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 } \propto \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 9 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 \times 10 ^ { 42 } h _ { 100 } ^ { -2 } erg 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 .