Daniel S. Hudson

What is a cool-core cluster? a detailed analysis of the cores of the X-ray flux-limited HIFLUGCS cluster sample

We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, some of them new. In order to identify the best parameter for characterizing cool-core clusters and quantify its relation to other parameters, we mainly use very high spatial resolution profiles of central gas density and temperature, and quantities derived from them. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, t cool , is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (t cool 50 h ―1 71 kpc) projected separation between their BCG and X-ray peak. In contrast, only two of the 56 clusters with a small separation between the BCG and X-ray peak (<50 h ―1 71 kpc) show large-scale radio emission. Based on this result, we argue that a large projected separation between the BCG and the X-ray peak is a good indicator of a major merger. The properties of weak cool-core clusters as an intermediate class of objects are discussed. Finally we describe individual properties of all 64 clusters in the sample.

AGN heating and ICM cooling in the HIFLUGCS sample of galaxy clusters

Active galactic nuclei (AGN) at the center of galaxy clusters with gas cooling times that are much shorter than the Hubble time have emerged as heating agents powerful enough to prevent further cooling of the intracluster medium (ICM). We carried out an intensive study of the AGN heating−ICM cooling network by comparing various cluster parameters to the integrated radio luminosity of the central AGN, LR, defined as the total synchrotron power between 10 MHz and 15 G Hz. This study is based on the HIFLUGCSsample comprising the 64 X-ray brightest galaxy clusters. We adopted the central cooling time, tcool, as the diagnostic to ascertain cooling properties of the HIFLUGCSsample and classify clusters with tcool 7.7 Gyr as non-cool-core (NCC) clusters. We find 48 out of 64 clusters (7 5%) contain cluster center radio sources (CCRS) cospatial with or within 50 h −1 71 kpc of the X-ray peak emission. Furthermore, we find that the p robability of finding a CCRS increases from 45% to 67% to 100% for NCC, WCC, and SCC clusters, respectively. We use a total of∼ 140 independent radio flux-density measurements, with data at more than two frequencies for more than 54% of the sources extending below 500 MHz, enabling the determination of accurate estimates of LR. We find that LR in SCC clusters depends strongly on the cluster scale such that more massive clusters harbor more powerful radio AGN. The same trend is observed between LR and the classical mass deposition rate, ˙ Mclassical in SCC and partly also in WCC clusters, and can be quantified as LR∝ ˙ M 1.69±0.25 classical . We also perform correlations of the luminosity for the brightest cluster galaxy, LBCG, close to the X-ray peak in all 64 clusters with LR and cluster parameters, such as the virial mass, M500, and the bolometric X-ray luminosity, LX. To this end, we use the 2MASS K-band magnitudes and invoke the near-infrared bulge luminosity-black hole mass relation to convert LBCG to supermassive black hole mass, MBH. We find a weak correlation between MBH and LR for SCC clusters, LR∼ M 4.10±0.42 BH , although with a few outliers. We find an excellent correlati on of LBCG with M500 and LX for the entire sample, the SCC clusters showing a tighter trend in both the cases. We discuss the plausible reasons behind these scaling relations in the context of cooling flows and AGN feedback. Our results strongly suggest an AGN-feedback machinery in SCC clusters, which regulates the cooling in the central regions. Since the dispersion in these correlations, such a s that between LR and ˙ Mclassical or LR and MBH, increases in going from SCC to WCC clusters, we conclude there must be secondary processes that work either in conjunction with the AGN heating or independently to counteract the radiative losses in WCC clusters.

Testing the low-mass end of X-ray scaling relations with a sample of Chandra galaxy groups

Context. Well-determined scaling relations between X-ray observab les and cluster mass are essential for using large cluster sa mples to constrain fundamental cosmological parameters. Scalin g relations between cluster masses and observables, such as the luminositytemperature, mass-temperature, luminosity-mass relatio ns, have been investigated extensively, however the questi on of whether these relations hold true also for poor clusters and groups remain s unsettled. Some evidence supports a ‘‘break’’ at the low en d of the group/cluster mass range, possibly caused by the stronger influen ce of non-gravitational physics on low-mass systems. Aims. The main goal of this work is to test local scaling relations f or the low-mass range in order to check whether or not there is a systematic difference between clusters and groups, and to thereby extend this method of reliable and convenient clu ster mass determination for future large samples down to the group reg im . Methods. We compiled a statistically complete sample of 112 X-ray gal axy groups, 26 of which have usable Chandra data. Temperature, metallicity, and surface brightness profile s w re created for these 26 groups, and used to determine the m ain physical quantities and scaling relations. We then compared the g roup properties to those of the HIFLUGCS clusters, as well as several other group and cluster samples. Results. We present radial profiles for the individual objects and sc aling relations of the whole sample ( Lx-T , M-T , Lx-M, Mg-M, M-Yx, Lx-Yx, fg-T ). Temperature and metallicity profiles behave universall y, except for the core regions. The Lx-T , M-T , Lx-M, Mg-M, M-Yx, andLx-Yx relations of the group sample are generally in good agreemen t with clusters. TheLx-T relation steepens for T < 3 keV, which could point to a larger impact of heating mechani sms on cooler systems. We found a significant drop in the gas m as fraction below. 1 keV, as well as a correlation with radius, which indicates t he ICM is less dominant in groups compared to clusters and the galaxies have a stronger influence on the global prop erties of the system. In all relations the intrinsic scatter for groups is larger than for clusters, which appears not to be correlated with merger activity but could be due to scatter caused by bar onic physics in the group cores. We also demonstrate the importance of sel ection effects. Conclusions. We have found some evidence for a similarity break between gr oups and clusters. However this does not have a strong effect on the scaling relations.

