Aims. We present the second ROSAT all-sky survey source catalogue, hereafter referred to as the 2RXS catalogue. This is the second publicly released ROSAT catalogue of point-like sources obtained ...from the ROSAT all-sky survey (RASS) observations performed with the position-sensitive proportional counter (PSPC) between June 1990 and August 1991, and is an extended and revised version of the bright and faint source catalogues. Methods. We used the latest version of the RASS processing to produce overlapping X-ray images of 6.4° × 6.4° sky regions. To create a source catalogue, a likelihood-based detection algorithm was applied to these, which accounts for the variable point-spread function (PSF) across the PSPC field of view. Improvements in the background determination compared to 1RXS were also implemented. X-ray control images showing the source and background extraction regions were generated, which were visually inspected. Simulations were performed to assess the spurious source content of the 2RXS catalogue. X-ray spectra and light curves were extracted for the 2RXS sources, with spectral and variability parameters derived from these products. Results. We obtained about 135 000 X-ray detections in the 0.1−2.4 keV energy band down to a likelihood threshold of 6.5, as adopted in the 1RXS faint source catalogue. Our simulations show that the expected spurious content of the catalogue is a strong function of detection likelihood, and the full catalogue is expected to contain about 30% spurious detections. A more conservative likelihood threshold of 9, on the other hand, yields about 71 000 detections with a 5% spurious fraction. We recommend thresholds appropriate to the scientific application. X-ray images and overlaid X-ray contour lines provide an additional user product to evaluate the detections visually, and we performed our own visual inspections to flag uncertain detections. Intra-day variability in the X-ray light curves was quantified based on the normalised excess variance and a maximum amplitude variability analysis. X-ray spectral fits were performed using three basic models, a power law, a thermal plasma emission model, and black-body emission. Thirty-two large extended regions with diffuse emission and embedded point sources were identified and excluded from the present analysis. Conclusions. The 2RXS catalogue provides the deepest and cleanest X-ray all-sky survey catalogue in advance of eROSITA.
We use a complete sample of about 140 000 galaxies from the Sloan Digital Sky Survey (SDSS) to study the size distribution of galaxies and its dependence on their luminosity, stellar mass and ...morphological type. The large SDSS data base provides statistics of unprecedented accuracy. For each type of galaxy, the size distribution at given luminosity (or stellar mass) is well described by a log-normal function, characterized by its median and dispersion σln R. For late-type galaxies, there is a characteristic luminosity at Mr,0∼−20.5 (assuming h= 0.7) corresponding to a stellar mass M0∼ 1010.6 M⊙. Galaxies more massive than M0 have and σln R∼ 0.3, while less massive galaxies have and σln R∼ 0.5. For early-type galaxies, the relation is significantly steeper, , but the σln R–M relation is similar to that of bright late-type galaxies. Faint red galaxies have sizes quite independent of their luminosities. We use simple theoretical models to interpret these results. The observed relation for late-type galaxies can be explained if the fraction of baryons that form stars is as predicted by the standard feedback model. Fitting the observed σln R–M relation requires in addition that the bulge/disc mass ratio be larger in haloes of lower angular momentum and that the bulge material transfers part of its angular momentum to the disc. This can be achieved if bulge formation occurs so as to maintain a marginally stable disc. For early-type galaxies, the observed σln R–M relation is inconsistent with formation through single major mergers of present-day discs. It is consistent with formation through repeated mergers, if the progenitors have properties similar to those of faint ellipticals or Lyman break galaxies and merge from relatively strongly bound orbits.
We present the catalogue of the REFLEX Cluster Survey providing information on the X-ray properties, redshifts, and some identification details of the clusters in the REFLEX sample. The catalogue ...describes a statistically complete X-ray flux-limited sample of 447 galaxy clusters above an X-ray flux of 3 x 10 super(-12) erg s super(-1) cm super(-2) (0.1 to 2.4 keV) in an area of 4.24 ster in the southern sky. The cluster candidates were first selected by their X-ray emission in the ROSAT-All Sky Survey and subsequently spectroscopically identified in the frame of an ESO key programme. Previously described tests have shown that the sample is more than 90% complete and there is a conservative upper limit of 9% on the fraction of clusters with a dominant X-ray contamination from AGN. In addition to the cluster catalogue we also describe the complete selection criteria as a function of the sky position and the conversion functions used to analyse the X-ray data. These are essential for the precise statistical analysis of the large-scale cluster distribution. This data set is at present the largest, statistically complete X-ray galaxy cluster sample. Together with these data set we also provide for the first time the full three-dimensional selection function. The sample forms the basis of several cosmological studies, one of the most important applications being the assessment of the statistics of the large-scale structure of the universe and the test of cosmological models. Part of these cosmological results have already been published.
