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Date and Time of the Query: 2019-07-16 T17:52:39 PDT
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For refcode 2000ApJ...534..114M:
Retrieve 284 NED objects in this reference.
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Copyright by American Astronomical Society. Reproduced by permission
2000ApJ...534..114M The RASSCALS: An X-Ray and Optical Study of 260 Galaxy Groups Andisheh Mahdavi Harvard-Smithsonian Center for Astrophysics, MS 10, 60 Garden Street, Cambridge, MA 02138 Hans Bohringer Max-Planck-Institut fur Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany Margaret J. Geller Harvard-Smithsonian Center for Astrophysics, MS 19, 60 Garden Street, Cambridge, MA 02138 and Massimo Ramella Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34131 Trieste, Italy Received 1999 August 7; accepted 1999 December 10 ABSTRACT 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 m_z_ = 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 {sigma}_p_ < 340 km s^-1^ are overluminous by several orders of magnitude compared to the familiar L_X_ is proportional to {sigma}^4^ law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions {sigma}_p_ < 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 {sigma}_p_ = 400 km s^-1^, and a median X-ray luminosity L_X_ = 3 x 10^42^ h_100_^-2^ ergs s^-1^, where the Hubble constant is H_0_ = 100 h_100_ km s^-1^ Mpc^-1^. We include a catalog of these properties, or the appropriate upper limits, for all 260 groups. Subject headings: galaxies: clusters: general-galaxies: statistics
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