Nearby Galaxy Clusters Research Articles

New evidence for a linear colour-magnitude relation and a single Schechter function for red galaxies in a nearby cluster of galaxies down to M*+ 8

The colour and luminosity distributions of red galaxies in the cluster Abell 1185 (z = 0.0325) were studied down to M ∗ + 8 in the B, V and R bands. The colour‐magnitude (CM) relation is linear without evidence for significant bending down to absolute magnitudes that are seldom probed in the literature (MR =− 12.5 mag). The CM relation is thin (±0.04 mag) and its thickness is quite independent of the magnitude. The luminosity function (LF) of red galaxies in Abell 1185 is adequately described by a Schechter function, with a characteristic magnitude and a faint end slope that also well describe the LF of red galaxies in other clusters. There is no passband dependence of the LF shape other than an obvious M ∗ shift due to the colour of the considered population. Finally, we conclude that, based on colours and luminosity, red galaxies form a homogeneous population over four decades in stellar mass, providing a second piece of evidence against faint red galaxies being a recent cluster population.

The Luminosity Function of Nearby Galaxy Clusters. II. Redshifts and Luminosity Function for Galaxies in the Region of the Centaurus Cluster

We acquired spectra for a random sample of galaxies within a 0.83 deg2 region centered on the core of the Centaurus Cluster. Radial velocities were obtained for 225 galaxies to limiting magnitudes of V -17.0 are considered dwarfs. We constructed the cluster galaxy luminosity function (LF) by using these spectroscopic results to calculate the expected fraction of cluster members in each magnitude bin. The faint-end slope of the LF using this method is shallower than the one obtained using a statistical method to correct for background galaxy contamination. We also use the spectroscopy results to define surface brightness criteria to establish membership for the full sample. Using these criteria, we find a LF very similar to the one constructed with the statistical background correction. For both, we find a faint-end slope α ~ -1.4. The error in faint-end slope for the statistically corrected LF is ~±0.2. Adjusting the surface brightness membership criteria, we find that the data are consistent with a faint-end slope as shallow as -1.22 or as steep as -1.50. We describe in this paper some of the limitations of using these methods for constructing the galaxy LF. This is Paper II in our investigation of the cluster galaxy LF.

The generation of magnetic fields by the Weibel instability

AbstractWe show that strong magnetic fields can be generated at shock waves associated with formation of galaxies or clusters of galaxies by the Weibel instability, an instability in collisionless plasmas. The estimated strength of the magnetic field generated through this mechanism is close to the order of values observed in galaxies or clusters of galaxies at present, which indicates that strong amplification of magnetic fields after formation of galaxies or clusters of galaxies is not required. This mechanism could have worked even at a redshift of ∼10, and therefore the generated magnetic fields may have affected the formation of stars at the early Universe. This model will be tested by future observations of nearby clusters of galaxies. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Environmental Effects on Late‐Type Galaxies in Nearby Clusters

ABSTRACTThe transformations that take place in late‐type galaxies in the environment of rich clusters of galaxies at z = 0 are reviewed. From the handful of late‐type galaxies that inhabit local clusters, whether they were formed in situ and survived as such, avoiding transformation or even destruction, or if they are newcomers that have recently fallen in from outside, we can learn an important lesson on the latest stages of galaxy evolution. We start by reviewing the observational scenario, covering the broadest possible stretch of the electromagnetic spectrum, from the gas tracers (radio and optical) to the star formation tracers (UV and optical), the old star tracers (near‐IR), and the dust (far‐IR). Strong emphasis is given to the three nearby, well‐studied clusters Virgo, A1367, and Coma, which are representative of different evolutionary stages, from unrelaxed and spiral‐rich (Virgo) to relaxed and spiral‐poor (Coma). We continue by providing a review of models of galaxy interactions that are relevant to clusters of galaxies. Prototypes of various mechanisms and processes are discussed, and their typical timescales are given in an a. Observations indicate the presence of healthy late‐type galaxies falling into nearby clusters individually or as part of massive groups. More rare are infalling galaxies belonging to compact groups, where significant preprocessing might take place. Once they have entered the cluster, they lose their gas and quench their star formation activity, becoming anemic. Observations and theory agree in indicating that the interaction with the intergalactic medium is responsible for the gas depletion. However, this process cannot be the origin of the cluster lenticular galaxy population. Physical and statistical properties of S0 galaxies in nearby clusters and at higher redshift indicate that they originate from spiral galaxies that have been transformed by gravitational interactions.

