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We examine the performance of a variety of different estimators for the mass of galaxy groups, based on their galaxy distribution alone. We draw galaxies from the Sloan Digital Sky Survey for a set of groups and clusters for which hydrostatic mass estimates based on high-quality Chandra X-ray data are available. These are used to calibrate the galaxy-based mass proxies, and to test their performance. Richness, luminosity, galaxy overdensity, rms radius and dynamical mass proxies are all explored. These different mass indicators all have their merits, and we argue that using them in combination can provide protection against being misled by the effects of dynamical disturbance or variations in star formation efficiency. Using them in this way leads us to infer the presence of significant non-statistical scatter in the X-ray based mass estimates we employ. We apply a similar analysis to a set of mock groups derived from applying a semi-analytic galaxy formation code to the Millennium dark matter simulation. The relations between halo mass and the mass proxies differ significantly in some cases from those seen in the observational groups, and we discuss possible reasons for this. [ABSTRACT FROM AUTHOR]

This paper is the second in a series in which we perform an extensive comparison of various galaxy-based cluster mass estimation techniques that utilize the positions, velocities and colours of galaxies. Our aim is to quantify the scatter, systematic bias and completeness of cluster masses derived from a diverse set of 25 galaxy-based methods using two contrasting mock galaxy catalogues based on a sophisticated halo occupation model and a semi-analytic model. Analysing 968 clusters, we find a wide range in the rms errors in log M200c delivered by the different methods (0.18-1.08 dex, i.e. a factor of ~1.5-12), with abundance-matching and richness methods providing the best results, irrespective of the input model assumptions. In addition, certain methods produce a significant number of catastrophic cases where the mass is under- or overestimated by a factor greater than 10. Given the steeply falling high-mass end of the cluster mass function, we recommend that richness- or abundance-matching-based methods are used in conjunction with these methods as a sanity check for studies selecting high-mass clusters. We see a stronger correlation of the recovered to input number of galaxies for both catalogues in comparison with the group/cluster mass, however, this does not guarantee that the correct member galaxies are being selected. We do not observe significantly higher scatter for either mock galaxy catalogues. Our results have implications for cosmological analyses that utilize the masses, richnesses, or abundances of clusters, which have different uncertainties when different methods are used. [ABSTRACT FROM AUTHOR]

We report the results of a study which assembles deep observations with the ACIS-I instrument on the Chandra observatory to study the evolution in the core properties of a sample of galaxy groups and clusters out to redshifts z ≈ 1.3. A search for extended objects within these fields yields a total of 62 systems for which redshifts are available, and we added a further 24 non-X-ray-selected clusters, to investigate the impact of selection effects and improve our statistics at high redshift. Six different estimators of cool core strength are applied to these data: the entropy (K) and cooling time (tcool) within the cluster core, the cooling time as a fraction of the age of the Universe (tcool/tUni) and three estimators based on the cuspiness of the X-ray surface brightness profile. A variety of statistical tests are used to quantify evolutionary trends in these cool core indicators. In agreement with some previous studies, we find that there is significant evolution in tcool/tUni, but little evolution in tcool, suggesting that gas is accumulating within the core, but that the cooling time deep in the core is controlled by active galactic nucleus (AGN) feedback. We show that this result extends down to the group regime and appears to be robust against a variety of selection biases (detection bias, archival biases and biases due to the presence of central X-ray AGN) which we consider. [ABSTRACT FROM AUTHOR]

We study intergalactic medium (IGM) heating in a sample of five fossil galaxy groups by using their radio properties at 610 MHz and 1.4 GHz. The power by radio jets introducing mechanical heating for the sampled objects is not sufficient enough to suppress the cooling flow. Therefore, we discussed shock-, vortex heating, and conduction as alternative heating processes. Further, the 1.4 GHz and 610 MHz radio luminosities of fossil groups are compared to a sample of normal galaxy groups of the same brightest group galaxies (BGGs), stellar mass, and total group stellar mass, quantified using the K-band luminosity. It appears that the fossil BGGs are under luminous at 1.4 GHz and 610 MHz for a given BGG stellar mass and luminosity, in comparison to a general population of the groups. In addition, we explore how the bolometric radio luminosity of fossil sample depends on clusters and groups characteristics. Using the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) as a control sample we found that the large-scale behaviours of fossil galaxy groups are consistent with their relaxed and virialized nature. [ABSTRACT FROM PUBLISHER]

