Aaron D. Lewis

XMM-NEWTON AND CHANDRA OBSERVATIONS OF THE GALAXY GROUP NGC 5044. I. EVIDENCE FOR LIMITED MULTIPHASE HOT GAS

Using new XMM and Chandra observations, we present an analysis of the temperature structure of the hot gas within a radius of 100 kpc of the bright nearby galaxy group NGC 5044. A spectral deprojection analysis of data extracted from circular annuli reveals that a two-temperature model (2T) of the hot gas is favored over single-phase or cooling flow ( = 4.5 ? 0.2 M? yr-1) models within the central ~30 kpc. Alternatively, the data can be fitted equally well if the temperature within each spherical shell varies continuously from ~Th to Tc ~ Th/2, but no lower. The high spatial resolution of the Chandra data allows us to determine that the temperature excursion Th ? Tc required in each shell exceeds the temperature range between the boundaries of the same shell in the best-fitting single-phase model. This is strong evidence for a multiphase gas having a limited temperature range. We do not find any evidence that azimuthal temperature variations within each annulus on the sky can account for the range in temperatures within each shell. We provide a detailed investigation of the systematic errors on the derived spectral models considering the effects of calibration, plasma codes, bandwidth, variable NH, and background rate. We find that the RGS gratings and the EPIC and ACIS CCDs give fully consistent results when the same models are fitted over the same energy ranges for each instrument. The cooler component of the 2T model has a temperature (Tc ~ 0.7 keV) similar to the kinetic temperature of the stars. The hot phase has a temperature (Th ~ 1.4 keV) characteristic of the virial temperature of the ~1013 M? halo expected in the NGC 5044 group. However, in view of the morphological disturbances and X-ray holes visible in the Chandra image within R ? 10 kpc, bubbles of gas heated to ~Th in this region may be formed by intermittent AGN feedback. Some additional heating at larger radii may be associated with the evolution of the cold front near R ~ 50 kpc, as suggested by the sharp edge in the EPIC images.

X-Ray Mass Estimates at z ~ 0.3 for the Canadian Network for Observational Cosmology Cluster Sample

Results are presented from the analysis of ROSAT High-Resolution Imager (HRI) and Position-Sensitive Proportional Counter (PSPC) observations of the Canadian Network for Observational Cosmology (CNOC) subsample of the Extended Medium-Sensitivity Survey (EMSS) high-redshift galaxy clusters. X-ray surface brightness profiles of 14 clusters with 0.17 < z < 0.55 are constructed and fit to isothermal ? models. Where possible, we use both the HRI and PSPC data to constrain the fit. Under the assumptions of isothermality, hydrostatic equilibrium, and spherical symmetry, we derive total X-ray masses within a range of radii from 141 to 526 h-1100 kpc. These masses are compared with both the dynamical masses obtained from galaxy velocities and the projected masses from published gravitational lensing studies. We find no systematic bias between X-ray and dynamical methods across the sample, with an average MDyn/MX=1.04 ? 0.07, although individual clusters exhibit mass discrepancies up to a factor of 2. We estimate that the systematic effects due to cooling flows, nonequilibrium systems, and temperature gradients affect the average mass ratio by no more than 15%-20%. Weak gravitational lensing masses appear to be systematically higher than X-ray results by factors of ~50%, while strong-lensing estimates show larger discrepancies (factors of ~2.5). However, these comparisons are complicated by the need to extrapolate the X-ray data to larger or smaller radii. We calculate X-ray-derived cluster gas masses, from which we obtain a cluster baryon fraction of ~5% h-3/2100, yielding ?0~0.3 h-1/2100.Results are presented from the analysis of ROSAT HRI and PSPC observations of the CNOC subsample of the EMSS high redshift galaxy clusters. X-ray surface brightness profiles of 14 clusters with 0.17 < z < 0.55 are constructed and fit to isothermal β models. Where possible, we use both the HRI and PSPC data to constrain the fit. Under the assumptions of isothermality, hydrostatic equilibrium, and spherical symmetry, we derive total X-ray masses within a range of radii from 141 to 526h 100 kpc. These masses are compared with both the dynamical masses obtained from galaxy velocities and the projected masses from published gravitational lensing studies. We find no systematic bias between X-ray and dynamical methods across the sample, with an average MDyn/MX = 1.04 ± 0.07, although individual clusters exhibit mass discrepancies up to a factor of 2. We estimate that the systematic effects due to cooling flows, non-equilibrium systems and temperature gradients affect the average mass ratio by no more than 15− 20%. Weak gravitational lensing masses appear to be systematically higher than X-ray results by factors of ∼ 50%, while strong lensing estimates show larger lewisad@colorado.edu e.elling@casa.colorado.edu Simon.Morris@hia.nrc.ca carlberg@moonray.astro.utoronto.ca

The Dark Matter Radial Profile in the Core of the Relaxed Cluster A2589

We present an analysis of a Chandra ACIS observation of the galaxy cluster A2589 to constrain the radial distribution of the total gravitating matter and the dark matter in the core of the cluster. A2589 is especially well suited for this analysis, because the hot gas in its core region (r 0.1rvir) is undisturbed by interactions with a central radio source. From the largest radius probed (r = 0.07rvir) down to r ≈ 0.02rvir, dark matter dominates the gravitating mass. Over this region, the radial profiles of the gravitating and dark matter are fitted well by the Navarro-Frenk-White (NFW) and Hernquist profiles predicted by cold dark matter (CDM). The density profiles are also described well by power laws, ρ ∝ r-α, where α = 1.37 ± 0.14 for the gravitating matter and α = 1.35 ± 0.21 for the dark matter. These values are consistent with profiles of CDM halos but are significantly larger than the α ≈ 0.5 found in low surface brightness galaxies and expected from self-interacting dark matter models.

