Diapositive 1

1
Dynamical state and star formation properties of
the merging galaxy cluster Abell 3921 C. Ferrari
1,2, C. Benoist 1, S. Maurogordato 1, A. Cappi 3,
E. Slezak 1 1 Observatoire de la Côte dAzur,
Nice, France - 2 Institut für Astrophysik,
Innsbruck, Austria - 3 Osservatorio Astronomico
di Bologna, Italy
Abstract In the framework of the hierarchical
model of structure formation, galaxy clusters
form through accretion and merging of
substructures of smaller mass. Merging clusters
are a privileged laboratory to test the physics
of evolutionary effects on their members. In
particular, it is still debated how star
formation is affected by the merging process
(e.g. Evrard 1991, Fujita et al. 1999). In this
context, after characterizing the dynamical state
of the merging cluster Abell 3921, we analyze its
star formation properties.

• Projected Morphology
• A3921 is characterized by
• the presence of several substructures
• a large scale bimodal morphology (A3921-A and
  A3921-B)
• an offset of the BCG from the main density peak
  of the clump A
• system out of dynamical equilibrium

A3921-B
A3921-A
BCG
Projected galaxy density map, built on the basis
of a multi-scale approach and using red sequence
galaxies, on a 34 ? 34 arcmin2 field centered on
A3921 and for three magnitude cuts. The two main
sub-clusters are indicated by rectangles. The
white crosses indicate the positions of the four
brightest cluster galaxies
Velocity distribution of the whole dataset

• Cluster Kinematics
• The kinematical properties of the whole sample
  and of the two subclusters do not show strong
  signatures of merging
• The dynamics of the central regions of the two
  clumps appears to be relatively unaffected by the
  merging event ?
• virial mass estimate
• MA/MB ? 5

Properties of the radial velocity distribution
and significance level of shape estimators for
the whole cluster and the two sub-clusters
Finding chart (27?20) of A3921 galaxies with
measured redshifts R-Band image from
ESO-WFI_at_2.2m Spectroscopy from ESO-EFOSC2_at_3.6m
15
Velocity distributions of A3921-A and of A3921-B
X-ray residuals after subtraction of a 2D-? model
(Belsole et al. 2004) overlaid on the red
sequence galaxy density map of the central part
of A3921 (22?? 18 arcmin2)

• Dynamical State
• Optical Morphology
• Cluster Kinematics
• Presence of an extended hot region oriented
  parallel to the line joining the centers of
  A3921-A and A3921-B (XMM observations, Belsole et
  al. 2004)
• The merger is in its central phase (0.0 ? 0.3
  Gyr)
• Collision in the plane of the sky

The bulk of X-ray emission in A3921-B is offset
towards SW from the main concentration of
galaxies A3921-B is tangentially traversing
A3921-A along a SW/NE direction

• Star Formation Properties of Cluster Members
• A3921 galaxies divided in different spectral
  types (following Dressler et al. 1999)
• k (passive, old population of stars) 71
• ka (recent star formation) 16
• e (ongoing star formation) 13
• - e-type galaxies
• share neither the same kinematics nor the same
  projected distribution than passive cluster
  members they are concentrated in the collision
  region
• The star formation episode could be the result of
  an interaction with the ICM, connected with the
  dynamical state of the cluster
• - ka galaxies
• show signature of an old star formation (1-1.5
  Gyr) activity that can hardly be related to the
  on-going merger
• Ongoing spectroscopic follow-up up to the virial
  radius may clarify this point

ka candidates (inversion of H-K lines)
confirmed ka emission line galaxies
Velocity distributions of the different spectral
types