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We present a study of the structural and scaling properties of the gas distributions in the intracluster medium (ICM) of 31 nearby ($z < 0.2$) clusters observed with XMM-Newton, which together comprise the Representative XMM-NewtonCluster Structure Survey (REXCESS). In contrast to previous studies, this sample is unbiased with respect to X-ray surface brightness and cluster dynamical state, and it fully samples the cluster X-ray luminosity function. The clusters cover a temperature range of 2.0-8.5 keV and possess a variety of morphologies. The sampling strategy allows us to compare clusters with a wide range of central cooling times on an equal footing. We applied a recently developed technique for the deprojection and PSF-deconvolution of X-ray surface brightness profiles to obtain non-parametric gas-density profiles out to distances ranging between $0.8~R_{500}$and $1.5~R_{500}$. We scaled the gas density distributions to allow for the systems' differing masses and redshifts. The central gas densities differ greatly from system to system, with no clear correlation with system temperature. At intermediate radii (~$0.3~R_{500}$), the scaled density profiles show much less scatter, with a clear dependence on system temperature. We find that the density at this radius scales proportionally to the square root of temperature, consistent with the presence of an entropy excess as suggested in previous literature. However, at larger scaled radii this dependence becomes weaker: clusters with $kT > 3$keV scale self-similarly, with no temperature dependence of gas-density normalisation. The REXCESS sample allows us to investigate the correlations between cluster properties and dynamical state. We find no evidence of correlations between cluster dynamical state and either the gas density slope in the inner regions or temperature, but do find some evidence of a correlation between dynamical state and outer gas density slope. We also find a weak correlation between dynamical state and both central gas normalisation and inner cooling times, but this is only significant at the 10% level. We conclude that, for the X-ray cluster population as a whole, both the central gas properties and the angle-averaged, large-scale gas properties are linked to the cluster dynamical state. We also investigate the central cooling times of the clusters. While the cooling times span a wide range, we find no evidence of a significant bimodality in the distributions of central density, density gradient, or cooling time. Finally, we present the gas mass-temperature relation for the REXCESS sample, finding that $h(z)M_{\rm gas} \propto T^{1.99\pm0.11}$, which is consistent with the expectation of self-similar scaling modified by the presence of an entropy excess in the inner regions of the cluster and consistent with earlier work on relaxed cluster samples. We measure a logarithmic intrinsic scatter in this relation of ~$10\%$, which should be a good measure of the intrinsic scatter in the Mgas-Trelation for the cluster population as a whole.

Context.The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass.

XMM-Newton background maps for the 3 EPIC instruments in their different instrument/mode/filter combinations and in several energy bands have been constructed using a superposition of 72 pointed observations. Event datasets, with point sources excised, for the different instrument/mode/filter combinations have also been assembled, with longer exposure times than previously available files. The construction of the background maps and event files, together with their properties and usage are described here. Also given are statistical properties of the photon and particle components of the XMM-Newton EPIC background, based on the analysis of the 72 datasets.

We report results obtained from analysis of the XMM-Newton observation of the compact group of galaxies HCG 16. It is a peculiar system composed of 7 spirals, 6 of which are active, and its nature as a bound system has been much debated. The EPIC camera observations give new insights into the X-ray parameters describing the physical status of the group. We detect diffuse X-ray emission with a rather elliptical morphology which extends to at least a radius of 135 $h^{-1}_{50}$kpc from the group centre. The spectrum within this region is well modelled by a thermal plasma with a temperature of 0.49 ±0.17 keV, and a non-zero metallicity. We measure a bolometric X-ray luminosity of $9.6\times 10^{40}$$h^{-2}_{50}$erg s-1which may be only a small fraction of the total luminosity because of the limit in spatial detection arising from the high background level. Despite its low temperature and luminosity, HCG 16 obeys the $L_{\rm X}-T$relation obtained for brighter galaxy groups even if it lies in a very extreme position. The properties of the diffuse emission confirm the bound nature of HCG 16 even if the gas trapped in the potential well may not yet be virialized. This reopens the debate about the real nature of spiral-dominated galaxy groups, and on their role in a more general cosmological context.

This paper presents the XMM-Newtonfirst-light observations of the Hickson-16 compact group of galaxies. Groups are possibly the oldest large-scale structures in the Universe, pre-dating clusters of galaxies, and are highly evolved. This group of small galaxies, at a redshift of 0.0132 (or 80 Mpc) is exceptional in the having the highest concentration of starburst or AGN activity in the nearby Universe. So it is a veritable laboratory for the study of the relationship between galaxy interactions and nuclear activity. Previous optical emission line studies indicated a strong ionising continuum in the galaxies, but its origin, whether from starbursts, or AGN, was unclear. Combined imaging and spectroscopy with the EPIC X-ray CCDs unequivocally reveals a heavily obscured AGN and a separately identified thermal (starburst) plasma, in NGC 835, NGC 833, & NGC 839. NGC 838 shows only starburst thermal emission. Starbursts and AGN can evidently coexist in members of this highly evolved system of merged and merging galaxies, implying a high probability for the formation of AGN as well as starbursts in post-merger galaxies.

We describe the first XMM-Newtonobservations of the starburst galaxy NGC 253. As known from previous X-ray observations, NGC 253shows a mixture of extended (disk and halo) and point-source emission. The high XMM-Newtonthroughput allows a detailed investigation of the spatial, spectral and variability properties of these components simultaneously. We characterize the brightest sources by their hardness ratios, detect a bright X-ray transient ~70´´SSW of the nucleus, and show the spectrum and light curve of the brightest point source (~30´´S of the nucleus, most likely a black-hole X-ray binary, BHXRB). The unresolved emission of two disk regions can be modeled by two thin thermal plasma components (temperatures of ~0.13 and 0.4 keV) plus residual harder emission, with the lower temperature component originating from above the disk. The nuclear spectrum can be modeled by a three temperature plasma (~0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus, as no non-thermal component, that would point at a significant contribution from an active nucleus (AGN), is needed. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the $E> 4$keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 yr-1. The unprecedented combination of RGS and EPIC also sheds new light on the emission of the complex nuclear region, the X-ray plume and the disk diffuse emission. In particular, EPIC images reveal that the limb-brightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured. The plume spectrum can again be modeled by a three temperature thermal plasma containing the two low temperature nuclear components (though less absorbed) plus an unabsorbed 0.15 keV component similar to the disk spectra. This points to new interpretations as to the make up of the starburst-driven outflow.

Context.XMM and Chandra opened a new area for the study of clusters of galaxies not only for cluster physics, but also for the detection of faint and distant clusters that were inaccessible with previous missions.