Structure and Dynamics of the Coma Cluster

Abstract

We examine the structure and dynamics of the galaxies in the Coma cluster using a catalog of 552 redshifts including 243 new measurements. The velocity distribution is shown to be non-Gaussian due to structure associated with the group of galaxies around NGC4839, 40 arcmin SW of the cluster core. We apply a mixture-modelling algorithm to the galaxy sample and obtain a robust partition into two subclusters which we use to examine the system's dynamics. We find that the late-type galaxies are freely-falling into a largely virialised cluster core dominated by early types. We obtain a virial mass for the main cluster in close agreement with the estimates derived from recent X-ray data. The mass of the NGC4839 group is about 5-10% the mass of the main cluster. Assuming the main cluster and the NGC4839 group follow a linear two-body orbit, the favored solution has the two clusters lying at 74 degrees to the line of sight at a true separation of 0.8 Mpc and moving together at 1700 km/s. The cluster core shows evidence of an ongoing merger between two subclusters centered in projection on the dominant galaxies NGC4874 and NGC4889 but offset in velocity by 300 km/s and 1100 km/s respectively. Combining these results with X-ray and radio observations, and an interpretation of the presence or lack of an extended halo around the dominant galaxies, we develop a merger history for the Coma cluster.Comment: To appear in Ap.J., 43 pages, Postscript of text only, see http://msowww.anu.edu.au/~colless/Preprints/coma.html for text, tables and figure

Evidence for a pressure discontinuity at the position of the Coma relic from Planck Sunyaev–Zel'dovich effect data

“…We put the centre of this second component at (RA, Dec) = (12 h 56 m 59 s , +27 • 29 27 ) and fix all the shape parameters of its pressure profile to the "universal" shape given by Arnaud et al (2010). The pressure amplitude of the group is fixed to 10% of the mass of the Coma cluster, which is the upper limit from dynamical observations (Colless & Dunn 1996). This leads to a PTE of only 0.005, implying that NGC 4839 with 10% of the Coma mass is insufficient to produce the observed pressure excess.…”

“…Observations with XMM-Newton (Neumann et al 2001) revealed a complex temperature structure around the group, which suggests the group to be at its first infall into Coma. We model the group as a second GNFW component with 1/10 the mass of the Coma cluster, which is the upper mass limit derived from a dynamical analysis (Colless & Dunn 1996). Alternatively, we let the mass be a free parameter, i.e.…”

Warm gas in and around simulated galaxy clusters as probed by absorption lines

“…In this paper, we use Rvir to define the virial radius of a galaxy cluster, such that the mean density inside this radius is 100 times the critical density of the universe at that redshift, consistent with the definitions in Bryan & Norman (1998 Colless & Dunn (1996a) 972 Table 1. The virial radius, virial mass, ICM gas temperature, and velocity dispersion of the Virgo and Coma galaxy clusters in comparison with the simulated Virgo-like and Coma-like clusters.…”

Clusters of Galaxies: Setting the Stage

“…At low redshifts, there are many other examples of merging clusters or galaxy groups falling onto clusters along filamentary structure. To mention a few: Coma (Colless and Dunn 1996;Adami et al 2005), A 521 , A 754 (Henry et al 2004), A 2199 (Rines et al 2001), A 2219 (Boschin et al 2004), A 3560 in the Shapley supercluster (Bardelli et al 2002), A 3921 . At high redshift the number of irregular and merging clusters increases substantially (Rosati 2004;Kodama et al 2007).…”