average (U – B) z = 0 of our sample. We also analyze the fraction of morphological early-type and late-type galaxies on the red sequence. We find that, while in the majority of the clusters most (80% to 90%) of the CMR population is composed of early-type galaxies, in the highest-redshift, low-mass cluster of our sample, the CMR late-type/early-type fractions are similar (50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the clusters X-ray luminosity, or with its velocity dispersion, and could be a real evolution with redshift.

The Detection of a Red Sequence of Massive Field Galaxies at z ~ 2.3 and Its Evolution to z ~ 0*

The existence of massive galaxies with strongly suppressed star formation at z similar to 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 3 sigma) red sequence, which hosts similar to 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 poststarburst phase, with typical ages of similar to 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 10(11)M(circle dot)) of the red sequence grow by factors of similar to 8 and similar to 6, respectively. We explore simple models to explain the observed evolution. Passive evolution models predict too-strong Delta(U - B) and produce z similar to 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 similar to 2.3 and the present.

The Dependence of Galaxy Morphology and Structure on Environment and Stellar Mass

From the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5), we extract a sample of 4594 galaxies at redshifts 0.02 < z < 0.03, complete down to a stellar mass of M = 1010 M☉. We quantify their structure (Sersic index), morphology (Sersic index + Bumpiness), and local environment. We show that morphology and structure are intrinsically different galaxy properties, and we demonstrate that this is a physically relevant distinction by showing that these properties depend differently on galaxy mass and environment. Structure mainly depends on galaxy mass whereas morphology mainly depends on environment. This is driven by variations in star formation activity, as traced by color, which only weakly affects the structure of a galaxy but strongly affects its morphological appearance. The implication of our results is that the existence of the morphology-density relation is intrinsic and not just due to a combination of more fundamental, underlying relations. Our findings have consequences for high-redshift studies, which often use some measure of structure as a proxy for morphology. A direct comparison with local samples selected through visually classified morphologies may lead to biases in the inferred evolution of the morphological mix of the galaxy population, and misinterpretations in terms of how galaxy evolution depends on mass and environment.

A Wide-Field Study of the z ~ 0.8 Cluster RX J0152.7–1357: The Role of Environment in the Formation of the Red Sequence

We present the first results from the largest spectroscopic survey to date of an intermediate redshift galaxy cluster, the z = 0.834 cluster RX J0152.7–1357. We use the colors of galaxies, assembled from a D ~ 12 Mpc region centered on the cluster, to investigate the properties of the red sequence as a function of density and clustercentric radius. Our wide-field multislit survey with a low-dispersion prism in the Inamori Magellan Areal Camera and Spectrograph at the 6.5 m Baade telescope allowed us to identify 475 new members of the cluster and its surrounding large-scale structure with a redshift accuracy of σ z /(1 + z) 1% and a contamination rate of ~2% for galaxies with i 4 × 1010 M ☉ (log M/M ☉>10.6). We find that the red galaxy fraction is 93 ± 3% in the two merging cores of the cluster and declines to a level of 64 ± 3% at projected clustercentric radii R 3 Mpc. At these large projected distances, the correlation between clustercentric radius and local density is nonexistent. This allows an assessment of the influence of the local environment on galaxy evolution, as opposed to mechanisms that operate on cluster scales (e.g., harassment, ram-pressure stripping). Even beyond R>3 Mpc we find an increasing fraction of red galaxies with increasing local density. The red galaxy fraction at the highest local densities in two large groups at R>3 Mpc matches the red galaxy fraction found in the two cores. Strikingly, galaxies at intermediate densities at R>3 Mpc, that are not obvious members of groups, also show signs of an enhanced red galaxy fraction. Our results point to such intermediate-density regions and the groups in the outskirts of the cluster, as sites where the local environment influences the transition of galaxies onto the red sequence.

Spatially Resolved Stellar Kinematics of Field Early-Type Galaxies at z = 1: Evolution of the Rotation Rate*

We use the spatial information of our previously published VLT/FORS2 absorption-line spectroscopy to measure mean stellar velocity and velocity dispersion profiles of 25 field early-type galaxies at a median redshift z = 0.97 (full range 0.6 < z < 1.2). This provides the first detailed study of early-type galaxy rotation at these redshifts. From surface brightness profiles from HST imaging we calculate two-integral oblate axisymmetric Jeans equation models for the observed kinematics. Fits to the data yield for each galaxy the degree of rotational support and the mass-to-light ratio M/LJeans. S0 and Sa galaxies are generally rotationally supported, whereas elliptical galaxies rotate less rapidly or not at all. Down to MB = − 19.5 (corrected for luminosity evolution), we find no evidence for evolution in the fraction of rotating early-type (E+S0) galaxies between z ~ 1 (63% ± 11% ) and the present (61% ± 5% ). We interpret this as evidence for little or no change in the field S0 fraction with redshift. We compare M/LJeans with M/Lvir inferred from the virial theorem and globally averaged quantities and assuming homologous evolution. There is good agreement for nonrotating (mostly E) galaxies. However, for rotationally supported galaxies (mostly S0) M/LJeans is on average ~40% higher than M/Lvir. We discuss possible explanations and the implications for the evolution of M/L between z = 1 and the present and its dependence on mass.