S. De Grandi

The ROSAT-ESO flux limited X-ray (REFLEX) galaxy cluster survey. I. The construction of the cluster sample ?

We discuss the construction of an X-ray flux-limited sample of galaxy clusters, the REFLEX survey catalogue, to be used for cosmological studies. This cluster identication and redshift survey was conducted in the frame of an ESO key programme and is based on candidates selected from the southern part of the ROSAT All-Sky Survey (RASS). For the rst cluster candidate selection from a flux-limited RASS source list, we make use of optical data from the COSMOS digital catalogue produced from the scans of the UK-Schmidt plates. To ensure homogeneity of the sample construction process, this selection is based only on this one well-dened optical data base. The nature of the candidates selected in this process is subsequently checked by a more detailed evaluation of the X-ray and optical source properties and available literature data. The nal identication and the redshift is then based on optical spectroscopic follow-up observations. In this paper we document the process by which the primary cluster candidate catalogue is constructed prior to the optical follow-up observations. We describe the reanalysis of the RASS source catalogue which enables us to impose a proper flux limit cut to the X-ray source list without introducing a severe bias against extended sources. We discuss the correlation of the X-ray and optical (COSMOS) data to nd galaxy density enhancements at the RASS X-ray source positions and the further evaluation of the nature of these cluster candidates. Based also on the results of the follow-up observations we provide a statistical analysis of the completeness and contamination of the nal cluster sample and show results on the cluster number counts. The nal sample of identied X-ray clusters reaches a flux limit of 3 10 12 erg s 1 cm 2 in the 0.1{2.4 keV band and comprises 452 clusters in an area of 4.24 ster. The results imply a completeness of the REFLEX cluster sample well in excess of 90%. We also derive for the rst time an upper limit of less than 9% for the number of clusters which may feature a dominant contribution to the X-ray emission from AGN. This accuracy is sucient for the use of this cluster sample for cosmological tests.

On the Iron content in rich nearby Clusters of Galaxies

In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX .W e fi nd that the global iron mass of clusters is tightly related to the cluster luminosity and that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron mass vs. luminosity and luminosity vs. temperature correlations. The iron mass is found to scale linearly with the intracluster gas mass, implying that the global iron abundance in clusters is roughly constant. This result suggests that enrichment mechanisms operate at a similar rate in all clusters. By employing population synthesis and chemical enrichment models, we show that the iron mass associated with the abundance excess which is always found in the centre of cool core clusters can be entirely produced by the brightest cluster galaxy (BCG), which is always found at the centre of cool core clusters. The iron mass associated with the excess, the optical magnitude of the BCG and the temperature of the cluster are found to correlate with one another suggesting a link between the properties of the BCG and the hosting cluster. These observational facts lends strength to current formation theories which envisage a strong connection between the formation of the giant BCG and its hosting cluster.

A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: The Catalog and log N-log S

We describe the selection of an X-ray flux-limited sample of bright clusters of galaxies in the southern hemisphere, based on the first analysis of the ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an identification of candidate clusters in RASS1, after which their X-ray fluxes are remeasured using the steepness-ratio technique. This method is better suited than the RASS1 standard algorithm to measuring flux from extended sources. The final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so that as a result of the distribution of NH, its effective flux limit varies between ~3 and 4×10−12 ergs cm-2 s-1 over the selected area. This covers the δ<25 part of the south Galactic cap region (b<-20°), excluding patches of low RASS1 exposure time and of the Magellanic Clouds area, for a total of 8235 deg2. One hundred and thirty candidate sources fulfill our selection criteria for bona fide clusters of galaxies in this area. Of these, 101 are Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of these clusters, 126 (97%) have a redshift, and for these we compute an X-ray luminosity. 20% of the cluster redshifts come from new observations, as part of the ESO Key Program survey of RASS clusters that is currently under completion. Considering the intrinsic biases and incompletenesses introduced by the flux selection and source identification processes, we estimate the overall completeness to be better than 90%. The observed number count distribution, log N-log S, is well fitted by a power law with slope α=1.34±0.15 and normalization A=11.87±1.04 sr-1 (10-11 ergs cm-2 s-1)α, in good agreement with other measurements.

