Interactions of radio galaxies and the intracluster medium in Abell 160 and Abell 2462

We present Chandra and Very Large Array observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and we test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores (∼30–40 kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C 465 and Hydra A. However, none of the published models of WAT formation offers a satisfactory explanation as to why this may be the case. Therefore, it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process.     

The X-ray properties of optically selected z > 0.6 clusters in the European Southern Observatory Distant Cluster Survey

We present XMM–Newton observations of three optically selected   z > 0.6  clusters from the European Southern Observatory (ESO) Distant Cluster Survey (EDisCS), comprising the first results of a planned X-ray survey of the full EDisCS high-redshift sample. The EDisCS clusters were identified in the Las Campanas Distant Cluster Survey as surface brightness fluctuations in the optical sky and their masses and galaxy populations are well described by extensive photometric and spectroscopic observations. We detect two of the three clusters in the X-ray and place a firm upper limit on diffuse emission in the third cluster field. We are able to constrain the X-ray luminosity and temperature of the detected clusters and estimate their masses. We find that the X-ray properties of the detected EDisCS clusters are similar to those of X-ray-selected clusters of comparable mass and – unlike other high-redshift, optically selected clusters – are consistent with the T –σ and   L _X–σ  relations determined from X-ray-selected clusters at low redshift. The X-ray determined mass estimates are generally consistent with those derived from weak-lensing and spectroscopic analyses. These preliminary results suggest that the novel method of optical selection used to construct the EDisCS catalogue may, like selection by X-ray luminosity, be well suited for identification of relaxed, high-redshift clusters whose intracluster medium is in place and stable by   z ∼ 0.8  .     

Galaxy cluster masses without non-baryonic dark matter

We apply the modified acceleration law obtained from Einstein gravity coupled to a massive skew symmetric field,   F _μνλ  , to the problem of explaining X-ray galaxy cluster masses without exotic dark matter. Utilizing X-ray observations to fit the gas mass profile and temperature profile of the hot intracluster medium (ICM) with King 'β-models', we show that the dynamical masses of the galaxy clusters resulting from our modified acceleration law fit the cluster gas masses for our sample of 106 clusters without the need of introducing a non-baryonic dark matter component. We are further able to show for our sample of 106 clusters that the distribution of gas in the ICM as a function of radial distance is well fitted by the dynamical mass distribution arising from our modified acceleration law without any additional dark matter component. In a previous work, we applied this theory to galaxy rotation curves and demonstrated good fits to our sample of 101 low surface brightness, high surface brightness and dwarf galaxies including 58 galaxies that were fitted photometrically with the single-parameter mass-to-light ratio ( M / L )_stars. The results obtained there were qualitatively similar to those obtained using Milgrom's phenomenological Modified Newtonian Dynamics (MOND) model, although the determined galaxy masses were quantitatively different, and MOND does not show a return to Keplerian behaviour at extragalactic distances. The results obtained here are compared to those obtained using Milgrom's phenomenological MOND model which does not fit the X-ray galaxy cluster masses unless an auxiliary dark matter component is included.     

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, focusing 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 nine clusters possessing cool cores (CCs) and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the CC clusters, which obey a nearly universal radial scaling in temperature of the form   T ∝ r ^∼0.4  , within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in CC clusters  ( t _cool∝ r ^∼1.3)  , reaching   t _cool < 1 Gyr  in all cases, although surprisingly low central cooling times (<5 Gyr) are found in many of the non-CC 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 CCs.     

Evidence of a link between the evolution of clusters and their AGN fraction

We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at   z ∼ 1  in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ∼200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K -band luminosities of   L_K = 1.7 ± 0.7 L *  for the X-ray sources and   L_K = 2.3 ± 0.1 L *  for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage.     

The 6.4-keV fluorescent iron line from cluster cooling flows

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τ_T. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 10²³ cm⁻². Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.     