Galaxy groups are not scaled down versions of massive galaxy clusters - the hot gas in groups [known as the intragroup medium (IGrM)] is, on average, less dense than the intracluster medium, implying that one or more non-gravitational processes (e.g. radiative cooling, star formation and/or feedback) has had a relatively larger effect on groups. In the present study, we compare a number of cosmological hydrodynamic simulations that form part of the OverWhelmingly Large Simulations project to isolate and quantify the effects of cooling and feedback from supernovae (SNe) and active galactic nuclei (AGN) on the gas. This is achieved by comparing Lagrangian thermal histories of the gas in the different runs, which were all started from identical initial conditions. While radiative cooling, star formation and SN feedback are all necessary ingredients, only runs that also include AGN feedback are able to successfully reproduce the optical and X-ray properties of groups and low-mass clusters. We isolate how, when and exactly what gas is heated by AGN. Interestingly, we find that the gas that constitutes the present-day IGrM is that which was not strongly heated by AGN. Instead, the low median density/high median entropy of the gas in present-day groups is achieved by the ejection of lower entropy gas from low-mass progenitor galaxies at high redshift (primarily ). This corresponds to the epoch when supermassive black holes accreted most of their mass, typically at a rate that is close to the Eddington limit (i.e. when the black holes are in a 'quasar mode'). [ABSTRACT FROM AUTHOR]

We study the luminosity gap, , between the first- and second-ranked galaxies in a sample of 59 massive () galaxy clusters, using data from the Hale Telescope, the Hubble Space Telescope, Chandra and Spitzer. We find that the distribution, ), is a declining function of to which we fitted a straight line: . The fraction of clusters with 'large' luminosity gaps is , which represents a excess over that obtained from Monte Carlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with 'extreme' luminosity gaps, , giving a fraction of . More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/discy brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/discy BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch and the recent infall history of the cluster. 'BCG dominance' is therefore a phase that a cluster may evolve through and is not an evolutionary 'cul-de-sac'. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models is able to reproduce all of the observational results on , underlining the inability of the current generation of models to match the empirical properties of BCGs. We identify the strength of active galactic nucleus feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies. [ABSTRACT FROM AUTHOR]

The relatively recent insight that energy input from supermassive black holes (BHs) can have a substantial effect on the star formation rates (SFRs) of galaxies motivates us to examine the effects of BH feedback on the scale of galaxy groups. At present, groups contain most of the galaxies and a significant fraction of the overall baryon content of the Universe and, along with massive clusters, they represent the only systems for which it is possible to measure both the stellar and gaseous baryonic components directly. To explore the effects of BH feedback on groups, we analyse two high-resolution cosmological hydrodynamic simulations from the OverWhelmingly Large Simulations (OWLS) project. While both include galactic winds driven by supernovae, only one of the models includes feedback from accreting BHs. We compare the properties of the simulated galaxy groups to a wide range of observational data, including the entropy and temperature profiles of the intragroup medium, hot gas mass fractions, the luminosity–temperature and mass–temperature scaling relations, the K-band luminosity of the group and its central brightest galaxy (CBG), SFRs and ages of the CBG, and gas- and stellar-phase metallicities. Both runs yield entropy distributions similar to the data, while the run without active galactic nucleus (AGN) feedback yields highly peaked temperature profiles, in discord with the observations. Energy input from supermassive BHs significantly reduces the gas mass fractions of galaxy groups with masses less than a few , yielding a gas mass fraction and X-ray luminosity scaling with system temperature that is in excellent agreement with the data, although the detailed scatter in the L– T relation is not quite correct. The run without AGN feedback suffers from the well-known overcooling problem – the resulting stellar mass fractions are several times larger than observed and present-day cooling flows operate uninhibitedly. By contrast, the run that includes BH feedback yields stellar mass fractions, SFRs and stellar age distributions in excellent agreement with current estimates, thus resolving the long-standing ‘cooling crisis’ of simulations on the scale of groups. Both runs yield very similar gas-phase metal abundance profiles that match X-ray measurements, but they predict very different stellar metallicities. Based on the above, galaxy groups provide a compelling case that feedback from supermassive BHs is a crucial ingredient in the formation of massive galaxies. [ABSTRACT FROM AUTHOR]

