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We present a systematic analysis of 43 nearby galaxy groups (kT 500 = 0.7 − 2.7 keV or M 500 = 10 13 − 10 14 h −1 M ⊙ , 0.012 < z < 0.12), based on Chandra archival data. With robust background subtraction and modeling, we trace gas properties to at least r 2500 for all 43 groups. For 11 groups, gas properties can be robustly derived to r 500 . For an additional 12 groups, we derive gas properties to at least r 1000 and estimate properties at r 500 from extrapolation. We show that in spite of the large variation in temperature profiles inside 0.15 r 500 , the temperature profiles of these groups are similar at > 0.15 r 500 and are consistent with a "universal temperature profile." We present the K − T relations at six characteristic radii (30 kpc, 0.15 r 500 , r 2500 , r 1500 , r 1000 and r 500 ), for 43 groups from this work and 14 clusters from the Vikhlinin et al. (2008) sample. Despite large scatter in the entropy values at 30 kpc and 0.15 r 500 , the intrinsic scatter at r 2500 is much smaller and remains the same (∼ 10%) to r 500 . The entropy excess at r 500 is confirmed, in both groups and clusters, but the magnitude is smaller than previous ROSAT and ASCA results. We also present scaling relations for the gas fraction. It appears that the average gas fraction between r 2500 and r 500 has no temperature dependence, ∼ 0.12 for 1 -10 keV systems. The group gas fractions within r 2500 are generally low and have large scatter. This work shows that the difference of groups from hotter clusters stems from the difficulty of compressing group gas inside of r 2500 . The large scatter of the group gas fraction within r 2500 causes large scatter in the group entropy around the center and may be responsible for the large scatter of the group luminosities. Nevertheless, the groups appear more regular and more like clusters beyond r 2500 , from the results on gas fraction and entropy. Therefore, mass proxies can be extended into low mass systems. The M 500 − T 500 and M 500 − Y X,500 relations derived in this work are indeed well behaved down to at least 2 ×10 13 h −1 M ⊙ .

The most successful cosmological models to date envision structure formation as a hierarchical process in which gravity is constantly drawing lumps of matter together to form increasingly larger structures. Clusters of galaxies currently sit atop this hierarchy as the largest objects that have had time to collapse under the influence of their own gravity. Thus, their appearance on the cosmic scene is also relatively recent. Two features of clusters make them uniquely useful tracers of cosmic evolution. First, clusters are the biggest things whose masses we can reliably measure because they are the largest objects to have undergone gravitational relaxation and entered into virial equilibrium. Mass measurements of nearby clusters can therefore be used to determine the amount of structure in the universe on scales of 10^14 to 10^15 solar masses, and comparisons of the present-day cluster mass distribution with the mass distribution at earlier times can be used to measure the rate of structure formation, placing important constraints on cosmological models. Second, clusters are essentially ``closed boxes'' that retain all their gaseous matter, despite the enormous energy input associated with supernovae and active galactic nuclei, because the gravitational potential wells of clusters are so deep. The baryonic component of clusters therefore contains a wealth of information about the processes associated with galaxy formation, including the efficiency with which baryons are converted into stars and the effects of the resulting feedback processes on galaxy formation. This article reviews our theoretical understanding of both the dark-matter component and the baryonic component of clusters. (Abridged)Comment: 54 pages, 15 figures, Rev. Mod. Phys. (in press

We present radial entropy profiles of the intracluster medium (ICM) for a collection of 239 clusters taken from the Chandra X-ray Observatory's Data Archive. Entropy is of great interest because it controls ICM global properties and records the thermal history of a cluster. Entropy is therefore a useful quantity for studying the effects of feedback on the cluster environment and investigating any breakdown of cluster self-similarity. We find that most ICM entropy profiles are well-fit by a model which is a power-law at large radii and approaches a constant value at small radii: K(r) = K 0 + K 100 (r/100 kpc) α , where K 0 quantifies the typical excess of core entropy above the best fitting power-law found at larger radii. We also show that the K 0 distributions of both the full archival sample and the primary HIFLUGCS sample of Reiprich (2001) are bimodal with a distinct gap between K 0 ≈ 30 − 50 keV cm 2 and population peaks at K 0 ∼ 15 keV cm 2 and K 0 ∼ 150 keV cm 2 . The effects of PSF smearing and angular resolution on best-fit K 0 values are investigated using mock Chandra observations and degraded entropy profiles, respectively. We find that neither of these effects is sufficient to explain the entropy-profile flattening we measure at small radii. The influence of profile curvature and number of radial bins on best-fit K 0 is also considered, and we find no indication K 0 is significantly impacted by either. For completeness, we include previously unpublished optical spectroscopy of Hα and [N II] emission lines discussed in Cavagnolo et al. (2008a). All data and results associated with this work are publicly available via the project web site.Abell 119 (z = 0.0442): This is a highly diffuse cluster without a prominent cool core. The large core region and slowly varying surface brightness made deprojection highly unstable. We have excluded a small source at the very center of the BCG. The exclusion region for the source is ≈ 2.2 ′′ in radius which at the redshift of the cluster is ∼ 2 kpc. This cluster required a double β-model.Abell 160 (z = 0.0447): The highly asymmetric, low surface brightness of this cluster resulted in a noisy surface brightness profile that could not be deprojected. This cluster required a double β-model. The BCG hosts a compact X-ray source. The exclusion region for the compact source has a radius of ∼ 5 ′′ or ∼ 4.3 kpc. The BCG for this cluster is not coincident with the X-ray centroid and hence is not at the zero-point of our radial analysis.Abell 193 (z = 0.0485): This cluster has an azimuthally symmetric and a very diffuse ICM centered on a BCG which is interacting with a companion galaxy. In Fig. 1 one can see that the central three bins of this cluster's surface brightness profile are highly discrepant from the best-fit β-model. This is a result of the BCG being coincident with a bright, compact X-ray source. As we have concluded in 3.5, compact X-ray sources are excluded from our analysis as they are not the focus of our study here. Hence we have used the best-fit β-model in d...

