A statistically-selected Chandra sample of 20 galaxy clusters -- I. Temperature and cooling time profiles

We present an analysis of 20 galaxy clusters observed with the Chandra X-ray satellite, focussing on the temperature structure of the intracluster medium and the cooling time of the gas. Our sample is drawn from a flux-limited catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their large angular size compared to the Chandra field-of-view. We describe a quantitative measure of the impact of central cooling, and find that the sample comprises 9 clusters possessing cool cores and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the cool core clusters, which obey a nearly universal radial scaling in temperature of the form T \propto r^~0.4, within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in cool core clusters (t_cool \propto r^~1.3), reaching t_cool <1Gyr in all cases, although surprisingly low central cooling times (<5Gyr) are found in many of the non-cool core systems. The scatter between the cooling time profiles of all the clusters is found to be remarkably small, implying a universal form for the cooling time of gas at a given physical radius in virialized systems, in agreement with recent previous work. Our results favour cluster merging as the primary factor in preventing the formation of cool cores.Comment: 14 pages, 9 figures; accepted for publication in MNRA

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

WMAP constraints on the Intra-Cluster Medium

We devise a Monte-Carlo based, optimized filter match method to extract the thermal Sunyaev-Zel'dovich (SZ) signature of a catalog of 116 low-redshift X-ray clusters from the first year data release of the Wilkinson Microwave Anisotropy Probe (WMAP). We detect an over-all amplitude for the SZ signal at the ~ 8-sigma level, yielding a combined constraint of f_{gas}h = 0.08 +/- 0.01 (ran) +/- 0.01 (sys) on the gas mass fraction of the Intra-Cluster Medium. We also compile X-ray estimated gas fractions from the literature for our sample, and find that they are consistent with the SZ estimates at the 2-sigma level, while both show an increasing trend with X-ray temperature. Nevertheless, our SZ estimated gas fraction is 30-40% smaller than the concordance LCDM cosmic average. We also express our observations in terms of the SZ flux-temperature relation, and compare it with other observations, as well as numerical studies. Based on its spectral and spatial signature, we can also extract the microwave point source signal of the clusters at the 3-sigma level, which puts the average microwave luminosity (at ~ 41 GHz) of bright cluster members (M_K < -21) at (2.4 +/- 0.8) x 10^{27} h^{-2} erg/s/Hz. Furthermore, we can constrain the average dark matter halo concentration parameter to c_{vir}=3.4+0.6-0.9, for clusters with T_x > 5 kev. Our work serves as an example for how correlation of SZ surveys with cluster surveys in other frequencies can significantly increase our physical understanding of the intra-cluster medium.Comment: 34 pages, 6 ps figures, Extended discussion of theoretical uncertainties, radio sources, and future prospects, Accepted for Publication in Ap

X-ray modelling of galaxy cluster gas and mass profiles

We present a parametric analysis of the intracluster medium and gravitating mass distribution of a statistical sample of 20 galaxy clusters using the phenomenological cluster model of Ascasibar and Diego. We describe an effective scheme for the estimation of errors on model parameters and derived quantities using bootstrap resampling. We find that the model provides a good description of the data in all cases and we quantify the mean fractional intrinsic scatter about the best-fit density and temperature profiles, finding this to have median values across the sample of 2 and 5 per cent, respectively. In addition, we demonstrate good agreement between r500 determined directly from the model and that estimated from a core-excluded global spectrum. We compare cool core and non-cool core clusters in terms of the logarithmic slopes of their gas density and temperature profiles and the distribution of model parameters and conclude that the two categories are clearly separable. In particular, we confirm the effectiveness of the logarithmic gradient of the gas density profile measured at 0.04 r500 in differentiating between the two types of cluster.Comment: 8 pages, 7 figures, accepted for publication in MNRA

Bayesian modelling of the cool core galaxy group NGC 4325

We present an X-ray analysis of the radio-quiet cool-core galaxy group NGC 4325 (z=0.026) based on Chandra and ROSAT observations. The Chandra data were analysed using XSPEC deprojection, 2D spectral mapping and forward-fitting with parametric models. Additionally, a Markov chain Monte Carlo method was used to perform a joint Bayesian analysis of the Chandra and ROSAT data. The results of the various analysis methods are compared, particularly those obtained by forward-fitting and deprojection. The spectral mapping reveals the presence of cool gas displaced up to 10 kpc from the group centre. The Chandra X-ray surface brightness shows the group core to be highly disturbed, and indicates the presence of two small X-ray cavities within 15 kpc of the group core. The XSPEC deprojection analysis shows that the group has a particularly steep entropy profile, suggesting that an AGN outburst may be about to occur. With the evidence of prior AGN activity, but with no radio emission currently observed, we suggest that the group in in a pre-outburst state, with the cavities and displaced gas providing evidence of a previous, weak AGN outburst.Comment: 12 pages, 10 figures; accepted for publication in MNRA