We present an analysis of deep XMM–Newton and Chandra observations of the galaxy cluster XLSSJ022403.9−041328 (hereafter XLSSC 029), detected in the XMM–Newton Large Scale Structure survey. Density and temperature profiles of the X-ray emitting gas were used to perform a hydrostatic mass analysis of the system. This allowed us to measure the total mass and gas fraction in the cluster and define overdensity radii R500 and R2500. The global properties of XLSSC 029 were measured within these radii and compared with those of the local population. The gas mass fraction was found to be consistent with local clusters. The mean metal abundance was , with the cluster core regions excluded, consistent with the predicted and observed evolution. The properties of XLSSC 029 were then used to investigate the position of the cluster on the and scaling relations. In all cases the observed properties of XLSSC 029 agreed well with the simple self-similar evolution of the scaling relations. This is the first test of the evolution of these relations at and supports the use of the scaling relations in cosmological studies with distant galaxy clusters. [ABSTRACT FROM AUTHOR]

We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5 deg2 out to a redshift of . The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well-defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses, obtained from dedicated spatial and spectral fitting. The cluster distribution peaks around and keV, half of the objects being groups with a temperature below 2 keV. Our LX– T( z) relation points towards self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well-monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the LX– T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10 deg2 has the potential to exclude the non-evolution hypothesis, but those constraints on more refined intracluster medium models will probably be limited by the large intrinsic dispersion of the LX– T relation, whatever be the sample size. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to σ8, Ωm, LX– T( z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass–flux and mass–count rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming Sunyaev–Zel'dovich surveys. [ABSTRACT FROM AUTHOR]

Following the presentation of the XMM-Large Scale Structure ( XMM-LSS) survey X-ray source detection package by Pacaud et al., we provide the source lists for the first surveyed 5.5 deg2. The catalogues pertain to the [0.5–2] and [2–10] keV bands and contain in total 3385 point-like or extended sources above a detection likelihood of 15 in either band. The agreement with deep log N– log S is excellent. The main parameters considered are position, count rate, source extent with associated likelihood values. A set of additional quantities such as astrometric corrections and fluxes are further calculated while errors on the position and count rate are deduced from simulations. We describe the construction of the band-merged catalogue allowing rapid subsample selection and easy cross-correlation with external multiwavelength catalogues. A small optical Canada–France–Hawaii Telescope Legacy Survey multiband subset of objects are associated with each source along with an X-ray/optical overlay. We make the full X-ray images available in FITS format. The data are available at the Centre de Données de Strasbourg and, in a more extended form, at the Milan XMM-LSS survey data base. [ABSTRACT FROM AUTHOR]

We present gas temperature, density, entropy and cooling time profiles for the cores of a sample of 15 galaxy groups observed with Chandra. We find that the entropy profiles follow a power-law profile down to very small fractions of R500. Differences between the gas profiles of groups with radio-loud and radio-quiet brightest group galaxies are only marginally significant, and there is only a small difference in the relations, for the central regions we study with Chandra, between the radio-loud and radio-quiet objects in our sample, in contrast to the much larger difference found on scales of the whole group in earlier work. However, there is evidence, from splitting the sample based on the mass of the central black holes, that repeated outbursts of active galactic nuclei (AGN) activity may have a long-term cumulative effect on the entropy profiles. We argue that, to first order, energy injection from radio sources does not change the global structure of the gas in the cores of groups, although it can displace gas on a local level. In most systems, it appears that AGN energy injection serves primarily to counter the effects of radiative cooling, rather than being responsible for the similarity breaking between groups and clusters. [ABSTRACT FROM AUTHOR]

We study the thermodynamic properties of the hot gas in a sample of groups in the 0.012–0.024 redshift range, using XMM–Newton observations. We present measurements of temperature, entropy, pressure and iron abundance. Non-parametric fits are used to derive the mean properties of the sample and to study dispersion in the values of entropy and pressure. The scaling of the entropy at 0.2 r500 matches well the results of Ponman, Sanderson & Finoguenov. However, compared to cool clusters, the groups in our sample reveal larger entropy at inner radii and a substantially flatter slope in the entropy in the outskirts, compared to both the prediction of pure gravitational heating and observations of clusters. This difference corresponds to the systematically flatter group surface brightness profiles, reported previously. The scaled pressure profiles can be well approximated with a Sérsic model with . We find that groups exhibit a systematically larger dispersion in pressure, compared to clusters of galaxies, while the dispersion in entropy is similar. [ABSTRACT FROM AUTHOR]