Spectral indications of thermal Sunyaev-Zel'dovich effect in ARCHEOPS and WMAP data

In this paper, we present a joint cross-correlation analysis of the Archeops CMB maps at 143 and 217 GHz and the WMAP CMB maps at 41, 61 and 94 GHz with sky templates of projected galaxy density constructed from the 2MASS Extended Source catalog. These templates have been divided in patches sorted in decreasing galaxy density with a fixed number of pixels (we considered patches having 32, 64 and 128 pixels) and the cross correlation has been performed independently for each of these patches. We find that the densest patch shows a strong temperature decrement in the Q, V, W bands of WMAP and in the 143 GHz channel of Archeops, but not in the 217 GHz channel. Furthermore, we find that the spectral behavior of the amplitude of this decrement is compatible with that expected for the non-relativistic thermal Sunyaev-Zel'dovich effect, and is incompatible (at 4.5 sigma level) with the null hypothesis of having only CMB, noise and a dust component (nu^2) in those pixels. We find that the first 32-pixel sized patch samples the cores of 11 known massive nearby galaxy clusters. Under the assumption that the decrement found in this patch is due entirely to the thermal Sunyaev-Zel'dovich effect, we obtain an average comptonization parameter for those sources of y = (0.41 +- 0.08) x 10^(-4) at 13 arcmin angular scales. This value is compatible at 1 sigma with the expectation, y = 0.49 x 10^(-4), from a model of the cluster flux number counts based on the standard Lambda-CDM cosmology. The observed value of y is slightly diluted when computed for the first patch of 64 and 128 pixels, presumably due to the inclusion of less massive clusters, and the dilution factor inferred is also compatible with the quoted model.

Effects of Mergers and Core Structure on the Bulk Properties of Nearby Galaxy Clusters

We use morphological measurements and the scatter of clusters about observed and simulated scaling relations to examine the impact of merging and core-related phenomena on the structure of galaxy clusters. All relations constructed from emission-weighted mean temperature and intracluster medium mass, X-ray luminosity, isophotal size, or near-IR luminosity show a separation between cool core (CC) and non-cool core (NCC) clusters. We attribute this partially to a temperature bias in CC clusters, and partially to other cool core-related structural changes. We attempt to minimize CC/NCC separation in scaling relations by applying a uniform scale factor to CC cluster temperatures and determining the scale factor for each relation that minimizes the separation between CC and NCC populations, and by introducing central surface brightness as a third parameter in relations. The latter approach reduces scatter in relations more than temperature scaling. We compare the scatter within subsamples split by CC/NCC and morphological merger indicators. CC clusters and clusters with less substructure generally exhibit higher scatter about relations. The larger structural variations in CC clusters exit well outside the core, suggesting that a process more global than core radiative instability is at work. Simulations without cooling mechanisms also show no correlation between substructure and larger scatter about relations, indicating that any merger-related scatter increases are subtle. The results indicate that cool core related phenomena, not merging processes, are the primary contributor to scatter in scaling relations. Our analysis does not appear to support the scenario in which clusters evolve cool cores over time unless they experience major mergers. (Abridged)

UV Dust Attenuation in Normal Star‐Forming Galaxies. I. Estimating theLTIR/LFUVRatio

We analyze the dust attenuation properties of a volume-limited, optically-selected sample of normal star forming galaxies in nearby clusters as observed by GALEX. The internal attenuation is estimated using three independent indicators, namely: the ratio of the total infrared to far-ultraviolet emission, the ultraviolet spectral slope beta and the Balmer decrement. We confirm that normal galaxies follow a L_TIR/L_FUV-beta relation offset from the one observed for starburst galaxies. This offset is found to weakly correlate with the birthrate parameter, thus with the galaxy star formation history. We study the correlations of dust attenuation with other global properties, such as the metallicity, dynamical mass, ionized gas attenuation, Halpha emission and mass surface density. Metal-rich, massive galaxies are, as expected, more heavily extinguished in the UV than are small systems. For the same gas metallicity normal galaxies have lower L_TIR/L_FUV ratio than starbursts, in agreement with the difference observed in the L_TIR/L_FUV-beta relation. Unexpectedly we find however that normal star forming galaxies follow exactly the same relationship between metallicity and ultraviolet spectral slope beta determined for starbursts, complicating our understanding of dust properties. This result might indicate a different dust geometry between normal galaxies and starbursts, but it could also be due to aperture effects eventually present in the IUE starbursts dataset. The present multiwavelength study allows us to provide some empirical relations from which the total infrared to far ultraviolet ratio (L_TIR/L_FUV) can be estimated when far infrared data are absent.