X-ray substructure studies of four galaxy clusters using XMM-Newton data

Mahdavi et al. find that the degree of agreement between weak lensing and X-ray mass measurements is a function of cluster radius. Numerical simulations also point out that X-ray mass proxies do not work equally well at all radii. The origin of the effect is thought to be associated with cluster mergers. Recent work presenting the cluster maps showed an ability of X-ray maps to reveal and study cluster mergers in detail. Here, we present a first attempt to use the study of substructure in assessing the systematics of the hydrostatic mass measurements using two-dimensional (2D) X-ray diagnostics. The temperature map is uniquely able to identify the substructure in an almost relaxed cluster which would be unnoticed in the intracluster medium electron number density and pressure maps. We describe the radial fluctuations in the 2D maps by a cumulative/differential scatter profile relative to the mean profile within/at a given radius. The amplitude indicates ~10% fluctuations in the temperature, electron number density, and entropy maps, and ~15% fluctuations in the pressure map. The amplitude of and the discontinuity in the scatter complement 2D substructure diagnostics, e.g., indicating the most disturbed radial range. There is a tantalizing link between the substructure identified using the scatter of the entropy and pressure fluctuations and the hydrostatic mass bias relative to the expected mass based on the M-Y X and M-M gas relations particularly at r 500. XMM-Newton observations with ~120,000 source photons from the cluster are sufficient to apply our substructure diagnostics via the spectrally measured 2D temperature, electron number density, entropy, and pressure maps.

Suzaku Observation of Abell 2204: Galaxy Cluster Gas Temperature Measurement Up to the Virial Radius

Measurements of the intracluster gas temperatures out to large radii, where much of the cluster mass resides, are of the utmost importance for the use of clusters in precision cosmology and for studies of cluster physics. Previous attempts to measure temperatures at the cluster virial radius have failed. The preliminary results from the Suzaku observation of Abell 2204 reported here show that such measurements appear feasible now for the first time, if care is taken to account for background and PSF effects.

Scrutinizing the AGN Heating Mechanism in Galaxy Clusters

We carried out an intensive study of the AGN heating‐intracluster medium (ICM) cooling network by comparing various cluster parameters to the integrated radio luminosity of the central AGN, LR. This study is based on the HIFLUGCS sample comprising the 64 X‐ray brightest galaxy clusters. Based on the central cooling time, tcool, we classify clusters with tcool 7.7 Gyr as non‐cool‐core (NCC) clusters. We find 48 out of 64 clusters contain cluster center radio sources (CCRS). Furthermore, we find that the probability of finding a CCRS increases from 45% to 67% to 100% for NCC, WCC, and SCC clusters, respectively.We find that LR in SCC clusters depends strongly on the cluster scale such that more massive clusters harbor more powerful radio AGN. The same trend is observed between LR and the classical mass deposition rate, Ṁclassical. We also perform correlations of the luminosity for the brightest...

Studying the Nature of Dark Energy with Galaxy Clusters

We report on the status of our effort to constrain the nature of dark energy through the evolution of the cluster mass function. Chandra temperature profiles for 31 clusters from a local cluster sample are shown. The X-ray appearance of the proto supermassive binary black hole at the center of the cluster Abell 400 is described. Preliminary weak lensing results obtained with Megacam@MMT for a redshift z = 0.5 cluster from a distant cluster sample are given.

Complex Physics in Cluster Cores: Showstopper for the Use of Clusters for Cosmology?

The influence of cool galaxy cluster cores on the X-ray luminosity--gravitational mass relation is studied with Chandra observations of 64 clusters in the HIFLUGCS sample. As preliminary results we find (i) a significant offset of cool core (CC) clusters to the high luminosity (or low mass) side compared to non-cool core (NCC) clusters, (ii) a smaller scatter of CC clusters compared to NCC clusters, (iii) a decreasing fraction of CC clusters with increasing cluster mass, (iv) a reduced scatter in the luminosity--mass relation for the entire sample if the luminosity is scaled properly with the central entropy. The implications of these results on the intrinsic scatter are discussed.