Cosmological constraints from Chandra X-ray observations of galaxy clusters

Chandra X-ray observations of rich, dynamically relaxed galaxy clusters allow the properties of the X-ray gas and the total gravitating mass to be determined precisely. Here, we discuss how Chandra observations may be used as a powerful tool for cosmological studies. By combining Chandra X-ray results on the X-ray gas mass fractions in clusters with independent measurements of the Hubble constant and the mean baryonic matter density of the universe, we obtain a tight constraint on the mean total matter density of the universe, Ο_m, and an interesting constraint on the cosmological constant, Ο_Λ. Using these results, together with the observed local X-ray luminosity function of the most X-ray luminous galaxy clusters, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we obtain a precise constraint on the normalization of the power spectrum of density fluctuations in the nearby universe,σ₈. We compare our results with those obtained from other, independent methods. This revised version was published online in July 2006 with corrections to the Cover Date.     

The optical/X-ray connection: intra-cluster medium iron content and galaxy optical luminosity in 20 galaxy clusters

X-ray observations of galaxy clusters have shown that the intra-cluster gas has iron abundances of about one-third of the solar value. These observations also show that part (if not all) of the intra-cluster gas metals was produced within the member galaxies. We present a systematic analysis of 20 galaxy clusters to explore the connection between the iron mass and the total luminosity of early- and late-type galaxies, and of the brightest cluster galaxies (BCGs). From our results, the intra-cluster medium (ICM) iron mass seems to correlate better with the luminosity of the BCGs than with that of the red and blue galaxy populations. As the BCGs cannot produce alone the observed amount of iron, we suggest that ram-pressure plus tidal stripping acts together to enhance, at the same time, the BCG luminosities and the iron mass in the ICM. Through the analysis of the iron yield, we have also estimated that SN Ia are responsible for more than 50 per cent of the total iron in the ICM. This result corroborates the fact that ram-pressure contributes to the gas removal from galaxies to the ICM, being very efficient for clusters in the temperature range  2 < kT (keV) < 10  .     

Note on a polytropic β-model to fit the X-ray surface brightness of clusters of galaxies

I suggest that the β -model used to fit the X-ray surface brightness profiles of extended sources, like groups and clusters of galaxies, has to be corrected when the counts are collected in a wide energy band comparable to the mean temperature of the source, and a significant gradient in the gas temperature is observed. I present a revised version of the β -model for the X-ray brightness that applies to an intracluster gas with temperature and density related by a polytropic equation and extends the standard version that is strictly valid for an isothermal gas. Given a temperature gradient observed through an energy window of 1–10 keV typical for the new generation of X-ray observatories, the β parameter can change systematically by up to 20 per cent from the value obtained under isothermal assumption, i.e. by an amount larger than any statistical uncertainty obtained from the present data. Within the virial regions of typical clusters of galaxies, these systematic corrections affect the total gravitating mass estimate by 5–10 per cent, the gas mass by 10–30 per cent and the gas fraction value up to 50 per cent, when compared with the measurements obtained under the isothermal assumption.     

X-ray colour maps of the cores of galaxy clusters

We present an analysis of X-ray colour maps of the cores of clusters of galaxies, formed from the ratios of counts in different X-ray bands. Our technique groups pixels lying between contours in an adaptively smoothed image of a cluster. We select the contour levels to minimize the uncertainties in the colour ratios, whilst preserving the structure of the object. We extend the work of Allen & Fabian by investigating the spatial distributions of cooling gas and absorbing material in cluster cores. Their sample is almost doubled: we analyse archive ROSAT Position Sensitive Proportional Counter (PSPC) data for 33 clusters from the sample of the 55 brightest X-ray clusters in the sky. Many of our clusters contain strong cooling flows. We present colour maps of a sample of the clusters, in addition to adaptively smoothed images in different bands. Most of the cooling flow clusters display little substructure, unlike several of the non-cooling-flow clusters.
We fitted an isothermal plasma model with galactic absorption and constant metallicity to the mid-over-high energy colours in our clusters. Those clusters with known strong cooling flows have inner contours which fit a significantly lower temperature than the outer contours. Clusters in the sample without strong cooling flows show no significant temperature variation. The inclusion of a metallicity gradient alone was not sufficient to explain the observations. A cooling flow component plus a constant temperature phase did account for the colour profiles in clusters with known strong cooling flow components. We also had to increase the levels of absorbing material to fit the low-over-high colours at the cluster centres. Our results provide more evidence that cooling flows accumulate absorbing material. No evidence for increased absorption was found for the non-cooling-flow clusters.     