Hαsurface photometry of galaxies in nearby clusters

We present the Halpha imaging observations of 273 late-type galaxies in the nearby rich galaxy clusters Virgo, A1367, Coma, Cancer, Hercules and in the Great Wall, carried out primarily with the 2.1m telescope of the San Pedro Martir Observatory (SPM) and with the ESO/3.6m telescope. We derived the Halpha+[NII] fluxes and equivalent widths. The Halpha survey reached completion for an optically selected sample of nearby galaxies in and outside rich clusters. Taking advantage of the completeness of the data set, the dependence of Halpha properties on the Hubble type was determined for late-type galaxies in the Virgo cluster. Differences in the gaseous content partly account for the large scatter of the Halpha E.W. within each Hubble-type class. We studied the radial distributions of the Halpha E.W. around Coma+A1367 and the Virgo clusters in two luminosity bins. Luminous galaxies show a decrease in their average Halpha E.W. in the inner 1 virial radius, while low-luminosity galaxies do not show this trend.

Structure and scaling of the entropy in nearby galaxy clusters

Using XMM-Newton observations, we investigate the scaling and structural properties of the ICM entropy in a sample of 10 nearby ($z < 0.2$) morphologically relaxed galaxy clusters in the temperature range 2-9 keV. We derive the local entropy-temperature ($S{-}T$) relation at $R = 0.1, 0.2, 0.3$ and $0.5R_{200}$. The logarithmic slope of the relation is the same within the $1\sigma$ error at all scaled radii. However, the intrinsic dispersion about the best fitting relation is significantly higher at $0.1R_{200}$. The slope is $0.64\pm0.11$ at $0.3\,R_{200}$, in excellent agreement with previous work. We also investigate the entropy-mass relation at density contrasts $\delta=5000, 2500$ and 1000. We find a shallower slope than that expected in simple self-similar models, which is in agreement with the observed empirically-determined entropy-temperature and mass-temperature scaling. The dispersion is smaller than for the $S{-}T$ relation. Once scaled appropriately, the entropy profiles appear similar beyond ~$0.1R_{200}$, with an intrinsic dispersion of ~15 per cent and a shape consistent with gravitational heating ($S(r) \lower 3pt \hbox{$\, \buildrel {\textstyle \propto}\over {\textstyle \sim}\,$}r^{1.1}$). However, the scatter in scaled entropy profiles increases with smaller scaled radius, to more than 60 per cent at $R \la 0.05 R_{200}$. Our results are in qualitative agreement with models which boost entropy production at the accretion shock. However, localised entropy modification may be needed to explain the dispersion in the inner regions.

The radio luminosity function of the NEP distant cluster radio galaxies

A complete sample of 18 X-ray selected clusters of galaxies belonging to the ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large Array at 1.4 GHz in B configuration. These are the most distant clusters in the X-ray survey with redshift in the range . Seventy-nine radio sources are detected within half an Abell radius with an observed peak brightness 0.17 mJy beam-1 , except for three sources, belonging to the same cluster, which have a higher peak brightness limit of 0.26 mJy beam-1 . The NEP field source counts are in good agreement with the source counts of a comparison survey, the VLA-VIRMOS deep field survey, indicating that the NEP sample is statistically complete. Thirty-two out of the 79 sources are within 0.2 Abell radii, twenty-two of them are considered cluster members based on spectroscopic redshifts or their optical magnitude and morphological classification. The cluster radio galaxies are used to construct the Radio Luminosity Function (RLF) of distant X-ray selected clusters. A comparison with two nearby cluster RLFs shows that the NEP RLF lies above the local ones, has a steeper slope at low radio powers (≤ 1024 W Hz-1 ) and shows no evidence for a break at W Hz-1 which is observed in the nearby cluster RLFs. We discuss briefly the origin and possible explanations of the differences observed in the radio properties of nearby and distant clusters of galaxies. The main result of this study is that the RLF of the distant X-ray clusters is very different from that of the local rich Abell clusters.

