A Spectroscopic Census of X-Ray Systems in the COSMOS Field

Abstract

We investigate spectroscopic properties of galaxy systems identified based on deep X-ray observations in the COSMOS field. The COSMOS X-ray system catalog we use (George et al. 2011) includes 180 X-ray systems to a limiting flux of 1.0 × 10 −15 erg cm −2 s −1 , an order of magnitude deeper than future e-ROSITA survey. We identify spectroscopic members of these X-ray systems based on the spectroscopic catalog constructed by compiling various spectroscopic surveys including 277 new measurements; 137 X-ray systems are spectroscopically identified groups with more than three spectroscopic members. We identify 1843 spectroscopic redshifts of member candidates in these Xray systems. The X-ray luminosity (L X ) -velocity dispersion (σ v ) scaling relation of the COSMOS X-ray systems is consistent with that of massive X-ray clusters. One of the distinctive features of the COSMOS survey is that it covers the X-ray luminosity range where poor groups overlap the range for extended emission associated with individual quiescent galaxies. We assess the challenges posed by the complex morphology of the distribution of low X-ray luminosity systems, including groups and individual quiescent galaxies, in the L x − σ v plane.

Scaling Properties of Galaxy Groups

“…Based upon a cleaned sample of 74 groups, ref. [143] showed that the relation for individual groups appears to follow a shallower relation than clusters (consistent with that found by previous studies, e.g., [137]). However, they note that this trend may be affected by a small number of groups that appear to have anomalously low velocity dispersions (at σ v 125 km s −1 ) for their measured X-ray luminosity (discussed further in Section 3.4).…”

“…One observation made by various authors studying the L x -σ v relation, is the presence of low velocity dispersion groups (appearing at σ v 200 km s −1 ) that have a high X-ray luminosity in comparison to their σ v (conversely, it can be stated that these groups have a low σ v for their L x ). These low velocity outliers have been noted in various studies in the literature (e.g., [137,162]), attributed as the cause of the flattening of the L x -σ v relation at the group scale (e.g., Vajgel et al [149], Sohn et al [143]). Although it has been shown in Section 3.2 that the group scale relations may be consistent with self-similar predictions when accounting for the differing emissivity, the presence of these outliers are extreme cases.…”

“…Ref. [143] use galaxy redshift information from a wide variety of surveys in the literature and associate them with the X-ray detected groups, compiling a final sample of 146 groups with at least three spectroscopic members. Based upon a cleaned sample of 74 groups, ref.…”

“…Unless otherwise stated, references consider (bolometric) luminosities and temperatures derived within an estimate of R 500 . Please note that the Sohn et al (2019) relation is based upon an analysis using 7 bins of using the full sample of 74 systems. * refers to references using 0.1-2.4 keV band luminosities.…”

“…This study constructs a catalog of galaxy groups based upon those identified in George et al (2011), using XMM-Newton and Chandra observations of the COSMOS field, reaching an X-ray flux limit of ∼10 −15 erg s −1 cm −2 . Sohn et al (2019) use galaxy redshift information from a wide variety of surveys in the literature and associate them with the X-ray detected groups, compiling a final sample of 146 groups with at least three spectroscopic members. Based upon a cleaned sample of 74 groups, Sohn et al (2019) showed that the relation for individual groups appears to follow a shallower relation than clusters (consistent with that found by previous studies, e.g., Mahdavi & Geller 2001).…”