Multiwavelenght study of cluster mergers and consequences for the radio emission properties of galaxy clusters
Creators: name: Giacintucci, Simona <1975>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2007
Subjects: FIS/05 Astronomia e astrofisica

description: In the present thesis a thourough multiwavelength analysis of a number of galaxy
clusters known to be experiencing a merger event is presented.
The bulk of the thesis consists in the analysis of deep radio observations of
six merging clusters, which host extended radio emission on the cluster scale. A
composite optical and X–ray analysis is performed in order to obtain a detailed and
comprehensive picture of the cluster dynamics and possibly derive hints about the
properties of the ongoing merger, such as the involved mass ratio, geometry and time
scale. The combination of the high quality radio, optical and X–ray data allows us
to investigate the implications of the ongoing merger for the cluster radio properties,
focusing on the phenomenon of cluster scale diffuse radio sources, known as radio
halos and relics.
A total number of six merging clusters was selected for the present study:
A3562, A697, A209, A521, RXCJ 1314.4–2515 and RXCJ 2003.5–2323.
All of them were known, or suspected, to possess extended radio emission on the
cluster scale, in the form of a radio halo and/or a relic.
High sensitivity radio observations were carried out for all clusters using the
Giant Metrewave Radio Telescope (GMRT) at low frequency (i.e. ≤ 610 MHz),
in order to test the presence of a diffuse radio source and/or analyse in detail the
properties of the hosted extended radio emission. For three clusters, the GMRT
information was combined with higher frequency data from Very Large Array (VLA)
observations. A re–analysis of the optical and X–ray data available in the public
archives was carried out for all sources. Propriety deep XMM–Newton and Chandra
observations were used to investigate the merger dynamics in A3562.
Thanks to our multiwavelength analysis, we were able to confirm the existence
of a radio halo and/or a relic in all clusters, and to connect their properties and
origin to the reconstructed merging scenario for most of the investigated cases.
• The existence of a small size and low power radio halo in A3562 was successfully
explained in the theoretical framework of the particle re–acceleration model
for the origin of radio halos, which invokes the re–acceleration of pre–existing
relativistic electrons in the intracluster medium by merger–driven turbulence.
• A giant radio halo was found in the massive galaxy cluster A209, which has
likely undergone a past major merger and is currently experiencing a new
merging process in a direction roughly orthogonal to the old merger axis. A
giant radio halo was also detected in A697, whose optical and X–ray properties
may be suggestive of a strong merger event along the line of sight. Given the
cluster mass and the kind of merger, the existence of a giant radio halo in both
clusters is expected in the framework of the re–acceleration scenario.
• A radio relic was detected at the outskirts of A521, a highly dynamically
disturbed cluster which is accreting a number of small mass concentrations.
A possible explanation for its origin requires the presence of a merger–driven
shock front at the location of the source. The spectral properties of the relic
may support such interpretation and require a Mach number M &lt;
∼ 3 for the
shock.
• The galaxy cluster RXCJ 1314.4–2515 is exceptional and unique in hosting two
peripheral relic sources, extending on the Mpc scale, and a central small size
radio halo. The existence of these sources requires the presence of an ongoing
energetic merger. Our combined optical and X–ray investigation suggests that
a strong merging process between two or more massive subclumps may be
ongoing in this cluster. Thanks to forthcoming optical and X–ray observations,
we will reconstruct in detail the merger dynamics and derive its energetics, to
be related to the energy necessary for the particle re–acceleration in this cluster.
• Finally, RXCJ 2003.5–2323 was found to possess a giant radio halo. This source
is among the largest, most powerful and most distant (z=0.317) halos imaged
so far. Unlike other radio halos, it shows a very peculiar morphology with
bright clumps and filaments of emission, whose origin might be related to the
relatively high redshift of the hosting cluster. Although very little optical and
X–ray information is available about the cluster dynamical stage, the results
of our optical analysis suggest the presence of two massive substructures which
may be interacting with the cluster. Forthcoming observations in the optical
and X–ray bands will allow us to confirm the expected high merging activity
in this cluster.
Throughout the present thesis a cosmology with H0 = 70 km s−1 Mpc−1, 
m=0.3
and 
=0.7 is assumed.