ISM gas removal from starburst galaxies and the premature death of star clusters

Recent observational studies of the age distribution of star clusters in nearby merging galaxies and starburst (SB) galaxies indicate a premature death of the young clusters. The fate of an evolving star cluster crucially depends of its gas content. This behaves like a glue that helps to keep the star system gravitationally bound. In SB systems where the rate of supernovae (SNe) explosions is elevated one should expect an efficient heating of the gas and its complete removal which could then favor the rapid dissociation of the evolving star clusters. Based on a contemporaneous study of the dynamical evolution of the interstellar gas in SB environments (Melioli & de Gouveia Dal Pino 2004, A&A, 424, 817) where it has been considered also the presence of dense clouds that may inhibit the heating efficiency of the interstellar gas by the SNe, we have here computed the timescales for gas removal from young clusters embedded in these systems and found that they are consistent with the very short timescales for cluster dissolution which are inferred from the observational studies above. Our results indicate that typical SB proto-clusters should start to disperse after less than 5 Myr. For a given total gas mass content, this result is nearly insensitive to the initial star formation efficiency.

Tracing the X-ray emitting intra-cluster medium of clusters of galaxies beyondR200

We present in this paper a study of a sample of 14 nearby clusters of galaxies (0.06 r200. In the center (r 0.4r200 we find that the ScEM profiles agree well within the error bars, suggesting self-similarity. Fitting beta-models to the overall ScEM profiles, we find for the different subsamples rc = 0.15−0.18r200 and β = 0.8, which is higher than the canonical value of β = 2/3 often found. The beta-model is generally a better representation for hotter than for cooler clusters. We see indications for continuous steepening of the profiles with increasing radius: at radii r > 0.8r200 the profiles are systematically below the beta-model curve with β = 0.8. We discuss our results with respect to the observed X-ray luminosity LX − T relation, the gas mass Mgas − T relation and the total mass M − T relation. We also address implications on the origin of the observed S − T relation. Furthermore, we discuss the observed steepness of the X-ray profiles, which falls off more rapidly than predicted from the NFW-profile for cold dark matter halos.

The late merging phase of a galaxy cluster: XMM EPIC observations of A 3266

We present a mosaic of five XMM-Newton observations of the nearby ($z=0.0594$) merging galaxy cluster Abell 3266. We use the spectro-imaging capabilities of \xmm to build precise (projected) temperature, entropy, pressure and Fe abundance maps. The temperature map exhibits a curved, large-scale hot region, associated with elevated entropy levels, very similar to that foreseen in numerical simulations. The pressure distribution is disturbed in the central region but is remarkably regular on large scales. The Fe abundance map indicates that metals are inhomogeneously distributed across the cluster. Using simple physical calculations and comparison with numerical simulations, we discuss in detail merging scenarios that can reconcile the observed gas density, temperature and entropy structure, and the galaxy density distribution.

Hubble Space Telescope imaging of globular cluster candidates in low surface brightness dwarf galaxies

Fifty-seven nearby low surface brightness dwarf galaxies (−10 > MV > −16) were searched for globular cluster candidates (GCCs) using Hubble Space Telescope WFPC2 imaging in V and I. The sample consists of 18 dwarf spheroidal (dSph), 36 irregular (dIrr), and 3 “transition” type (dIrr/dSph) galaxies with angular sizes less than 3.7 kpc situated at distances 2−6 Mpc in the field and in the nearby groups: M 81, Centaurus A, Sculptor, Canes Venatici I cloud. We find that ∼50% of dSph, dIrr/dSph, and dIrr galaxies contain GCCs. The fraction of GCCs located near the center of dwarf spheroidal galaxies is >2 times higher than for dIrrs. The mean integral color of GCCs in dSphs, (V − I)0 = 1.04 ± 0.16 mag, coincides with the corresponding value for Galactic globular clusters and is similar to the blue globular cluster sub-populations in massive early-type galaxies. The color distribution for GCCs in dIrrs shows a clear bimodality with peaks near (V − I)0 = 0.5 and 1.0 mag. Blue GCCs are presumably young with ages t −6.5 mag in both dSph and dIrr galaxies, reminiscent of excess populations of faint globular clusters in nearby Local Group spiral galaxies. The measurement of structural parameters using King-profile fitting reveals that most GCCs have structural parameters similar to extended outer halo globular clusters in the Milky Way and M 31, as well as the recently discovered population of “faint fuzzy” clusters in nearby lenticular galaxies.