We present the XMM Large-Scale Structure Survey ( XMM-LSS) cluster catalogue corresponding to the Canada–France–Hawaii Telescope Legacy Survey D1 area. The list contains 13 spectroscopically confirmed, X-ray selected galaxy clusters over to a redshift of unity and so constitutes the highest density sample of clusters to date. Cluster X-ray bolometric luminosities range from 0.03 to . In this study, we describe our catalogue construction procedure: from the detection of X-ray cluster candidates to the compilation of a spectroscopically confirmed cluster sample with an explicit selection function. The procedure further provides basic X-ray products such as cluster temperature, flux and luminosity. We detected slightly more clusters with (0.5–2.0 keV) X-ray fluxes of than we expected based on expectations from deep ROSAT surveys. We also present the luminosity–temperature relation for our nine brightest objects possessing a reliable temperature determination. The slope is in good agreement with the local relation, yet compatible with a luminosity enhancement for the objects having , a population that the XMM-LSS is identifying systematically for the first time. The present study permits the compilation of cluster samples from XMM images whose selection biases are understood. This allows, in addition to studies of large-scale structure, the systematic investigation of cluster scaling law evolution, especially for low mass X-ray groups which constitute the bulk of our observed cluster population. All cluster ancillary data (images, profiles, spectra) are made available in electronic form via the XMM-LSS cluster data base. [ABSTRACT FROM AUTHOR]

We present the X-ray pipeline developed for the purpose of the cluster search in the XMM Large-Scale Structure ( XMM-LSS) survey. It is based on a two-stage procedure via a dedicated handling of the Poisson nature of the signal: (i) source detection on multiresolution wavelet filtered images and (ii) source analysis by means of a maximum likelihood fit to the photon images. The source detection efficiency and characterization are studied through extensive Monte Carlo simulations. This led us to define two samples of extended sources: the C1 class that is uncontaminated, and the less restrictive C2 class that allows for 50 per cent contamination. The resulting predicted selection function is presented and the comparison to the current XMM-LSS confirmed cluster sample shows very good agreement. We arrive at average predicted source densities of about seven C1 and 12 C2 per deg2, which is higher than any available wide field X-ray survey. We finally note a substantial deviation of the predicted redshift distribution for our samples from the one obtained using the usual assumption of a flux-limited sample. [ABSTRACT FROM AUTHOR]

We present X-ray and optical spectroscopic observations of 12 galaxy groups and clusters identified within the X-ray Multi-Mirror ( XMM) Large-Scale Structure (LSS) survey. Groups and clusters are selected as extended X-ray sources from a 3.5 deg2 XMM image mosaic above a flux limit in the [0.5–2] keV energy band. Deep BVRI images and multi-object spectroscopy confirm each source as a galaxy concentration located within the redshift interval . We combine line-of-sight velocity dispersions with the X-ray properties of each structure computed from a two-dimensional surface brightness model and a single temperature fit to the XMM spectral data. The resulting distribution of X-ray luminosity, temperature and velocity dispersion indicate that the XMM–LSS survey is detecting low-mass clusters and galaxy groups to redshifts . Confirmed systems display little or no evidence for X-ray luminosity evolution at a given X-ray temperature compared to lower-redshift X-ray group and cluster samples. A more complete understanding of these trends will be possible with the compilation of a statistically complete sample of galaxy groups and clusters anticipated within the continuing XMM–LSS survey. [ABSTRACT FROM AUTHOR]