An SZ temperature decrement–X-ray luminosity relation for galaxy clusters

We present the observed relation between Δ T _SZ, the cosmic microwave background (CMB) temperature decrement due to the Sunyaev–Zeldovich (SZ) effect, and L , the X-ray luminosity of galaxy clusters. We discuss this relation in terms of the cluster properties, and show that the slope of the observed Δ T _SZ– L relation is in agreement with both the L – T _e relation based on numerical simulations and X-ray emission observations, and the M _gas– L relation based on observation. The slope of the Δ T _SZ– L relation is also consistent with the M _tot– L relation, where M _tot is the cluster total mass based on gravitational lensing observations. This agreement may be taken to imply a constant gas mass fraction within galaxy clusters, however, there are large uncertainties, dominated by observational errors, associated with these relations. Using the Δ T _SZ– L relation and the cluster X-ray luminosity function, we evaluate the local cluster contribution to arcmin-scale cosmic microwave background anisotropies. The Compton distortion y -parameter produced by galaxy clusters through the SZ effect is roughly two orders of magnitude lower than the current upper limit based on FIRAS observations.     

BeppoSAX observations of three distant, highly luminous clusters of galaxies: RXJ1347−1145, Zwicky 3146 and Abell 2390

We present an analysis of BeppoSAX observations of three clusters of galaxies that are amongst the most luminous in the Universe: RXJ1347−1145, Zwicky 3146 and Abell 2390. Using data from both the Low Energy (LECS) and Medium Energy (MECS) Concentrator Spectrometers, and a joint analysis with the Phoswich Detection System (PDS) data above 10 keV, we constrain, with a relative uncertainty of between 7 and 42 per cent (90 per cent confidence level), the mean gas temperature in the three clusters. These measurements are checked against any possible non-thermal contribution to the plasma emission and are shown to be robust.
We confirm that RXJ1347−1145 has a gas temperature that lies in the range between 13.2 and 22.3 keV at the 90 per cent confidence level, and is larger than 12.1 keV at 3 σ level. The existence of such a hot galaxy cluster at redshift of about 0.45 implies an upper limit on the mean mass density in the Universe, Ω_m, of 0.5.
Combining the BeppoSAX estimates for gas temperature and luminosity of the three clusters presented in this work with ASCA measurements available in the literature, we obtain a slope of 2.7 in the L – T relation once the physical properties are corrected from the contamination from the central cooling flows.     

The distribution of active galactic nuclei in a large sample of galaxy clusters

We present an analysis of the X-ray point source populations in 182 Chandra images of galaxy clusters at   z > 0.1  with exposure time >10 ks, as well as 44 non-cluster fields. The analysis of the number and flux of these sources, using a detailed pipeline to predict the distribution of non-cluster sources in each field, reveals an excess of X-ray point sources associated with the galaxy clusters. A sample of 148 galaxy clusters at  0.1 < z < 0.9  , with no other nearby clusters, shows an excess of 230 cluster sources in total, an average of ∼1.5 sources per cluster. The lack of optical data for these clusters limits the physical interpretation of this result, as we cannot calculate the fraction of cluster galaxies hosting X-ray sources. However, the fluxes of the excess sources indicate that over half of them are very likely to be active galactic nuclei (AGN), and the radial distribution shows that they are quite evenly distributed over the central 1 Mpc of the cluster, with almost no sources found beyond this radius. We also use this pipeline to successfully reproduce the results of previous studies, particularly the higher density of sources in the central 0.5 Mpc of a few cluster fields, but show that these conclusions are not generally valid for this larger sample of clusters. We conclude that some of these differences may be due to the sample properties, such as the size and redshift of the clusters studied, or a lack of publications for cluster fields with no excess sources. This paper also presents the basic X-ray properties of the galaxy clusters, and in subsequent papers in this series the dependence of the AGN population on these cluster properties will be evaluated.
In addition the properties of over 9500 X-ray point sources in the fields of galaxy clusters are tabulated in a separate catalogue available online or at http://www.sc.eso.org~rgilmour .     

