The stellar and hot gas content of low-mass galaxy clusters

Abstract

We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3 × 1014 < M/M⊙ < 6 × 1014 (h= 0.7), as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, Anglo-Australian Telescope and Canada–France–Hawaii Telescope to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7–20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor of 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction

The Complete Local Volume Groups Sample – I. Sample selection and X-ray properties of the high-richness subsample

“…Their sample includes higher mass groups than ours (kT=0.6-2.8 keV), but the key to their high detection rate appears to be the very high quality optical data available from GAMA, which supports the use of detailed substructure tests to exclude unrelaxed systems. The Pearson et al detection rate is actually superior to the 70% achieved by Balogh et al (2011) for a sample of low-mass clusters (3-6×10 14 M , equivalent to ∼3-5 keV). Figure 6 shows a histogram of the numbers of X-ray detections against our optical richness estimator R, and a comparison with Xray luminosity.…”

On optical mass estimation methods for galaxy groups

“…Groups selected in this way have the advantage of avoiding the inclusion of spurious groups, arising from line-of-sight superposition of unrelated galaxies; however, the groups in our sample will be more evolved and relaxed than many groups in optically selected samples. This is seen clearly in the studies of Rasmussen et al (2006) and Balogh et al (2011) which found optically selected samples to contain many groups with little or no X-ray emission. This could result from lack of virialization, or from the effects of strong feedback raising the gas entropy (e.g.…”

“…Zabludoff & Mulchaey 1998;Osmond & Ponman 2004;Rasmussen et al 2006;Balogh et al 2011). In the light of this, there are considerable advantages to employing not just a single mass proxy, but a whole set of them, since different proxies will be robust against different effects.…”

Direct observational evidence for a large transient galaxy population in groups at 0.85 < z < 1

“…This potentially introduces a scientifically interesting bias, if groups with X‐ray emission have fundamentally different populations from groups of similar mass without such emission. However, the main difference between optically and X‐ray selected samples seems to be primarily just that the latter preferentially selects more massive systems (Finoguenov et al 2009; Jeltema et al 2009; Balogh et al 2010). We thus use this catalogue as the basis of our group selection, which we describe in the following subsection.…”