The structural and scaling properties of nearby galaxy clusters

Using a sample often nearby (z ≤ 0.15), relaxed galaxy clusters in the temperature range [2-9] keV, we have investigated the scaling relation between the mass at various density contrasts (6 = 2500, 1000, 500, 200) and the cluster temperature. The masses are derived from NFW-type model fits to mass profiles, obtained under the hydrostatic assumption using precise measurements, with XMM-Newton, at least down to 6 = 1000. The logarithmic slope of the M-T relation is well constrained and is the same at all 6, reflecting the self-similarity of the mass profiles. At 6 = 500, the slope of the relation for the sub-sample of hot clusters (kT > 3.5 keV) is consistent with the standard self-similar expectation: a = 1.49 ± 0.15. The relation steepens when the whole sample is considered: a = 1.71 ± 0.09. The normalisation of the relation is discrepant (by ∼30 per cent), at all density contrasts, with the prediction from purely gravitation based models. Models that take into account radiative cooling and galaxy feedback are generally in better agreement with our data. We argue that remaining discrepancies, in particular at low 6, are more likely due to problems with models of the ICM thermal structure rather than to an incorrect estimate of the mass from X-ray data.

Z-bursts from the Virgo cluster

Resonant annihilation of ultrahigh energy cosmic neutrinos ($\mathrm{UHEC}\ensuremath{\nu}$) on the cosmic neutrino background ($\mathrm{C}\ensuremath{\nu}\mathrm{B}$) into $Z$ bosons---the $Z$-burst mechanism---and its associated absorption and emission phenomenology provide a unique, albeit indirect, probe of the $\mathrm{C}\ensuremath{\nu}\mathrm{B}$ in its present state. In this paper, we examine the implications of gravitational clustering of the $\mathrm{C}\ensuremath{\nu}\mathrm{B}$ in nearby galaxy clusters for the $Z$-burst phenomenology. In particular, we study the emission features of the $Z$-decay products originating from the Virgo cluster, and the potential of future cosmic ray experiments to observe clustering-enhanced $Z$-burst rates. We find that a detector with an exposure equivalent to three years of observations at the Extreme Universe Space Observatory (EUSO) will very likely measure these enhanced rates together with the associated $\mathrm{UHEC}\ensuremath{\nu}$ flux, provided that the latter saturates current observational limits and the neutrino masses are quasidegenerate, ${m}_{{\ensuremath{\nu}}_{i}}\ensuremath{\gtrsim}0.1\text{ }\text{ }\mathrm{eV}$. In the case of $\mathrm{UHEC}\ensuremath{\nu}$ fluxes below the electromagnetic cascade limit, or a hierarchical neutrino mass spectrum, an experimental sensitivity exceeding that of EUSO by at least 2 orders of magnitude is required to detect the clustering enhancements with any certainty.

A possible origin of magnetic fields in galaxies and clusters: strong magnetic fields at z 10?

We propose that strong magnetic fields should be generated at shock waves associated with the formation of galaxies or clusters of galaxies by the Weibel instability, an instability in collisionless plasmas. The strength of the magnetic fields generated through this mechanism is close to the order of those observed in galaxies or clusters of galaxies at present. If the generated fields do not decay rapidly, this indicates that strong amplification of magnetic fields after the formation of galaxies or clusters of galaxies is not required. This mechanism could have worked even at a redshift of ~10, and therefore the generated magnetic fields may have affected the formation of stars in protogalaxies. This model will partially be confirmed by future observations of nearby clusters of galaxies. Mechanisms that preserve the magnetic fields for a long time without considerable decay are discussed.