We use hydrodynamical simulations of galaxy clusters and groups to study the effect of pre-heating on the entropy structure of the intracluster medium. Our simulations account for non-gravitational heating of the gas either by imposing a minimum entropy floor at redshift in adiabatic simulations, or by considering feedback by galactic winds powered by supernova (SN) energy in runs that include radiative cooling and star formation. In the adiabatic simulations we find that the entropy is increased out to the external regions of the simulated haloes as a consequence of the transition from clumpy to smooth accretion induced by extra heating. This result is in line with the predictions of the semi-analytical model by Voit et al. However, the introduction of radiative cooling substantially reduces this entropy amplification effect. While we find that galactic winds of increasing strength are effective in regulating star formation, they have a negligible effect on the entropy profile of cluster-sized haloes. Only in models where the action of the winds is complemented with diffuse heating corresponding to a pre-collapse entropy do we find a sizeable entropy amplification out to the virial radius of the groups. Observational evidence for entropy amplification in the outskirts of galaxy clusters and groups therefore favours a scenario for feedback that distributes heating energy in a more diffuse way than predicted by the model for galactic winds from SN explosions explored here. [ABSTRACT FROM AUTHOR]

We present XMM–Newton observations of the wake–radio galaxy system 4C 34.16, which shows a cool and dense wake trailing behind the host galaxy of 4C 34.16. A comparison with numerical simulations is enlightening, as they demonstrate that the wake is produced mainly by ram pressure stripping during the galactic motion through the surrounding cluster. The mass of the wake is a substantial fraction of the mass of the X-ray halo of an elliptical galaxy. This observational fact supports a wake formation scenario similar to that recently demonstrated numerically by Acreman et al.: the host galaxy of 4C 34.16 has fallen into its cluster, and is currently crossing its central regions. A substantial fraction of its X-ray halo has been stripped by ram pressure, and remains behind to form the galaxy wake. [ABSTRACT FROM AUTHOR]

The superb spatial resolution ofChandrais utilized to study the X-ray morphology of the dwarf starburst galaxy NGC 1800 embedded in a small group of galaxies. Diffuse galactic emission is detected, extending several kiloparsec above the galactic plane, with an overall morphology similar to the galactic winds seen in nearby X-ray-bright starburst galaxies. This makes NGC 1800 the most distant dwarf starburst with a clear detection of diffuse X-ray emission. The diffuse X-ray luminosity ofaccounts for at least 60 per cent of the total soft X-ray output of the galaxy. A hot gas temperature ofkeV and metallicityare derived, the latter being consistent with results from optical spectroscopy of the interstellar medium. Our failure to detect any hot gas associated with the embedding galaxy group translates into an upper limit to the group X-ray luminosity of. There is no convincing evidence that the outflowing wind of NGC 1800 is currently interacting with any intragroup gas, and mechanical considerations indicate that the wind can escape the galaxy and its surrounding H ihalo, eventually delivering energy and metals to the intragroup gas. Properties of NGC 1800 are compared to those of other dwarf starburst galaxies, and a first detailed discussion of the X-ray scaling properties of this population of objects is given, set against the equivalent results obtained for normal starburst galaxies. Results indicate that dwarf starbursts to a large degree behave as down-scaled versions of normal starburst galaxies. [ABSTRACT FROM AUTHOR]

Using the high sensitivity of XMM–Newton, we have studied the X-ray emission of the two prototypical late-stage merger remnants, NGC 3921 and 7252. In the case of NGC 7252, this is complemented by archival Chandra data. We investigate the nature of the discrete X-ray point source populations and the hot diffuse gas components in these two galaxies, and compare them in the light of their different merger ages and histories. We detect three candidate ultraluminous X-ray point sources (ULXs) in NGC 3921 and at least six in NGC 7252, for which we have high spatial resolution Chandra data. These have luminosities ranging from (for ). We expect these ULXs to be high-mass X-ray binaries, associated with the recent star formation in these two galaxies. Extended hot gas is observed in both galaxies. We have sufficient counts in the XMM–Newton data to fit two-component hot plasma models to their X-ray spectra, and estimate the X-ray luminosities of the hot diffuse gas components to be and in NGC 3921 and 7252, respectively. These luminosities are low compared with the luminosities observed in typical mature elliptical galaxies ), into which these merger remnants are expected to evolve. We do not see evidence that the X-ray haloes of these galaxies are currently being regenerated to the masses and luminosities seen in typical elliptical galaxies. The mass of atomic gas available to fall back into the main bodies of these galaxies and shock-heat to X-ray temperatures is insufficient for this to be the sole halo regeneration mechanism. We conclude that halo regeneration is most likely a long-term (>10 Gyr) process, occurring predominantly via mass loss from evolving stars, in a subsonic outflow stage commencing ∼2 Gyr after the merging event. [ABSTRACT FROM AUTHOR]

