Ann I. Zabludoff

Galaxy Star Formation as a Function of Environment in the Early Data Release of the Sloan Digital Sky Survey

We study the galaxy star formation rate (SFR) as a function of environment using the SDSS EDR data. We nd that the SFR is depressed in dense environments (clusters and groups) compared to the eld. We nd that the suppression of the SFR starts to be noticeable at around 4 virial radii. We nd no evidence for SF triggering as galaxies fall into the clusters. We also present a project to study these eects in cluster pairs systems where the eects of lamen ts and large scale structure may be noticeable.

INTRACLUSTER LIGHT IN NEARBY GALAXY CLUSTERS: RELATIONSHIP TO THE HALOS OF BRIGHTEST CLUSTER GALAXIES

We present a detailed analysis of the surface brightness distribution of the brightest cluster galaxy (BCG) in each of 24 galaxy clusters at 0.03 < z < 0.13. We use two-dimensional profile fitting to model the surface brightness out to r = 300 kpc for each BCG, comparing r1/4, r1/n, and double r1/4 models. We obtain statistically superior fits using a two-component model consisting of a pair of r1/4 profiles with independent scale lengths, ellipticities, and orientations. The two-component model can simply reproduce the observed position angle and ellipticity gradients, which cannot generally be explained purely by triaxiality. The inner component of our two-component model has properties similar to those of a typical massive elliptical galaxy and is clearly associated with the BCG. The outer component is 10-40 times larger in scale, has ~10 times the total luminosity of the inner component, and exhibits a steeper μ-re relation than that of the elliptical fundamental plane. We interpret this outer component as a population of intracluster stars that trace the cluster potential. The two components are strongly aligned (|Δθ| < 10°) in roughly 40% of the clusters. When they are not aligned, the components tend toward high degrees of misalignment, suggesting that accretion of infalling material may change the orientation of some BCGs for a time. The extent of the outer component and its similar elongation to published cluster galaxy distributions indicates that the evolution of the intracluster light is tied to the cluster as a whole rather than to the BCG.

The Properties of Poor Groups of Galaxies. II. X-Ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h-1 kpc. The location and extent of this component, combined with its X-ray temperature (~0.7- 0.9 keV) and luminosity (~1041-42 h-2 ergs s-1), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least ~100-300 h-1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (LX), and optical velocity dispersion (σr) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of β derived from the σr-T relationship for groups and clusters is ~0.99 ± 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.

The Properties of Poor Groups of Galaxies. III. The Galaxy Luminosity Function

The form of the galaxy luminosity function (GLF) in poor groups—regions of intermediate galaxy density that are common environments for galaxies—is not well understood. Multiobject spectroscopy and wide-field CCD imaging now allow us to measure the GLF of bound group members directly (i.e., without statistical background subtraction) and to compare the group GLF with the GLFs of the field and of rich clusters. We use R-band images in 1.5 × 1.5 degree2 mosaics to obtain photometry for galaxies in the fields of six nearby (2800 -19 + 5 log h) to giants (MR ≤ -19 + 5 log h) is significantly larger for the five groups with luminous X-ray halos than for the one marginally X-ray-detected group; (2) the composite GLF for the luminous X-ray groups is consistent in shape with two measures of the composite R-band GLF for rich clusters (Trentham; Driver et al.) and flatter at the faint end than another (α ≈ -1.5; Smith et al.); (3) the composite group GLF rises more steeply at the faint end than the R-band GLF of the Las Campanas Redshift Survey (LCRS; α = -0.7 from Lin et al.), a large volume survey dominated by galaxies in environments more rarefied than luminous X-ray groups; (4) the shape difference between the LCRS field and composite group GLFs results mostly from the population of non-emission line galaxies (EW [O II] < 5 A), whose dwarf-to-giant ratio is larger in the denser group environment than in the field (cf. Ferguson & Sandage; Bromley et al.); and (5) the non-emission line dwarfs are more concentrated about the group center than the non-emission line giants, except for the central, brightest (MR < M) group elliptical (BGG). This last result indicates that the dwarfs, giants, and BGGs occupy different orbits (i.e., have not mixed completely) and suggests that at least one of these populations formed at a different time. Our results show that the shape of the GLF varies with environment and that this variation is due primarily to an increase in the dwarf-to-giant ratio of quiescent galaxies in higher density regions, at least up to the densities characteristic of X-ray luminous poor groups. This behavior suggests that, in some environments, dwarfs are more biased than giants with respect to dark matter. This trend conflicts with the prediction of standard biased galaxy formation models.

