X-ray–optical classification of cluster mergers and the evolution of the cluster merger fraction

By A. Mann, H. Ebeling. 2011

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We present the results of a simple but robust morphological classification of a statistically complete sample of 108 of the most X-ray-luminous clusters at 0.15 ≤z≤ 0.7 observed with Chandra. Our aims are to (a) identify the most disturbed massive clusters to be used as gravitational lenses for studies of the distant Universe and as probes of particle acceleration mechanisms resulting in non-thermal radio emission; (b) find cluster mergers featuring subcluster trajectories that make them suitable for quantitative analyses of cluster collisions; and (c) constrain the evolution with redshift of the cluster merger fraction. Finally, (d) this paper represents the third public release of clusters from the MAssive Cluster Survey sample, adding 24 clusters to the 46 published previously. To classify clusters by the degree of relaxation, we use the projected offset of the brightest cluster galaxy from the peak (or the global centroid) of the X-ray emission as a measure of the segregation between the intracluster gas and dark matter, and also perform a visual assessment of the optical and X-ray morphology on all scales. Regarding (a), we identify 10 complex systems likely to have undergone multiple merger events in the recent past. Regarding (b), we identify 11 systems likely to be post-collision, binary, head-on mergers (BHOMs), as well as another six mergers that are possible BHOMs but probably harder to interpret because of non-negligible impact parameters and merger axes closer to our line of sight. Regarding (c), we find a highly significant increase with redshift in the fraction of morphologically disturbed clusters (and thus a clear decrease in the number of fully relaxed systems) starting at z∼ 0.4, in spite of a detection bias in our sample against very disturbed systems at high redshift. Since our morphological diagnostics are all based on imaging data and thus sensitive to projection effects, the measured merger fractions should be considered lower limits and our list of mergers incomplete, as we are likely to miss systems forming along axes close to our line of sight. A larger sample of clusters with high-quality X-ray data in particular at high redshift will be needed to trace the evolutionary history of cluster growth and relaxation closer to the primary epoch of cluster formation z∼ 1.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/86f460f6adc8321cf286040f5bedc75828f1ff79

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ARE ULTRA-FAINT GALAXIES AT z = 6–8 RESPONSIBLE FOR COSMIC REIONIZATION? COMBINED CONSTRAINTS FROM THE HUBBLE FRONTIER FIELDS CLUSTERS AND PARALLELS (2015, methodology): “Recently, using HST imaging of 25 Xray-selected clusters in the Abell (Abell et al. 1989) and MACS (Ebeling et al. 2001, 2007, 2010; Mann & Ebeling 2012) catalogs, the CLASH program (Postman et al. 2012) has made important progress in the characterization of the lensing properties of clusters.”
LENS MODELS AND MAGNIFICATION MAPS OF THE SIX HUBBLE FRONTIER FIELDS CLUSTERS (2014, unknown): “5+2223 (z = 0.543), Abell S1063 (z = 0.348), and Abell 370 (z = 0.375) (Babyk et al. 2012; Mann & Ebeling 2012; Ebeling et al. 2007, 2001; Böhringer et al. 2004; White 2000; Struble & Rood 1987; Abell et al. 1989; Abell 1958).”
Mass and magnification maps for the Hubble Space Telescope Frontier Fields clusters: implications for high-redshift studies (2014, unknown): “1−2403 0.396 Mann & Ebeling (2012) MACS J0717.”
CLASH: THE ENHANCED LENSING EFFICIENCY OF THE HIGHLY ELONGATED MERGING CLUSTER MACS J0416.1−2403 (2012, background): “Mann & Ebeling (2012) classified it as merging based on the double-peaked X-ray structure, where the southern peak is offset by a few arcseconds from the corresponding (second) BCG, as expected in mergers (e.g. Bradač et al. 2006; Merten et al. 2011).”
Weighing the Giants - I. Weak-lensing masses for 51 massive galaxy clusters: project overview, data analysis methods and cluster images (2012, result): “We therefore confirm previous studies that noted the correlation between BCG – X-ray offset and dynamical state of the cluster (e.g. Allen 1998; Smith et al. 2005; Bildfell et al. 2008; Sanderson et al. 2009; Hudson et al. 2010; Mann & Ebeling 2012).”
RELICS: Reionization Lensing Cluster Survey (2019, background): “…Galaxy Cluster Survey (RXC; Ebeling et al. 1998, 2000a); the ROSAT MAssive Cluster Survey (MACS and SMACS; Ebeling et al. 2001, 2007, 2010, 2013; Mann & Ebeling 2012; Repp et al. 2016; Repp & Ebeling 2018); an extended radio source 2 http://archive.stsci.edu/hst/search.php 3 The Frontier Fields…”
HUBBLE SPACE TELESCOPE COMBINED STRONG AND WEAK LENSING ANALYSIS OF THE CLASH SAMPLE: MASS AND MAGNIFICATION MODELS AND SYSTEMATIC UNCERTAINTIES (2014, background): “Although M0429 (z = 0.399) has been subject to various X-ray studies (Schmidt & Allen 2007; Comerford & Natarajan 2007; Maughan et al. 2008; Allen et al. 2008; Mann & Ebeling 2012), we did not find any previous SL, nor WL, analyses for this cluster outside the CLASH framework.”
The Frontier Fields: Survey Design and Initial Results (2016, background): “Its bolometric X-ray luminosity is Lx = 1.02 × 10 45 erg s−1, with a double-peaked profile suggestive of a merging cluster (Mann & Ebeling 2012).”
The SPTpol Extended Cluster Survey (2019, background): “This should generally have a small effect; studies with X-ray centering proxies (see, e.g., Mann & Ebeling 2012) have found an additional 20–60 kpc (~ l lR R0.”