Using the Fundamental Plane to Estimate the Total Binding Mass in A2626

We use fundamental plane (FP) distance estimates to the components of the double cluster A2626 (cz~17,500 km/s) to constrain cluster kinematics and estimate total binding mass. The FP coefficients for a sample of 24 early type and S0 cluster members (alpha=1.30+/-0.36 and beta=0.31+/-0.06) are consistent with others reported in the literature. We examine the Mg_b distributions within both subclusters and find them to be indistinguishable. Lacking evidence for stellar population differences, we interpret the FP zeropoint offset (\log(D_B/D_A)=-0.037+/-0.046, where D_{cl} is distance to subcluster cl) as a measure of the distance difference. This measurement is consistent with the subclusters being at the same distance, and it rules out the Hubble flow hypothesis (distances proportional to velocity) with 99% confidence; analysis of the subcluster galaxy magnitude distributions rules out Hubble flow at 93% confidence. Both results favor a kinematic model where the subclusters are bound and infalling. We estimate the total cluster binding mass by modelling the subcluster merger as radial infall. The minimum possible total binding mass is 1.65 times higher than the sum of the standard virial masses, a difference statistically significant at the ~3sigma level. We discuss explanations for the inconsistency including (1) biases in the standard virial mass estimator, (2) biases in our radial infall mass estimate, and (3) mass beyond the virialized cluster region; if the standard virial mass is significantly in error, the cluster has an unusually high mass to light ratio (~1000h). Because observational signatures of departures from radial infall are absent, we explore the implications of mass beyond the virialized, core regions. (abridged)Comment: 14 pages and 5 figures, Latex, Accepted for publication in A

A Cold Front in a Preheated Galaxy Cluster

We present a simulated cluster of galaxies, modeled with a pre-heated intracluster medium, that exhibits X-ray features similar to the `cold fronts' seen in Chandra observations. Mock observations at a particular epoch show factor two discontinuities in X-ray temperature and factor four in surface brightness on a spatial scale <= 100 kpc. Analysis of the cluster's dynamical history reveals that the front is a transient contact discontinuity created by an ongoing merger of two roughly equal mass subgroups. The cold front feature in this realization is amplified by the adiabatic expansion of one of the subgroups following its ablation from the center of its local dark matter potential. The presence of cold front features in a cluster modeled without radiative cooling or magnetic fields implies that such relatively complex physics is not a necessary element of the phenomenon and suggests that the prevalence of such features in high resolution X-ray images of clusters may simply reflect the high frequency of ongoing mergers driven by gravity and comparatively simple hydrodynamics.Comment: 4 pages, 4 figures, submitted to ApJ

Possible AGN Shock Heating in the Cool Core Galaxy Cluster Abell 478

We present a detailed X-ray study of the intracluster medium (ICM) of the nearby, cool-core galaxy cluster Abell 478, with Chandra and XMM observations. Using a wavelet smoothing hardness analysis, we derive detailed temperature maps of A478, revealing a surprising amount of temperature structure. The broad band Chandra spectral fits yield temperatures which are significantly hotter than those from XMM, but the Fe ionization temperature shows good agreement. We show that the temperature discrepancy is slightly reduced when comparing spectra from regions selected to enclose nearly isothermal gas. However, by simulating multi-temperature spectra and fitting them with a single temperature model, we find no significant difference between Chandra and XMM, indicating that non-isothermality cannot fully explain the discrepancy. We have discovered 4 hot spots located between 30--50 kpc from the cluster center, where the gas temperature is roughly a factor of 2 higher than in the surrounding material. We estimate the combined excess thermal energy present in these hot spots to be (3+/-1)x10^59 erg. The location of and amount of excess energy present in the hot spots are suggestive of a common origin within the cluster core, which hosts an active galactic nucleus. This cluster also possesses a pair of X-ray cavities coincident with weak radio lobes, as reported in a previous analysis, with an associated energy <10% of the thermal excess in the hot spots. The presence of these hot spots could indicate strong-shock heating of the ICM from the central radio source -- one of the first such detections in a cool core cluster. We also probe the mass distribution in the core and find it to be characterized by a logarithmic slope of -0.35+/-0.22, which is significantly flatter than an NFW cusp of -1. (abridged)Comment: 15 pages, 15 figures; extra section on simulating effect of multiphase gas, plus some restructuring of discussion section. Accepted by ApJ; corrected typo in equation

On Galaxy-Cluster Sizes and Temperatures

We show that the distribution of the sizes and temperatures of clusters can be used to constrain cosmological models. The size-temperature (ST) distribution predicted in a flat Gaussian cluster-abundance-normalized Omega_0=0.3 model agrees well with the fairly tight ST relation observed. A larger power-spectrum amplitude sigma_8 would give rise to a larger scatter about the ST relation as would a larger value of Omega_0 and/or long non-Gaussian high-density tails in the probability density function. For Gaussian initial conditions, the ST distribution suggests a constraint sigma_8 Omega_0^{0.26} \simeq 0.76. The ST relation is expected to get tighter at high redshifts. In the process, we derive a simple formula for the halo formation-redshift distribution for non-Gaussian models. We also suggest that the discrepancy between the naive zero-redshift ST relation and that observed may be due, at least in part, to the fact that lower-mass clusters form over a wider range of redshifts. An Appendix derives an equation for the formation-redshift distribution of halos.Comment: 6 pages, 7 figures; replacement to match the published versio

A Chandra Study of the Effects of a Major Merger on the Structure of Abell 2319

We present an analysis of a Chandra observation of the massive, nearby galaxy cluster Abell 2319. A sharp surface brightness discontinuity--suggested by previous, lower angular resolution X-ray imaging--is clearly visible in the ACIS image. This roughly 300kpc feature suggests that a major merger is taking place with a significant velocity component perpendicular to the line of sight. The cluster emission-weighted mean temperature is 11.8+/-0.6kev, somewhat higher than previous temperature measurements. The Chandra temperature map of A2319 reveals substructure resembling that anticipated based on hydrodynamic simulations of cluster mergers. The merger feature shows a pressure change across the surface brightness discontinuity by a factor of <=2.5. The higher density side of the front has a lower temperature, suggesting the presence of a cold front similar to those in many other merging clusters. The velocity of the front is roughly sonic. We compare bulk properties of the ICM and galaxies in A2319 to the same properties in a large sample of clusters as a way of gauging the effects of the major merger. Interestingly, by comparing A2319 to a sample of 44 clusters studied with the ROSAT PSPC we find that the X-ray luminosity, isophotal size, and ICM mass are consistent with the expected values for a cluster of its temperature; in addition, the K-band galaxy light is consistent with the light--temperature scaling relation derived from a sample of about 100 clusters studied with 2MASS. Together, these results indicate either that the merger in A2319 has not been effective at altering the bulk properties of the cluster, or that there are large but correlated displacements in these quantities.Comment: 11 pages, 8 figures, ApJ Submitte