We present XMM–Newton observations of three X-ray-underluminous elliptical galaxies, NGC 3585, 4494 and 5322. All three galaxies have relatively large optical luminosities but have X-ray luminosities consistent with emission from discrete sources only. In conjunction with a Chandra observation of NGC 3585, we analyse the XMM–Newton data and show that the three galaxies are dominated by discrete source emission, but do possess some X-ray-emitting gas. The gas is at relatively low temperatures, . All three galaxies show evidence of recent dynamical disturbance and formation through mergers, including kinematically distinct cores, young stellar ages and embedded stellar discs. This leads us to conclude that the galaxies formed relatively recently and have yet to build up large X-ray haloes. They are likely to be in a developmental phase where the X-ray gas has a very low density, making it undetectable outside the galaxy core. However, if the gas is a product of stellar mass loss, as seems most probable, we would expect to observe supersolar metal abundances. While abundance is not well constrained by the data, we find best-fitting abundances for single-temperature models, and it seems unlikely that we could underestimate the metallicity by such a large factor. [ABSTRACT FROM AUTHOR]

We report on the results of XMM–Newton observations of the galaxy group WARPJ0943.7+1644 and the nearby poor cluster WARPJ0943.5+1640 at . Tracing the X-ray gas out to ∼425 and in the two systems, corresponding to roughly 70 per cent of the estimated virial radii, we find the surface-brightness profile of both groups to be well described by standard β-models across the entire radial range, but with a significantly lower value of β in the cooler WARPJ0943.7+1644. A Navarro–Frenk–White (NFW) model for the gas density can also provide a good fit to the surface-brightness data, but a large scale-radius is required, and models in which gas traces mass are strongly ruled out. Both systems fall close to the observed relation for clusters. Gas mass fractions increase with radius out to the radii probed, reaching extrapolated values at the virial radius of 0.08 and 0.09 , respectively. The latter is concordant with results obtained for more massive systems. Gas entropy profiles show evidence for excess entropy out to at least , and are inconsistent with predictions of simple pre-heating models. This study emphasizes the need for observing galaxy groups out to large radii, to establish their global properties securely. [ABSTRACT FROM AUTHOR]

We report on the results of XMM–Newton observations of the galaxy group WARPJ0943.7+1644 and the nearby poor cluster WARPJ0943.5+1640 at . Tracing the X-ray gas out to ∼425 and in the two systems, corresponding to roughly 70 per cent of the estimated virial radii, we find the surface-brightness profile of both groups to be well described by standard β-models across the entire radial range, but with a significantly lower value of β in the cooler WARPJ0943.7+1644. A Navarro–Frenk–White (NFW) model for the gas density can also provide a good fit to the surface-brightness data, but a large scale-radius is required, and models in which gas traces mass are strongly ruled out. Both systems fall close to the observed relation for clusters. Gas mass fractions increase with radius out to the radii probed, reaching extrapolated values at the virial radius of 0.08 and 0.09 , respectively. The latter is concordant with results obtained for more massive systems. Gas entropy profiles show evidence for excess entropy out to at least , and are inconsistent with predictions of simple pre-heating models. This study emphasizes the need for observing galaxy groups out to large radii, to establish their global properties securely. [ABSTRACT FROM AUTHOR]

We present an X-ray study of the galaxy group or poor cluster MKW 4. Working with XMM–Newton data we examine the distribution and properties of the hot gas which makes up the group halo. The inner halo shows some signs of structure, with circular or elliptical beta models providing a poor fit to the surface brightness profile. This may be evidence of large-scale motion in the inner halo, but we do not find evidence of sharp fronts or edges in the emission. The temperature of the halo declines in the core, with deprojected spectral fits showing a central temperature of ∼1.3 keV compared to ∼3 keV at 100 kpc. In the central ∼30 kpc of the group, multitemperature spectral models are required to fit the data, but they indicate a lack of gas at low temperatures. Steady-state cooling flow models provide poor fits to the inner regions of the group and the estimated cooling time of the gas is long except within the central dominant galaxy, NGC 4073. Abundance profiles show a sharp increase in the core of the group, with mean abundance rising by a factor of 2 in the centre of NGC 4073. Fitting individual elements shows the same trend, with high values of Fe, Si and S in the core. We estimate that ∼50 per cent of the Fe in the central 40 kpc was injected by Type Ia supernovae, in agreement with previous ASCA studies. Using our best-fitting surface brightness and temperature models, we calculate the mass, gas fraction, entropy and mass-to-light ratio of the group. At 100 kpc (∼0.1 virial radius) the total mass and gas entropy of the system ( and ∼300 keV cm2) are quite comparable to those of other systems of similar temperature, but the gas fraction is rather low (∼1 per cent). We conclude that MKW 4 is a fairly relaxed group, which has developed a strong central temperature gradient but not a large-scale cooling flow. [ABSTRACT FROM AUTHOR]

