The RASSCALS:: An X-ray and optical study of 260 galaxy groups

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 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 ct = 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.