The RASSCALS: an X-ray and optical study of 260 galaxy groups.
MAHDAVI A., BOHRINGER H., GELLER M.J. and RAMELLA M.
Abstract (from CDS):
We describe the ROSAT All-Sky Survey–Center for Astrophysics Loose Systems (RASSCALS), the largest X-ray and optical survey of low-mass galaxy groups to date. We draw 260 groups from the combined Center for Astrophysics and Southern Sky Redshift Surveys, covering one-quarter of the sky to a limiting Zwicky magnitude of mz=15.5. We detect 61 groups (23%) as extended X-ray sources. The statistical completeness of the sample allows us to make the first measurement of the X-ray selection function of groups, along with a clean determination of their fundamental scaling laws. We find robust evidence of similarity breaking in the relationship between the X-ray luminosity and velocity dispersion. Groups with σp<340 km.s–1 are overluminous by several orders of magnitude compared to the familiar LX∝σ4 law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions σp<150 km.s–1. After accounting for selection effects, we conclude that only 40% of the optical groups are extended X-ray sources. The remaining 60% are either accidental superpositions or systems devoid of X-ray emitting gas. Combining our results with group statistics from N-body simulations, we find that the fraction of real, bound systems in our objectively selected optical catalog is between 40%-80%. The X-ray detections have a median membership of nine galaxies, a median recession velocity cz=7250 km.s–1, a median projected velocity dispersion σp=400 km.s–1, and a median X-ray luminosity LX=3x1042 h–2100 ergs.s–1, where the Hubble constant is H0=100 h100 km.s–1.Mpc–1. We include a catalog of these properties, or the appropriate upper limits, for all 260 groups.
Galaxies: Clusters: General - Galaxies: Statistics
VizieR on-line data:
<Available at CDS (J/ApJ/534/114): table2.dat table3.dat table4.dat>