Abundance profiles and cool cores in galaxy groups

Using data from the Two Dimensional XMM-Newton Group Survey (2dXGS), we have examined the abundance profile properties of both cool core (CC) and non cool core (NCC) galaxy groups. The ten NCC systems in our sample represent a population which to date has been poorly studied in the group regime. Fitting the abundance profiles as a linear function of log radius, we find steep abundance gradients in cool core (CC) systems, with a slope of -0.54+/-0.07. In contrast, non cool core (NCC) groups have profiles consistent with uniform metallicity. Many CC groups show a central abundance dip or plateau, and we find evidence for anticorrelation between the core abundance gradient and the 1.4 GHz radio power of the brightest group galaxy (BGG) in CC systems. This may indicate the effect of AGN-driven mixing within the central ~0.1r_500. It is not possible to discern whether such behaviour is present in the NCC groups, due to the small and diverse sample with the requisite radio data. The lack of strong abundance gradients in NCC groups, coupled with their lack of cool core, and evidence for enhanced substructure, leads us to favour merging as the mechanism for disrupting cool cores, although we cannot rule out disruption by a major AGN outburst. Given the implied timescales, the disruptive event must have occurred within the past few Gyrs in most NCC groups.Comment: 15 pages, 12 figures, accepted for publication in MNRA

A statistically-selected Chandra sample of 20 galaxy clusters -- II. Gas properties and cool-core/non-cool core bimodality

We investigate the thermodynamic and chemical structure of the intracluster medium (ICM) across a statistical sample of 20 galaxy clusters analysed with the Chandra X-ray satellite. In particular, we focus on the scaling properties of the gas density, metallicity and entropy and the comparison between clusters with and without cool cores (CCs). We find marked differences between the two categories except for the gas metallicity, which declines strongly with radius for all clusters (Z ~ r^{-0.31}), outside ~0.02 r500. The scaling of gas entropy is non-self-similar and we find clear evidence of bimodality in the distribution of logarithmic slopes of the entropy profiles. With only one exception, the steeper sloped entropy profiles are found in CC clusters whereas the flatter slope population are all non-CC clusters. We explore the role of thermal conduction in stabilizing the ICM and conclude that this mechanism alone is sufficient to balance cooling in non-CC clusters. However, CC clusters appear to form a distinct population in which heating from feedback is required in addition to conduction. Under the assumption that non-CC clusters are thermally stabilized by conduction alone, we find the distribution of Spitzer conduction suppression factors, f_c, to be log-normal, with a log (base 10) mean of -1.50+/-0.03 (i.e. f_c=0.032) and log standard deviation 0.39+/-0.02.Comment: 13 pages, 13 figures; accepted for publication in MNRA

The dark haloes of early-type galaxies in low-density environments: XMM-Newton and Chandra observations of NGC 57, NGC 7796 and IC 1531

We present analysis of Chandra and XMM-Newton observations of three early-type galaxies, NGC 57, NGC 7796 and IC 1531. All three are found in very low density environments, and appear to have no neighbours of comparable size. NGC 57 has a halo of kT~0.9 keV, solar metallicity gas, while NGC 7796 and IC 1531 both have ~0.55 keV, 0.5-0.6 Zsol haloes. IC 1531 has a relatively compact halo, and we consider it likely that gas has been removed from the system by the effects of AGN heating. For NGC 57 and NGC 7796 we estimate mass, entropy and cooling time profiles and find that NGC 57 has a fairly massive dark halo with a mass-to-light ratio of 44.7 (4.0,-8.5) Msol/Lsol (1 sigma uncertainties) at 4.75 Re. This is very similar to the mass-to-light ratio found for NGC 4555 and confirms that isolated ellipticals can possess sizable dark matter haloes. We find a significantly lower mass-to-light ratio for NGC 7796, 10.6 (+2.5,-2.3) Msol/Lsol at 5 Re, and discuss the possibility that NGC 7796 hosts a galactic wind, causing us to underestimate its mass.Comment: 14 pages, 9 figures, accepted for publication in MNRA