Issue |
A&A
Volume 399, Number 3, March I 2003
|
|
---|---|---|
Page(s) | 813 - 828 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20021741 | |
Published online | 14 February 2003 |
Multiple merging events in Abell 521 *,**
1
CERGA, UMR 6527, CNRS, Observatoire de la Côte d'Azur, BP 4229, Le Mont-Gros, 06304 Nice Cedex 4, France
2
INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
Corresponding author: C. Ferrari, chiara.ferrari@obs-nice.fr
Received:
12
August
2002
Accepted:
12
November
2002
We present a detailed spatial and dynamical analysis of
the central
~2.2 Mpc region of the galaxy cluster Abell 521 (
0.247), based on
238 spectra (of which 191 new measurements) obtained
at the 3.6 m Telescope of the European Southern Observatory and at
the Canada-France-Hawaii Telescope.
From the analysis of the 125 galaxies that are confirmed members of the
cluster, we derive a location (“mean” velocity) of
km s-1 and detect a complex
velocity distribution with high velocity scale (“dispersion”,
km s-1), but clear departure
from a single Gaussian component. When excluding a possible background
group of four galaxies, the velocity dispersion
remains still large (~1200 km s-1).
The general structure of the
cluster follows a North-West/South-East direction, crossed by a
perpendicular high
density “ridge” of galaxies in the core region.
The Northern region of the cluster is characterized by a lower
velocity dispersion as compared to the whole cluster value; it hosts
the BCG and a dynamically bound complex of galaxies, and it is
associated with a group detected in X-ray (Arnaud et al. 2000).
This region could be in a stage of pre-merger onto the
main cluster. The small offset (
+250 km s-1) in the mean
velocity of the northern region as compared to the whole cluster
suggests that the merging occurs partly in the plane of the sky.
These results, taken together with the fact that most of the
clumps detected on the isodensity maps, as well as
the early-type galaxies and the brightest ones
(
) are aligned, suggest that this North-West/South-East direction
is the preferred one for the formation of this cluster.
The central high dense region (“ridge”) shows a lower velocity location
(
km s-1) and significantly
higher scale (
km s-1) as compared to the whole
cluster values. This is due to the presence of a low-velocity
group of galaxies with a high fraction of emission line objects. This
can be explained in a scenario in which a merging of subclusters has
recently occurred along the direction of the “ridge” with a
significant component along the line of sight. The low-velocity group
would then be a high-speed remnant of the collision
which would have also triggered an episode of intense star formation
responsible for the large fraction of late-type objects in this
region.
Key words: galaxies: clusters: general / galaxies: clusters: individual: Abell 521 / galaxies: distances and redshifts / cosmology: observations
© ESO, 2003
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