Towards a holistic view of the heating and cooling of the intracluster medium

X-ray clusters are conventionally divided into two classes: 'cool core' (CC) clusters and 'non-cool core' (NCC) clusters. Yet relatively little attention has been given to the origins of this apparent dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either CC or NCC clusters. The inferred entropy profiles of NCC clusters require that material is 'pre-heated' prior to cluster collapse in order to explain the absence of low-entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their entropy profiles at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent active galactic nuclei (AGN) feedback loop in which the lowest entropy, most rapidly cooling material is heated and rises buoyantly to mix with material at larger radii. The resulting model does not require fine-tuning and is in excellent agreement with a wide variety of observational data from Chandra and XMM–Newton , including entropy and gas density profiles, the luminosity–temperature relation and high-resolution spectra. The spread in cluster core morphologies is seen to arise because of the steep dependence of the central cooling time on the initial level of pre-heating. Some of the other implications of this model are briefly discussed.     

V407 Vul: a direct impact accretor

Chandra and XMM-Newton observations have confirmed the presence of large temperature gradients within the cores of many relaxed clusters of galaxies. Here we investigate whether thermal conduction operating over those gradients can supply sufficient heat to offset radiative cooling. Narayan & Medredev and Gruzinov have noted, using published results on cluster temperatures, that conduction within a factor of a few of the Spitzer rate is sufficient to balance bremsstrahlung cooling. From a detailed study of the temperature and emission measure profiles of Abell 2199 and Abell 1835, we find that the heat flux required by conduction is consistent with or below the rate predicted by Spitzer in the outer regions of the core. Conduction may therefore explain the lack of observational evidence for large mass cooling rates inferred from arguments based simply on radiative cooling, provided that conductivity is suppressed by no more than a factor of 3 below the full Spitzer rate. To stem cooling in the central 20 kpc however, would necessitate conductivity values of at least a factor of 2 larger than the Spitzer values, which we consider implausible. This may provide an explanation for the observed star formation and optical nebulosities in cluster cores. The solution is likely to be time-dependent. We briefly discuss the possible origin of the cooler gas and the implications for massive galaxies.     

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r ₂₅₀₀, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe,     , and measure a positive cosmological constant,     . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.     

Mass and gas profiles in A1689: joint X-ray and lensing analysis

We carry out a comprehensive joint analysis of high-quality HST /ACS and Chandra measurements of A1689, from which we derive mass, temperature, X-ray emission and abundance profiles. The X-ray emission is smooth and symmetric, and the lensing mass is centrally concentrated indicating a relaxed cluster. Assuming hydrostatic equilibrium we deduce a 3D mass profile that agrees simultaneously with both the lensing and X-ray measurements. However, the projected temperature profile predicted with this 3D mass profile exceeds the observed temperature by ∼30 per cent at all radii, a level of discrepancy comparable to the level found for other relaxed clusters. This result may support recent suggestions from hydrodynamical simulations that denser, more X-ray luminous small-scale structure can bias observed temperature measurements downward at about the same (∼30 per cent) level. We determine the gas entropy at  0.1 r _vir  (where r _vir is the virial radius) to be ∼800 keV cm², as expected for a high-temperature cluster, but its profile at  >0.1 r _vir  has a power-law form with index ∼0.8, considerably shallower than the ∼1.1 index advocated by theoretical studies and simulations. Moreover, if a constant entropy 'floor' exists at all, then it is within a small region in the inner core,   r < 0.02 r _vir  , in accord with previous theoretical studies of massive clusters.