Download C. Ferrari et al.

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 d’Azur, 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:
A3921-B
1) the presence of several substructures
2) a large scale bimodal morphology (A3921-A and A3921-B)
3) an offset of the BCG from the main density peak of the
clump A
A3921-A
BCG
 system out of dynamical equilibrium
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
1) The kinematical properties
of the whole sample and of the
two subclusters do not show
strong signatures of merging
2) The dynamics of the central
regions of the two clumps
appears to be relatively
unaffected by the merging
event 
Subsample
Ngal
Vmean
(km/s)
v
(km/s)
Skewness
(%)
Kurtosis
(%)
Whole
cluster
104
28047  76
831  90
 10
 10
A3921-A
41
28017  150
1008  130
> 10
> 20
A3921-B
20
27920  87
451  150
< 10
> 20
Properties of the radial velocity distribution and
significance level of shape estimators for the whole
cluster and the two sub-clusters
virial mass estimate:
MA/MB  5
Finding chart (27’20’) of A3921 galaxies
with measured redshifts
R-Band image from [email protected]
1
5
’
Spectroscopy from [email protected]
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
The bulk of X-ray emission in A3921B is offset towards SW from the
main concentration of galaxies:
+
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)
A3921-B is tangentially traversing
A3921-A along a SW/NE direction
 The merger is in its central phase (0.0  0.3 Gyr)
 Collision in the plane of the sky
Star Formation Properties of Cluster Members
A3921 galaxies divided in different spectral types (following Dressler et al. 1999):
1) k (passive, old population of stars) – 71 %
2) k+a (recent star formation) – 16 %
3) e (ongoing star formation) – 13 %
- e-type galaxies
= k+a candidates (inversion of H-K lines)
= confirmed k+a
= emission line 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
- k+a 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
Velocity distributions of
the different spectral
types