We investigate the spatial distribution of the baryonic and non-baryonic mass components in a sample of 66 virialized systems. We have used X-ray measurements to determine the deprojected temperature and density structure of the intergalactic medium and have employed these to map the underlying gravitational potential. In addition, we have measured the deprojected spatial distribution of galaxy luminosity for a subset of this sample, spanning over two decades in mass. With this combined X-ray/optical study, we examine the scaling properties of the baryons and address the issue of mass-to-light ( M/ L) ratio in groups and clusters of galaxies. We measure a median mass-to-light ratio of in the rest frame band, in good agreement with other measurements based on X-ray determined masses. There is no trend in M/ L with X-ray temperature and no significant trend for mass to increase faster than luminosity: . This implied lack of significant variation in star formation efficiency suggests that gas cooling cannot be greatly enhanced in groups, unless it drops out to form baryonic dark matter. Correspondingly, our results indicate that non-gravitational heating must have played a significant role in establishing the observed departure from self-similarity in low-mass systems. The median baryon fraction for our sample is , which allows us to place an upper limit on the cosmological matter density, , in good agreement with the latest results from WMAP. We find evidence of a systematic trend towards higher central density concentration in the coolest haloes, indicative of an early formation epoch and consistent with hierarchical formation models. [ABSTRACT FROM AUTHOR]

ABSTRACT We describe the properties of a sample of galaxy groups with very unusual distributions of galaxy luminosities. The most extreme example has an X-ray luminosity similar to that of the Virgo cluster but has a very low richness, with only one galaxy brighter than L *, compared with six in Virgo. That one galaxy, however, is optically more luminous than any galaxy in Virgo and has an optical luminosity as bright as many of the central cD galaxies in rich Abell clusters. The characteristic feature of the fossil groups we study is that most of the light arises from one dominant, central galaxy. We define a fossil system and, based on this definition, construct a small X-ray-selected, flux-limited sample of fossil groups with well-known selection criteria. We confirm that these systems are indeed groups of galaxies, but dominated by one central luminous giant elliptical galaxy and with few, or no, L * galaxies. We find that fossil systems represent 8–20 per cent of all systems of the same X-ray luminosity. Fossil groups are at least as numerous as all poor and rich clusters combined, and are thus a possible site for the formation of luminous central cluster galaxies before infall into clusters occurs. The fossil systems in our sample have significantly higher X-ray luminosities than normal groups of similar total optical luminosities (or similar X-ray temperature, where the latter can be measured). These enhanced X-ray luminosities may be due to relatively cool gas in the innermost regions or due to a low central gas entropy. We interpret fossil groups as old, undisturbed systems which have avoided infall into clusters, but where galaxy merging of most of the L * galaxies has occurred. An early formation epoch, before that of most groups, could explain low central gas entropies and high X-ray luminosities. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine profiles and scaling properties of the entropy of the intergalactic gas in a sample of 66 virialized systems, ranging in mass from single elliptical galaxies to rich clusters, for which we have resolved X-ray temperature profiles. Some of the properties we derive appear to be inconsistent with any of the models put forward to explain the breaking of self-similarity in the baryon content of clusters. In particular, the entropy profiles, scaled to the virial radius, are broadly similar in form across the sample, apart from a normalization factor that differs from simple self-similar scaling with temperature. Low-mass systems do not show the large isentropic cores predicted by preheating models, and the high entropy excesses reported at large radii in groups by Finoguenov et al. are confirmed, and found to extend even to moderately-rich clusters. We discuss the implications of these results for the evolutionary history of hot gas in clusters, and suggest that preheating may affect the entropy of intracluster gas primarily by reducing the density of material accreting into groups and clusters along cosmic filaments. [ABSTRACT FROM AUTHOR]

