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 σ_p<340 km s^-1 are overluminous by several orders of magnitude compared to the familiar L_X~σ⁴ law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions σ_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 σ_p=400 km s^-1, and a median X-ray luminosity L_X=3×10⁴² 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.