We constructed the composite luminosity function (LF) of clusters of galaxies in the five SDSS photometric bands u, g, r, i and z from the RASS-SDSS galaxy cluster catalog. Background and foreground ...galaxies are subtracted using both a local and a global background correction to take in account the presence of large scale structures and field to field variations, respectively. The composite LF clearly shows two components: a bright-end LF with a classical slope of -1.25 in each photometric band, and a steeper faint-end LF (-2.1 less than or equal to alpha less than or equal to -1.6) in the dwarf galaxy region. The observed upturn of the faint galaxies has a location ranging from -16 + 5 log(h) in the g band to -18.5 + 5 log(h) in the z band. To study the universality of the cluster LF we compare the individual cluster LFs with the composite luminosity function. In agreement with the composite LF, a single Schechter component is not a good fit for the majority of the clusters. We fit a Schechter function to the bright-end of the individual cluster LFs in the magnitude region brighter than the observed upturn of the dwarf galaxies. The bright-end of the galaxy clusters shows the same shape in all the systems. To study the behavior of the individual faint-end LF we define the Dwarf to Giant galaxy Ratio (DGR) of the single clusters. The distribution of DGR has a spread much larger than the statistical errors. The DGR clearly anti-correlates with both X-ray and optical cluster luminosities. This anti-correlation is most likely due to the choice of a fixed metric aperture for all the clusters. Therefore, because of this effect, the different cluster physical sizes must be taken into account before comparing the LF of different clusters.
We use the RASS-SDSS galaxy cluster sample to compare the quality of optical and X-ray luminosities as predictors of other cluster properties such as their masses, temperatures, and velocity ...dispersions. We use the SDSS spectroscopic data to estimate the velocity dispersions and the virial masses of a subsample of 69 clusters within r sub(500) and r sub(200). The ASCA temperature of the intra-cluster medium, T sub(x), is retrieved from the literature for a subsample of 49 clusters. For this subsample we estimate the cluster masses also by using the mass-temperature relation. We show that the optical luminosity, L sub(op), correlates with the cluster mass much better than the X-ray luminosity, L sub(x). L sub(op) can be used to estimate the cluster mass with an accuracy of 40% while L sub(x) can predict the mass only with a 55% accuracy. We show that correcting L sub(x) for the effect of a cool core at the center of a cluster lowers the scatter of the L sub(x) - M relation only by 3%. We find that the scatter observed in the L sub(op) - L sub(x) relation is determined by the scatter of the L sub(x) - M relation. The mass-to-light ratio in the SDSS i band clearly increases with the cluster mass with a slope 0.2 plus or minus 0.08. The optical and X-ray luminosities correlate in an excellent way with both T sub(x) and sigma sub(v) with an orthogonal scatter of 20% in both relations. Moreover, L sub(op) and L sub(x) can predict variables with the same accuracy both. We conclude that the cluster optical luminosity is a key cluster parameter since it can give important information about fundamental cluster properties such as the mass, the velocity dispersion, and the temperature of the intra-cluster medium.
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 identification 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 first 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-defined 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 final identification 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 find 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 final cluster sample and show results on the cluster number counts. The final sample of identified 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 first 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 sufficient for the use of this cluster sample for cosmological tests.
We present a 90 per cent flux-complete sample of the 201 X-ray-brightest clusters of galaxies in the northern hemisphere (δ ≥ 0°), at high Galactic latitudes (ǀbǀ ≥ 20°), with measured redshifts z ≤ ...0.3 and fluxes higher than 4.4 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All-Sky Survey data and is the largest X-ray-selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X-ray-brightest Zwicky clusters and other clusters selected from their X-ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non-cluster X-ray sources. X-ray fluxes are computed using an algorithm tailored for the detection and characterization of X-ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1σ error). We find the cumulative log N—log S distribution of clusters to follow a power law κ S−α with α = 1.31+0.06−0.03 (errors are the 10th and 90th percentiles) down to fluxes of 2 × 10−12 erg cm−2 s−1, i.e. considerably below the BCS flux limit. Although our best-fitting slope disagrees formally with the canonical value of −1.5 for a Euclidean distribution, the BCS log N—log S distribution is consistent with a non-evolving cluster population if cosmological effects are taken into account. Our sample will allow us to examine large-scale structure in the northern hemisphere, determine the spatial cluster—cluster correlation function, investigate correlations between the X-ray and optical properties of the clusters, establish the X-ray luminosity function for galaxy clusters, and discuss the implications of the results for cluster evolution.
We present an essentially complete, all-sky, X-ray flux-limited sample of 242 Abell clusters of galaxies (six of which are double) compiled from ROSAT All-Sky Survey data. Our sample is ...uncontaminated in the sense that systems featuring prominent X-ray point sources such as AGN or foreground stars have been removed. The sample is limited to high Galactic latitudes (∣b∣≥ 20°), the nominal redshift range of the ACO catalogue of z ≤ 0.2, and X-ray fluxes above 5.0 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. Owing to the X-ray flux limit, our sample consists, at intermediate and high redshifts, exclusively of very X-ray-luminous clusters. Since the latter tend to be also optically rich, the sample is not affected by the optical selection effects and, in particular, not by the volume incompleteness known to be present in the Abell and ACO catalogues for richness class 0 and 1 clusters. Our sample is the largest X-ray flux-limited sample of galaxy clusters compiled to date and will allow investigations of unprecedented statistical quality into the properties and distribution of rich clusters in the local Universe.
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