Gravitating mass profiles of nearby galaxy clusters and relations with x-ray gas temperature, luminosity and mass

We consider a sample of 22 nearby clusters of galaxies observed with the Medium Energy Concentrator Spectrometer (MECS) on board BeppoSAX . They cover the range in gas temperature between 3 and 10 keV, with bolometric X-ray luminosity between 2 10 44 erg s 1 and 6 10 45 erg s 1 . Using the de-projected gas temperature and density profiles resolved in a number of bins between 5 and 7 and obtained from this dataset only, we recover the total gravitating mass profiles for 20 objects just applying the (i) spherical symmetry and (ii) hydrostatic equilibrium assumptions. We investigate the correlations between total mass, gas temperature and luminosity at several overdensities values and find that the slopes of these relations are independent of the considered overdensity and consistent with what is predicted from the cluster scaling laws. The best-fit results on the normalization of the M T relation are slightly lower, but still consistent considering the large errors that we measure, with hydrodynamical simulations. A segregation between relaxed and non-relaxed systems is present in each plane of these relations pointing out a significant component in their intrinsic scatter. This segregation becomes more evident at higher overdensities and when physical quantities, like Mgas and L, that are direct functions of the amount of gas observed, are considered.

The ROSAT-ESO Flux-Limited X-ray (REFLEX) galaxy cluster survey — II. The spatial correlation function

We report the results of the spatial two-point correlation functioncc(r) for the new X- ray galaxy cluster survey REFLEX, which comprises of 452 X-ray selected clusters (449 with redshifts) detected by the ROSAT satellite during the ROSAT All-Sky-Survey (RASS). The REFLEX cluster sample is flux limited to 3 × 10 12 erg s 1 cm 2 in the ROSAT energy band (0.1 − 2.4 keV) and spans 3 decades in X-ray luminosity (10 42 −10 45 h 2 erg s 1 ), containing galaxy groups and rich clusters out to a redshift z ≤ 0.3. Covering a contiguous area of 4.24 sr REFLEX is the largest X-ray cluster sample to date for which spatial clustering has been analysed. Correlation studies using clusters selected on the basis of their X-ray emission are particularly interesting as they are largely free from the projection biases inherent to optical studies. For the entire flux-limited sample we find that the correlation length (the scale at which the correlation amplitude passes through unity) r0 ≃ 20h 1 Mpc. For example, if a power-law fit is made to �(r) over the range 4 − 40h 1 Mpc then r0 = 18.8 ± 0.9. An indication of the robustness of this result comes from the high degree of isotropy seen in the clustering pattern on scales close to the correlation length. On larger scalescc(r) deviates from a power-law, crossing zero at ≃ 45h 1 Mpc. From an examination of 5 volume-limited cluster sub-samples we find no significant trend of r0 with limiting X-ray luminosity. A comparison with recent model predictions for the clustering properties of X-ray flux-limited samples, indicates that Cold Dark Matter models with the matter density m = 1 fail to produce sufficient clustering to account for the data, while m ≃ 0.3 models provide an excellent fit.

The ROSAT-ESO Flux-Limited X-ray (REFLEX) Galaxy Cluster Survey. IV. The X-ray luminosity function

The X-ray galaxy cluster sample from the REFLEX Cluster Survey, which covers the X-ray brightest galaxy clusters detected in the ROSAT All-Sky Survey in the southern sky, is used to construct the X-ray luminosity function of clusters in the local universe. With 452 clusters detected above an X-ray flux limit of 3 × 10-12 ergs s-1 cm-2 in 4.24 sr of the sky, this sample is the most comprehensive X-ray cluster sample with a well-documented selection function, providing the best current census of the local X-ray galaxy cluster population. In this paper we discuss the construction of the luminosity function and the effects of flux measurement errors and of variations with sample region, and we compare the results with those from previous surveys.

The ROSAT-ESO Flux-Limited X-Ray (REFLEX) galaxy cluster survey III. The power spectrum ?