The radial entropy profile of the hot gas in clusters of galaxies tends to follow a power law in radius outside of the cluster core. Here we present a simple formula giving both the normalization and slope for the power‐law entropy profiles of clusters that form in the absence of non‐gravitational processes such as radiative cooling and subsequent feedback. It is based on 71 clusters drawn from four separate cosmological simulations, two using smoothed particle hydrodynamics and two using adaptive‐mesh refinement (AMR), and can be used as a baseline for assessing the impact of non‐gravitational processes on the intracluster medium outside of cluster cores. All the simulations produce clusters with self‐similar structure in which the normalization of the entropy profile scales linearly with cluster temperature, and these profiles are in excellent agreement outside of 0.2r200. Because the observed entropy profiles of clusters do not scale linearly with temperature, our models confirm that non‐gravitational processes are necessary to break the self‐similarity seen in the simulations. However, the core entropy levels found by the two codes used here significantly differ, with the AMR code producing nearly twice as much entropy at the centre of a cluster.

We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the representative XMM-Newton cluster structure survey (REXCESS), a sample in the temperature range 2−9 keV selected in X-ray luminosity only, with no bias toward any particular morphological type. The entropy profiles are robustly measured at least out to R 1000 in all systems and out to R 500 in thirteen systems. Compared to theoretical expectations from non-radiative cosmological simulations, the observed distributions show a radial and mass-dependent excess entropy, such that the excess is greater and extends to larger radii in lower mass systems. At R 500 , the mass dependence and entropy excess are both negligible within the large observational and theoretical uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R 500 . There is a large dispersion in scaled entropy in the inner regions, apparently linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by approximately a factor of two to 30 per cent, and the dichotomy between subsamples disappears. There are two peaks in the distribution of both inner slope and, after parameterising the profiles with a power law plus constant model, in central entropy K 0 . However, we are unable to distinguish between a bimodal or a left-skewed distribution of K 0 with the present data. The distribution of outer slopes is unimodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles by the gas mass fraction profile f gas (

Our Chandra X-Ray Observatory archival study of intracluster entropy in a sample of 222 galaxy clusters shows that Ha and radio emission from the brightest cluster galaxy are much more pronounced when the cluster's core gas entropy is Շ30 keV cm 2 . The prevalence of Ha emission below this threshold indicates that it marks a dichotomy between clusters that can harbor multiphase gas and star formation in their cores and those that cannot. The fact that strong central radio emission also appears below this boundary suggests that AGN feedback turns on when the intracluster medium starts to condense, strengthening the case for AGN feedback as the mechanism that limits star formation in the universe's most luminous galaxies.

We present a set of cluster models that link the present-day properties of clusters to the processes that govern galaxy formation. These models treat the entropy distribution of the intracluster medium as its most fundamental property. Because convection strives to establish an entropy gradient that rises with radius, the observable properties of a relaxed cluster depend entirely on its dark-matter potential and the entropy distribution of its uncondensed gas. Guided by simulations, we compute the intracluster entropy distribution that arises in the absence of radiative cooling and supernova heating by assuming that the gas-density distribution would be identical to that of the dark matter. The lowest-entropy gas would then fall below a critical entropy threshold at which the cooling time equals a Hubble time. Radiative cooling and whatever feedback is associated with it must modify the entropy of that low-entropy gas, changing the overall entropy distribution function and thereby altering the observable properties of the cluster. Using some phenomenological prescriptions for entropy modification based on the existence of this cooling threshold, we construct a remarkably realistic set of cluster models. The surface-brightness profiles, masstemperature relation, and luminosity-temperature relation of observed clusters all naturally emerge from these models. By introducing a single adjustable parameter related to the amount of intracluster gas that can cool within a Hubble time, we can also reproduce the observed temperature gradients of clusters and the deviations of cooling-flow clusters from the standard luminosity-temperature relation.