The kinematics of Abell clusters

Velocity histograms, galaxy positions, and velocity dispersions are presented for 69 nearby Abell clusters. The shape of the cumulative distribution for R = 1 or more clusters does not match the predictions of standard CDM models for any biasing parameter b. The only models consistent with the median and maximum dispersions of the sample are those of b about 1.6-2.0 and in which clusters are identified in three dimensions. Velocity dispersions of high-redshift systems appear fundamentally different from those of nearby clusters. The median dispersion of the subset of 25 cD clusters is similar to that of non-cD systems. A substantial fraction of the cD galaxies have velocities significantly different from the mean of their parent clusters. Three of these have 4 s or more measured redshifts. These last two results suggest that the formation of cD galaxies is dominated by the local, rather than global, cluster environment. 71 refs.

GALAXY CLUSTER BARYON FRACTIONS REVISITED

We measure the baryons contained in both the stellar and hot-gas components for 12 galaxy clusters and groups at z ~ 0.1 with M = 1-5 ? 1014 M ?. This paper improves upon our previous work through the addition of XMM-Newton X-ray data, enabling measurements of the total mass and masses of each major baryonic component?intracluster medium, intracluster stars, and stars in galaxies?for each system. We recover a mean relation for the stellar mass versus halo mass, , that is 1? shallower than in our previous result. We confirm that the partitioning of baryons between the stellar and hot-gas components is a strong function of M 500; the fractions of total mass in stars and X-ray gas within a sphere of radius r 500 scale as and , respectively. We also confirm that the combination of the brightest cluster galaxy and intracluster stars is an increasingly important contributor to the stellar baryon budget in lower halo masses. Studies that fail to fully account for intracluster stars typically underestimate the normalization of the stellar baryon fraction versus M 500 relation by ~25%. Our derived stellar baryon fractions are also higher, and the trend with halo mass weaker, than those derived from recent halo occupation distribution and abundance matching analyses. One difference from our previous work is the weak, but statistically significant, dependence here of the total baryon fraction upon halo mass: . For M 500 2 ? 1014, the total baryon fractions within r 500 are on average 18% below the universal value from the seven year Wilkinson Microwave Anisotropy Probe (WMAP) analysis, or 7% below for the cosmological parameters from the Planck analysis. In the latter case, the difference between the universal value and cluster baryon fractions is less than the systematic uncertainties associated with the M 500 determinations. The total baryon fractions exhibit significant scatter, particularly at M 500 < 2 ? 1014 M ? where they range from 60%-90%, or 65%-100%, of the universal value for WMAP7 and Planck, respectively. The ratio of the stellar-to-gas mass within r 500 (M /M gas), a measure of integrated star-formation efficiency, strongly decreases with increasing M 500. This relation is tight, with an implied intrinsic scatter of 12%. The fact that this relation remains tight at low mass implies that the larger scatter in the total baryon fractions at these masses arises from either true scatter in the total baryon content or observational scatter in M 500? rather than late-time physical processes such as redistribution of gas to beyond r 500. If the scatter in the baryon content at low mass is physical, then our results imply that in this mass range, the integrated star-formation efficiency rather than the baryon fraction that is constant at fixed halo mass.

The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions

We investigate the dependence of the galaxy infrared luminosity function (LF) and the associated stellar mass function (SMF) on environment and spectral type using photometry from the Two Micron All Sky Survey and redshifts from the Las Campanas Redshift Survey for galaxies brighter than MJ < -19 + 5 log h. In the field environment, the LFs of galaxies with emission lines have much steeper faint-end slopes (?J = -1.39) than those of galaxies without emission lines (?J = -0.59). In the cluster environment, however, even the non-emission-line galaxies have a steep faint-end LF (?J = -1.22). There is also a significant (95%) difference between the overall cluster and field LFs, ??J = -0.34 and ?M*J = -0.54. All these variations are more pronounced in the SMFs, which we compute by relating the strength of the 4000 ? break in the optical spectra to a stellar mass-to-light ratio.