THE X-RAY SIZE-TEMPERATURE RELATION FOR INTERMEDIATE REDSHIFT GALAXY CLUSTERS

We present the first measurements of the X-ray size-temperature (ST) relation in intermediate-redshift (z ~ 0.30) galaxy clusters. We interpret the local ST relation (z ~ 0.06) in terms of underlying scaling relations in the cluster dark matter properties, and then we use standard models for the redshift evolution of those dark matter properties to argue that the ST relation does not evolve with redshift. We then use ROSAT HRI observations of 11 clusters to examine the intermediate-redshift ST relation; for currently favored cosmological parameters, the intermediate-redshift ST relation is consistent with that of local clusters. Finally, we use the ST relation and our evolution model to measure angular diameter distances; with these 11 distances we evaluate constraints on ΩM and ΩΛ that are consistent with those derived from studies of Type Ia supernovae. The data rule out a model with ΩM = 1 and ΩΛ = 0 with 2.5 σ confidence. When limited to models where ΩM + ΩΛ = 1, these data are inconsistent with ΩM = 1 with 3 σ confidence.

Circulation Flows: Cooling Flows with Bubble Return

The failure of the XMM-Newton and Chandra X-ray telescopes to detect cooling gas in elliptical galaxies and clusters of galaxies has led many to adopt the position that the gas is not cooling at all and that heating by an active nucleus in the central E or cD galaxy is sufficient to offset radiative cooling. In this paper we explore an idealized limiting example of this point of view in which hot, buoyant bubbles formed near the center return the inflowing, radiatively cooling gas to distant regions in the flow. We show that idealized steady state, centrally heated non-cooling flows can indeed be constructed. In addition, the emission-weighted temperature profiles in these circulating flows resemble those of normal cooling flows. However, these solutions are valid only (1) for a range of bubble parameters for which there is no independent justification, (2) for a limited spatial region in the cooling flow, and (3) for a limited period of time after which cooling seems inevitable. Our exploration of non-cooling flows is set in the context of galaxy/group flows.

Very Isolated Early-Type Galaxies

We use the Karachentseva Catalogue of Very Isolated Galaxies to investigate a candidate list of more than 100 very isolated early-type galaxies. Broadband imaging and low-resolution spectroscopy are available for a large fraction of these candidates and result in a sample of 102 very isolated early-type galaxies, including 65 elliptical (E) and 37 S0 galaxies. Many of these systems are quite luminous, and the resulting optical luminosity functions of the E and early-type (E+S0) galaxies show no statistical differences when compared to luminosity functions dominated by group and cluster galaxies. However, whereas S0 galaxies outnumber E galaxies 4 : 1 in the CfA survey, isolated E outnumber S0 galaxies by nearly 2 : 1. We conclude that very isolated elliptical galaxies show no evidence of a different formation and/or evolution process compared to those formed in groups or clusters, but that most S0 galaxies are formed by a mechanism (e.g., gas stripping) that occurs only in groups and rich clusters. Our luminosity function results for elliptical galaxies are consistent with very isolated elliptical galaxies being formed by merger events, in which no companions remain. Chandra observations were proposed specifically to test the merger hypothesis for isolated elliptical galaxies. However, this program has resulted in the observation of only one isolated early-type galaxy, the S0 KIG 284, which was not detected at a limit well below that expected for a remnant group of galaxies. Therefore, the hypothesis remains untested that very isolated elliptical galaxies are the remains of a compact group of galaxies that have completely merged.

New X-Ray Clusters in the Einstein Extended Medium Sensitivity Survey. II. Optical Properties

We present optical images for 9 new clusters of galaxies we have found in a reanalysis of the Einstein IPC images comprising the Extended Medium Sensitivity Survey (EMSS). Based on the presence of a red sequence of galaxies in a color-magnitude (CM) diagram, a redshift is estimated for each cluster. Galaxy overdensities (cluster richnesses) are measured in each field using the B_gc statistic which allows their plausible identification with the X-ray emission. The nature of our X-ray detection algorithm suggests that most of these clusters have low X-ray surface brightness (LSB) compared to the previously known EMSS clusters. We compare the optical and X-ray observations of these clusters with the well-studied Canadian Network for Observational Cosmology (CNOC) subsample of the EMSS, and conclude that the new clusters exhibit a similar range of optical richnesses, X-ray luminosities, and, somewhat surprisingly, galaxy populations as the predominantly rich, relaxed EMSS/CNOC clusters.We present optical images for nine new clusters of galaxies we have found in a reanalysis of the Einstein IPC images comprising the Extended Medium Sensitivity Survey (EMSS). Based on the presence of a red sequence of galaxies in a color-magnitude (CM) diagram, a redshift is estimated for each cluster. Galaxy overdensities (cluster richnesses) are measured in each field using the Bgc statistic, which allows their plausible identification with the X-ray emission. The nature of our X-ray detection algorithm suggests that most of these clusters have low X-ray surface brightness (LSB) compared to the previously known EMSS clusters. We compare the optical and X-ray observations of these clusters with the well-studied Canadian Network for Observational Cosmology (CNOC) subsample of the EMSS, and conclude that the new clusters exhibit a similar range of optical richnesses, X-ray luminosities, and, somewhat surprisingly, galaxy populations as the predominantly rich, relaxed EMSS/CNOC clusters.