ABSTRACT We present near-infrared K-band imaging of a large sample of candidate merger remnant galaxies and Hickson Compact Group ellipticals. We derive light profile indices, effective radii and surface brightnesses, as well as total K-band magnitudes. We find that the light distributions of the merger remnant candidates are consistent with those of ‘normal’ ellipticals, and scatter around a mean profile index of (1/n) = 0.20 . Many of our sample galaxies have surface brightness profiles that are not well described by a de Vaucouleurs law (1/n= 0.25) , and we discuss the implications of this on the derived total magnitudes. Comparing the total K magnitudes calculated by extrapolating a de Vaucouleurs profile and those derived using a generalized Sérsic form, we find that a significant bias is introduced if the de Vaucouleurs law is not a good description of the actual light profile. [ABSTRACT FROM AUTHOR]

We have assembled a large sample of virialized systems, comprising 66 galaxy clusters, groups and elliptical galaxies with high-quality X-ray data. To each system we have fitted analytical profiles describing the gas density and temperature variation with radius, corrected for the effects of central gas cooling. We present an analysis of the scaling properties of these systems and focus in this paper on the gas distribution and M-T[sub x] relation. In addition to clusters and groups, our sample includes two early-type galaxies, carefully selected to avoid contamination from group or cluster X-ray emission. We compare the properties of these objects with those of more massive systems and find evidence for a systematic difference between galaxy-sized haloes and groups of a similar temperature. We derive a mean logarithmic slope of the M-T[sub x] relation within R[sub 200] of 1.84 ± 0.06, although there is some evidence of a gradual steepening in the M-T[sub x] relation, with decreasing mass. We recover a similar slope using two additional methods of calculating the mean temperature. Repeating the analysis with the assumption of isothermality, we find the slope changes only slightly, to 1.89 ± 0.04, but the normalization is increased by 30 per cent. Correspondingly, the mean gas fraction within R[sub 200] changes from (0.13 ± 0.01) h[sup -3/2, sub 70] to (0.11 ± 0.01) h[sup -3/2, sub 70], for the isothermal case, with the smaller fractional change reflecting different behaviour between hot and cool systems. There is a strong correlation between the gas fraction within 0.3R[sub 200] and temperature. This reflects the strong (5.8σ) trend between the gas density slope parameter, β, and temperature, which has been found in previous work. These findings are interpreted as evidence for self-similarity breaking from galaxy feedback processes, active galactic nuclei heating or possibly gas cooling. We discuss the implications of our... [ABSTRACT FROM AUTHOR]

The group of galaxies RXJ1340.6+4018 has approximately the same bolometric X-ray luminosity as other bright galaxy groups and poor clusters such as the Virgo cluster. However, 70 per cent of the optical luminosity of the group comes from a dominant giant elliptical galaxy, compared with 5 per cent from M87 in Virgo.The second brightest galaxy in RXJ1340.6+4018 is a factor of 10 fainter (Δm[sub 12] = 2.5 mag) than the dominant elliptical, and the galaxy luminosity function has a gap at about L[sup *]. We interpret the properties of the system as a result of galaxy merging within a galaxy group. We find that the central galaxy lies on the Fundamental Plane of ellipticals, has an undisturbed, non-cD morphology, and has no spectral features indicative of recent star formation, suggesting that the last major merger occurred ≤ 4 Gyr ago. The deviation of the system from the cluster Lx — T relation in the opposite sense to most groups may be caused by an early epoch of formation of the group or a strong cooling flow. The unusual elongation of the X-ray isophotes and the similarity between the X-ray and optical ellipticities at large radii (∼230 kpc) suggest that both the X-ray gas and the outermost stars of the dominant galaxy are responding to an elongated dark matter distribution. RXJ1340.6+4018 may be part of a filamentary structure related to infall in the outskirts of the cluster A1774. [ABSTRACT FROM AUTHOR]

Presents a correction to the paper "The XMM Large-Scale Structure Survey: An initial sample pf galaxy groups and clusters to a redshift" that was published in 2005 issue.

A correction to the article "Active Galactic Nuclei Heating in the Centres of Galaxy Groups: A Statistical Study" that was published in 2007 issue is presented.