The Importance of Lens Galaxy Environments

It is suspected that many strong gravitational lens galaxies lie in poor groups or rich clusters of galaxies, which modify the lens potentials. Unfortunately, little is actually known about the environments of most lenses, so environmental effects in lens models are often unconstrained and sometimes ignored. We show that such poor knowledge of environments introduces significant biases and uncertainties into a variety of lensing applications. Specifically, we create a mock poor group of 13 galaxies that resembles real groups, generate a sample of mock lenses associated with each member galaxy, and then analyze the lenses with standard techniques. We find that standard models of two-image (double) lenses, which neglect environment, grossly overestimate both the ellipticity of the lens galaxy (Δe/e ~ 0.5) and the Hubble constant (Δh/h ~ 0.22). Standard models of four-image (quad) lenses, which approximate the environment as a tidal shear, recover the ellipticity reasonably well (|Δe/e| 0.24) but overestimate the Hubble constant (Δh/h ~ 0.15) and have significant (~30%) errors in the millilensing analyses used to constrain the amount of substructure in dark matter halos. For both doubles and quads, standard models slightly overestimate the velocity dispersion of the lens galaxy (Δσ/σ ~ 0.06) and underestimate the magnifications of the images (Δμ/μ ~ -0.25). Standard analyses that use the statistics of lens populations to place limits on the dark energy overestimate ΩΛ (by 0.05-0.14) and underestimate the ratio of quads to doubles (by a factor of 2). The systematic biases related to environment help explain some long-standing puzzles (such as the high observed quad/double ratio) but aggravate others (such as the low value of H0 inferred from lensing). Most of the biases are caused by neglect of the convergence (gravitational focusing) from the mass associated with the environment, but additional uncertainty is introduced by neglect of higher order terms in the lens potential. Fortunately, we show that directly observing and modeling lens environments should make it possible to remove the biases and reduce the uncertainties. Such sophisticated lensing analyses will require finding the other galaxies that are members of the lensing groups and measuring the group centroids and velocity dispersions, but they should reduce systematic effects associated with environments to the few percent level.

The Las Campanas/AAT Rich Cluster Survey — II. The environmental dependence of galaxy colours in clusters at z ∼ 0.1

We present a photometric investigation of the variation in galaxy colour with environment in 11 X-ray-luminous clusters at 0.07 less than or equal to z less than or equal to 0.16 taken from the Las Campanas/AAT Rich Cluster Survey. We study the properties of the galaxy populations in individual clusters, and take advantage of the homogeneity of the sample to combine the clusters together to investigate weaker trends in the composite sample. We find that modal colours of galaxies lying on the colour-magnitude relation in the clusters become bluer by d(B - R)/dr(p) = -0.022 +/- 0.004 from the cluster core out to a projected radius of r(p) = 6 Mpc, further out in radius than any previous study. We also examine the variation in modal galaxy colour with local galaxy density, 2, for galaxies lying close to the colour-magnitude relation, and find that the median colour shifts bluewards by d(B - R)/d log(10)(Sigma) = -0.076 +/- 0.009 with decreasing local density across three orders of magnitude. We show that the position of the red envelope of galaxies in the colour-magnitude relation does not vary as a function of projected radius or density within the clusters, suggesting that the change in the modal colour results from an increasing fraction of bluer galaxies within the colour-magnitude relation, rather than a change in the colours of the whole population. We show that this shift in the colour-magnitude relations with projected radius and local density is greater than that expected from the changing morphological mix based on the local morphology-density relation. We therefore conclude that we are seeing a real change in the properties of galaxies on the colour-magnitude relation in the outskirts of clusters. The simplest interpretation of this result (and similar constraints in local clusters) is that an increasing fraction of galaxies in the lower density regions at large radii within clusters exhibit signatures of star formation in the recent past, signatures which are not seen in the evolved galaxies in the highest density regions.

The galaxy populations of X-ray-detected, poor groups

We determine the quantitative morphology and star formation properties of galaxies in six nearby X-ray-detected, poor groups using multiobject spectroscopy and wide-field R imaging. The mean recessional velocities of the galaxy groups range from 2843 to 7558 km s-1. Each group has 15-38 confirmed members ranging in luminosity from dwarfs to giants (-13.7 ≥ MR - 5 log h ≥ -21.9). We measure structural parameters for each galaxy by fitting a PSF-convolved, two-component model to their surface brightness profiles. To compare the samples directly, we fade, smooth, and rebin each galaxy image so that we effectively observe each galaxy at the same redshift (9000 km s-1) and physical resolution (0.87 h-1 kpc). The structural parameters are combined with [O II] measurements to test for correlations between morphological characteristics and current star formation in these galaxies. We compare results for the groups to a sample of field galaxies. We find that: (1) Galaxies spanning a wide range in morphological type and luminosity are fit well by a de Vaucouleurs bulge with exponential disk profile. (2) Morphologically classifying these nearby group galaxies by their bulge fraction (B/T) is fairly robust on average, even when their redshift has increased by up to a factor of 4 and the effective resolution of the images is degraded by up to a factor of 5. (3) The fraction of bulge-dominated systems in these groups is higher than in the field (~50% versus ~20%). (4) The fraction of bulge-dominated systems in groups decreases with increasing radius, similar to the morphology-radius (~density) relation observed in galaxy clusters. (5) Current star formation in group galaxies is correlated with significant morphological asymmetry for disk-dominated systems (B/T < 0.4). (6) The group galaxies that are most disk dominated (B/T < 0.2) are less star forming and asymmetric on average than their counterparts in the field.