We present a measure of the power spectrum on scales from 15 to 800h 1 Mpc using the ROSAT- ESO Flux-Limited X-Ray (REFLEX) galaxy cluster catalogue. The REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of galaxies with the nominal flux limit S =3 : 01 0 12 erg s 1 cm 2 for the ROSAT energy band (0:1 2:4) keV. Several tests are performed showing no signicant incompletenesses of the REFLEX clusters with X-ray luminosities brighter than 10 43 erg s 1 up to scales of about 800h 1 Mpc. They also indicate that cosmic variance might be more important than previous studies suggest. We regard this as a warning not to draw general cosmological conclusions from cluster samples with a size smaller than REFLEX. Power spectra, P (k), of comoving cluster number densities are estimated for flux- and volume-limited subsamples. The most important result is the detection of a broad maximum within the comoving wavenum- ber range 0:022 k 0:030h Mpc 1 . The data suggest an increase of the power spectral amplitude with X-ray luminosity. Compared to optically selected cluster samples the REFLEX P (k) is flatter for wavenumbers k 0:05h Mpc 1 thus shifting the maximum of P (k) to larger scales. The smooth maximum is not consis- tent with the narrow peak detected at k =0 :05h Mpc 1 using the Abell/ACO richness0 data. In the range 0:02 k 0:4h Mpc 1 general agreement is found between the slope of the REFLEX P (k) and those obtained with optically selected galaxies. A semi-analytic description of the biased nonlinear power spectrum in redshift space gives the best agreement for low-density Cold Dark Matter models with or without a cosmological constant.

Simulating Chandra observations of galaxy clusters

Although trivial in principle, direct comparison of galaxy clusters X-ray observations to numerical hydro-N-body simulations is not always simple, because of many possible artefacts introduced by the instrument response, sky background and instrumental noise. To address these problems, we constructed the software package X-MAS (X-ray Map Simulator), a tool devoted to simulate X-ray observations of galaxy clusters obtained from hydro-N-body simulations. One of the main features of X-MAS is the ability to generate event files following the same standards used for real observations. This implies that its simulated observations can be analysed in the same way as - and with the same tools of - real observations. In this paper we present how the X-MAS package works, and discuss its application to the simulation of Chandra ACIS-S3 observations. Using the results of high-resolution hydro-N-body simulations, we generate fake Chandra observations of a number of simulated clusters. We then compare some of the main physical properties of the input data to those derived from the simulated observations after performing a standard imaging and spectral analysis. We find that, because of the sky background, the lower surface brightness spatial substructures, which can be easily identified in the simulations, are no longer detected in the simulated observations. We also show that, when a cluster has a complex (i.e. not isothermal) thermal structure along the line of sight, then the projected spectroscopic temperature obtained from the observation is significantly lower than the emission-weighed value inferred directly from hydrodynamical simulation. This implies that much attention should be paid in the theoretical interpretation of observed temperatures.

The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 steradian around the South Galactic Pole

A field of 1.013 sr in the ROSAT All-Sky Survey (RASS), centered on the south Galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources in the field with the distribution of galaxies in the COSMOS digitized database, which was obtained from scanning the plates of the UK Schmidt IIIa-J optical survey of the southern sky. The study used the second ROSAT survey database (RASS-2) and included several optical observing campaigns to measure cluster redshifts. The search, which is a precursor to the larger REFLEX survey encompassing the whole southern sky, reached the detection limits of both the RASS and the COSMOS data and yielded a catalog of 186 clusters in which the lowest flux is 1:5 � 10 � 12 ergs cm � 2 s � 1 in the 0.1–2.4 keV band. Of these 157 have measured redshifts. Using a flux limit of 3:0 � 10 � 12 ergs cm � 2 s � 1 a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to a redshift of 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster ðz <� 0:07Þ, and an orthogonal structure at higher redshift ð0:07 < z < 0:15Þ. This result is consistent with large-scale structure suggested by optical surveys. Subject headings: catalogs — galaxies: clusters: general — X-rays: galaxies: clusters On-line material: machine-readable table

Abell 2142 at large scales: An extreme case for sloshing?

We present results obtained with a new XMM-Newton observation of A2142, a famous textbook example of cluster with multiple cold fronts, which has been studied in detail with Chandra but whose large scale properties are presented here for the first time. We report the discovery of a a new cold front, the most distant one ever detected in a galaxy cluster, at about one Mpc from the center to the SE. Residual images, thermodynamics and metal abundance maps are qualitatively in agreement with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similarly to what recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon, not confined in the cores, which extends well beyond the cooling region involving a large fraction of the ICM up to almost half of the virial radius. The absence of a cool core and a newly discovered giant radio halo in A2142, in spite of its relaxed X-ray morphology, suggest that large scale sloshing, or the intermediate merger which caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core