Optical and X‐ray structures of galaxy clusters. I

Abstract

The differences between optical and X-ray structures of galaxy clusters are discussed. We analyse in detail 7 Abell clusters. There is an increasing number of arguments in favour of different distributions of galaxies, gas, and dark matter in many dusters. We argue that most clusters present sub-structures at least in the gaseous and galactic components and, moreover, the subclustering of different components does not always coincide. Such arguments strongly support the idea that most galaxy clusters are by far more complex systems than accepted until now and are usually not in an hydrostatic isothermal equilibrium.Key words: galaxy clusters -structures -dominant galaxies A A A subject classification: 160 IntroductionGalaxy clusters are normally characterized by the BM types I, 11, and I11 or by the RS types cD, B, L, F, and I. Correlations between these morphological cluster types and the physical properties of the clusters are expected. This expectation concerns among others the X-ray properties. However many authors find only marginal evidence for such a correlation. In this connection the question arises which kind of observation is better suited for the description of the physical properties of galaxy clusters. Very often it is argued that optical observations are plagued by projection effects and by small number statistics. It is assumed that X-ray observations of clusters can overcome these problems because the relaxation time-scale of the X-ray emitting gas is much shorter than the age of a cluster. Thus, it is usually assumed that the gas is relaxed and the X-ray emission can be used as a tracer of the gravitational potential of clusters. A second point is that many galaxy clusters are observed to be commonly bright in X-rays due to the high gas densities in their cores. Therefore, they should be detected out to large redshifts making them useful probes of cosmological studies.Since all components of a given cluster share the total gravitational potential, it is assumed that both the intra-cluster medium and galaxy number densities are related to the potential well of the cluster in a similar way, or that dark matter, hot gas, and galaxy distribution evolve in a tightly coupled way (Frenk et al. 1995). Traditionally an hydrostatic equilibrium of both the hot intra-cluster gas and the galaxies with the binding cluster potential is assumed (e.g., Wu 1994, Lubin and Bahcalll993).In connection with all these theoretical considerations the distributions of different components in clusters (galaxies, gas, and dark matter) should show the same behaviour. The usefulness of the observation of one or more components is considered to be determined only by the possibility of observations. In principle, results from all such kinds of observations -galaxy distribution or X-ray emissivity, for example -should lead to equivalent results.However, the very observation of sub-structures in the majority of rich clusters, the different scale lengths of the various components in a cluster, or the so-called &problem, point to ...

Substructure and dynamical state of 2092 rich clusters of galaxies derived from photometric data

“…'merger' means the cluster shows merging signatures; 'radio' means the cluster has radio halo/relic; 'cool' means the cluster has a cool core in X-ray. Reference for 'merger', 'radio' and 'cool' in column (10): ak13 (Akamatsu & Kawahara 2013); all00 (Allen 2000); am04 (Andersson & Madejski 2004); ars+08 (Allen et al 2008); bbg+07 (Barrena et al 2007); bbg09 ); bbv+12 (Bonafede et al 2012); bfs+05 ; bgb+04 (Boschin et al 2004); bgb+09 ); bhs05 (Buote et al 2005); blw+96 (Baier et al 1996); bps+04 (Belsole et al 2004); bro+94 (Burns et al 1994); brp+95 (Burns et al 1995 Data Release 8 1 (SDSS DR8, Aihara et al 2011) to discriminate luminous member galaxies and to estimate cluster richness and radius which will be used in the quantification of cluster dynamical state. If any objects in the SDSS DR8 have deblending problems and saturated, they are discarded using the flags 2 .…”

“…'merger' means the cluster shows merging signatures; 'radio' means the cluster has radio halo/relic; 'cool' means the cluster has a cool core in X-ray. Reference for 'merger', 'radio' and 'cool' in column (10): ak13 (Akamatsu & Kawahara 2013); all00 (Allen 2000); am04 (Andersson & Madejski 2004); ars+08 (Allen et al 2008); bbg+07 (Barrena et al 2007); bbg09 (Boschin et al 2009); bbv+12 (Bonafede et al 2012); bfs+05 (Bauer et al 2005); bgb+04 (Boschin et al 2004); bgb+09 (Barrena et al 2009); bhs05 (Buote et al 2005); blw+96 (Baier et al 1996); bps+04 (Belsole et al 2004); bro+94 (Burns et al 1994); brp+95 (Burns et al 1995); cc11 (Cohen & Clarke 2011); cls+05 (Cypriano et al 2005); cmk+02 (Czoske et al 2002); cpl+08 (Capelato et al 2008); dfj+01 (Donnelly et al 2001); dk04 (David & Kempner 2004); dsc03 (De Filippis et al 2003); dvo+05 (Donahue et al 2005); fbb+81 (Forman et al 1981); fff+98 (Fisher et al 1998); fgg+12 (Feretti et al 2012); gbb06 (Girardi et al 2006); gff+12 (Govoni et al 2012); gmv+04 (Govoni et al 2004); gtv+08 (Gastaldello et al 2008); hmd10 (Hicks et al 2010); hsd09 (Holhjem et al 2009); jsc+08 (Juett et al 2008); ksm03 (Kempner et al 2003); ksr02 (Kempner et al 2002); mbz+04 (Marini et al 2004); me12 (Mann & Ebeling 2012); mnr+04 (Majerowicz et al 2004); mol+03 (Morrison et al 2003); mp...…”

Substructures and galaxy orientations in clusters II. Cluster Abell 14

“…Because of its low X-ray luminosity the Abell 14 cluster is comparable with clusters Abell 569 and Abell 2666 discussed by Baier et al (1996). All of the three clusters should be characterized as poor clusters.…”

“…Barnes 1989;Bode et al 1994; We cannot exclude the possibility of encounters between these clusters and a following merging one. Therefore, the future formation of a structure similar to that in the case of Abell 754 (Baier et al 1996;Godłowski et al 1998) is possible. In this connection, we should consider such clusters as Abell 14 to be stepping stones for the formation of larger structures on the basis of the bottom-up scenario for the evolution of cosmic structures.…”

X-ray morphological estimators for galaxy clusters

“…For example, the morphological classes identified by Jones & Forman (1992) were: 'single','double', 'primary with small secondary', 'complex', 'elliptical' (referred to the X-ray contours), 'off-center' (either presenting a difference between the centers in optical and Xray or showing an X-ray tail extended only in one sector off the X-ray peak), and 'galaxy' (when the X-ray emission is dominated by the central galaxy emission). A very similar classification was proposed by Baier et al (1996) who compared the substructures detected in X-ray with those known in optical wavelengths (see also Kolokotronis et al 2001). These visual estimates appeared to be robust (Jones & Forman 1999) when compared with a more quantitative measurement of Xray-morphology disturbance such as the power ratios (Buote & Tsai 1995, or axial ratios .…”