The RASSCALS: An X‐Ray and Optical Study of 260 Galaxy Groups

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 σ p < 340 km s −1 are overluminous by several orders of magnitude compared to the familiar L X ∝ σ 4 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 9 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 × 10 42 h −2 100 erg s −1 , where the Hubble constant is H 0 = 100h 100 km s −1 Mpc −1 . We include a catalog of these properties, or the appropriate upper limits, for all 260 groups.

X-Ray-Selected Galaxy Groups in Boötes

“…But, what is observed from the upper panel of Figure 8 is that groups with σ gr ≤ 200 km s −1 have higher L X in comparison with their σ gr . This same phenomenon was observed before by dell 'Antonio et al (1994); Mahdavi et al (2000); Mahdavi & Geller (2001); Helsdon et al (2005); Rines & Diaferio (2010).…”

“…In studies of poor systems, Mahdavi et al (2000) and Xue & Wu (2000) found L X ∝ σ 1.38±0.4 and L X ∝ σ 2.35±0.21 , flatter than the theoretical expectation for a self-similar model, L X ∝ σ 4 . Helsdon & Ponman (2000) (L X ∝ σ 2.4±0.4 ) and Osmond & Ponman (2004) (L X ∝ σ 2.31±0.62 ) also found significantly flatter relations in groups.…”

Shakhbazian compact galaxy groups

“…Dell'Antonio et al (1994) and Ponman et al (1996) showed that rich groups follow the L X ∼ σ 4 v relation, but groups with smaller RVD (<300 km s −1 ) have a more shallow slope. Mahdavi et al (2000) showed that the correlation between L X and σ v may be best represented by a broken power law with a ∼ 1.4 for galaxy groups and a ∼ 4 for galaxy clusters. Figure 20 shows that ShCGs represent the low-mass extension of clusters and have sufficiently large dispersion 5 .…”

AnXMM-Newtonstudy of the RGH 80 galaxy group

“…It was identified as an extended X-ray source in the ROSAT All-Sky Survey with an X-ray luminosity of 3.33 × 10 43 h −2 50 erg s −1 in the 0.1-2.4 keV band within an aperture of 1.0 h −1 50 Mpc (NRGs241; Mahdavi et al 2000). Global fitting of the ASCA SIS and GIS data by Davis et al (1999) yielded a mean gas temperature of 1.02 ± 0.05 keV, an average abundance of 0.26 that a two-temperature spectral model provides a better fit to the ASCA spectra of RGH 80 and has a metallicity that is substantially higher than that obtained by the single temperature spectral model, Z = 0.67…”