XMM‐NewtonEPIC Background Modeling for Extended Sources

We use XMM-Newton blank-sky and closed-cover background data to explore background subtraction methods for large extended sources filling the EPIC field of view, such as nearby galaxy clusters, for which local background estimation is difficult. In particular, we investigate the uncertainties of the background modeling in the 0.8-7.0 keV band that affect cluster analyses. To model the background, we have constructed composite data sets from the blank-sky observations and compared them to the individual blank-sky observations to evaluate the modeling error. Our results apply to data obtained with thin and medium optical filters and in full frame and extended full frame modes. As expected, the modeling uncertainty is determined by how the EPIC background flares are filtered. We find that to keep this uncertainty tolerable, one has to use a much more restrictive filter than that commonly applied. In particular, because flares have highly variable spectra, not all of them are identified by filtering the E > 10 keV light curve. We tried using the outer part of the EPIC field of view for monitoring the background in a softer band (1-5 keV). We find that one needs to discard the time periods when either the hard-band or the soft-band rate exceeds the nominal value by more than 20% in order to limit the 90% CL background uncertainty to between ±5% at E = 4-7 keV and ±20% at E = 0.8-1 keV, for both MOS and PN. This compares to a 10%-30% respective PN uncertainty when only the hard-band light curve is used for filtering, and to a 15%-45% PN uncertainty when applying the commonly used 2-3 σ filtering method. Adding such a soft-band filter on average results in only a 5%-10% reduction of the useful exposure time. We illustrate our method on the nearby cluster A1795. The above background uncertainties convert into systematic temperature uncertainties between about ±1% at r = 3'-4' and ±20%-25% (about ±1 keV for A1795) at r = 10'-15'. For comparison, the commonly applied 2-3 σ clipping of the hard-band light curve misses a significant number of flares, rendering the temperatures beyond r = 10' unconstrained. Thus, the background uncertainties do not prohibit the EPIC temperature profile analysis of low-brightness regions, such as outer regions of galaxy clusters, provided a conservative flare filtering such as the double-filtering method with ±20% limits is used.

ISO's Contribution to the Study of Clusters of Galaxies

Starting with nearby galaxy clusters like Virgo and Coma, and continuing out to the furthest galaxy clusters for which ISO results have yet been published ($z=0.56$), we discuss the development of knowledge of the infrared and associated physical properties of galaxy clusters from early IRAS observations, through the "ISO-era" to the present, in order to explore the status of ISO's contribution to this field. Relevant IRAS and ISO programmes are reviewed, addressing both the cluster galaxies and the still-very-limited evidence for an infrared-emitting intra-cluster medium. ISO made important advances in knowledge of both nearby and distant galaxy clusters, such as the discovery of a major cold dust component in Virgo and Coma cluster galaxies, the elaboration of the correlation between dust emission and Hubble-type, and the detection of numerous Luminous Infrared Galaxies (LIRGs) in several distant clusters. These and consequent achievements are underlined and described. We recall that, due to observing time constraints, ISO's coverage of higher-redshift galaxy clusters to the depths required to detect and study statistically significant samples of cluster galaxies over a range of morphological types could not be comprehensive and systematic, and such systematic coverage of distant clusters will be an important achievement of the Spitzer Observatory.

Tracing the X-ray emitting intra-cluster medium of clusters of galaxies beyondr$\mathsf{_{200}}$

(Abridged) We present in this paper a sample of 14 nearby clusters of galaxies observed with the ROSAT/PSPC. We only select clusters with low galactic nH in order to trace the X-ray emitting intra-cluster medium (ICM) out to large radii. We convert the X-ray surface brightness profiles of the clusters into emission measure profiles scaled to the classical scaling relations based on the spherical collapse model. We sort the clusters into different temperature bins and stack the scaled emission measure (ScEM) profiles of clusters together. The stacked profiles allow us to observe out to radii r>r_200. In the center we find that the ScEM profiles deviate from predicted scaling laws. This result is in very good agreement with current studies on the L_X-T relation and the entropy - temperature relation (S \propto T^0.65) found recently. At radii r>0.4r_200 we find that the ScEM profiles agree well within the error bars, suggesting self-similarity. Fitting beta-models to the overall ScEM profiles we find for the different sub samples r_c=0.15-0.18 r_200 and beta=0.8, which is higher than beta=2/3 often found. The beta-model is generally a better representation for hotter than for cooler clusters. We see indications for continuous steepening of the profiles with increasing radius: at radii r>0.8r_200 the profiles are systematically below the beta-model curve with beta=0.8. We discuss our results with respect to the observed X-ray luminosity L_X-T relation, the gas mass M_gas-T relation and the total mass M-T relation. We also address implications on the origin of the observed S-T relation. Furthermore we discuss the observed steepness of the X-ray profiles, which falls off more rapidly than predicted from the NFW-profile for cold dark matter halos.