9,421 research outputs found
The compact group--fossil group connection: observations of a massive compact group at z=0.22
It has been suggested that fossil groups could be the cannibalized remains of
compact groups, that lost energy through tidal friction. However, in the nearby
universe, compact groups which are close to the merging phase and display a
wealth of interacting features (such as HCG 31 and HCG 79) have very low
velocity dispersions and poor neighborhoods, unlike the massive, cluster-like
fossil groups studied to date. In fact, known z=0 compact groups are very
seldom embedded in massive enough structures which may have resembled the
intergalactic medium of fossil groups. In this paper we study the dynamical
properties of CG6, a massive compact group at z=0.220 that has several
properties in common with known fossil groups. We report on new g' and i'
imaging and multi-slit spectroscopic performed with GMOS on Gemini South. The
system has 20 members, within a radius of 1 h_70^-1 Mpc, a velocity dispersion
of 700 km/s and has a mass of 1.8 x 10^14 h_70^-1 Msun, similar to that of the
most massive fossil groups known. The merging of the four central galaxies in
this group would form a galaxy with magnitude M_r' ~ -23.4, typical for
first-ranked galaxies of fossil groups. Although nearby compact groups with
similar properties to CG 6 are rare, we speculate that such systems occurred
more frequently in the past and they may have been the precursors of fossil
groups.Comment: 7 pages, 3 figures (one color, low resolution), uses emulateapj.sty.
Accepted for publication in ApJ Lette
Multiple merging in the Abell cluster 1367
We present a dynamical analysis of the central ~1.3 square degrees of the
cluster of galaxies Abell 1367, based on 273 redshift measurements (of which
119 are news). From the analysis of the 146 confirmed cluster members we derive
a significantly non-Gaussian velocity distribution, with a mean location C_{BI}
= 6484+/-81 km/s and a scale S_{BI} = 891+/-58 km/s. The cluster appears
elongated from the North-West to the South-East with two main density peaks
associated with two substructures. The North-West subcluster is probably in the
early phase of merging into the South-East substructure (~ 0.2 Gyr before core
crossing). A dynamical study of the two subclouds points out the existence of a
group of star-forming galaxies infalling into the core of the South-East
subcloud and suggests that two other groups are infalling into the NW and SE
subclusters respectively. These three subgroups contain a higher fraction of
star-forming galaxies than the cluster core, as expected during merging events.
Abell 1367 appears as a young cluster currently forming at the intersection of
two filaments.Comment: 15 pages, 13 figures, 7 tables. Accepted for publication on A&A. High
resolution figures at http://goldmine.mib.infn.it/papers/a1367.htm
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