66 research outputs found
Central galaxy growth and feedback in the most massive nearby cool core cluster
We present multi-wavelength observations of the centre of RXCJ1504.1-0248 -
the galaxy cluster with the most luminous and relatively nearby cool core at
z~0.2. Although there are several galaxies within 100 kpc of the cluster core,
only the brightest cluster galaxy (BCG), which lies at the peak of the X-ray
emission, has blue colours and strong line-emission. Approximately 80 Msun/yr
of intracluster gas is cooling below X-ray emitting temperatures, similar to
the observed UV star formation rate of ~140 Msun/yr. Most star formation occurs
in the core of the BCG and in a 42 kpc long filament of blue continuum, line
emission, and X-ray emission, that extends southwest of the galaxy. The
surrounding filamentary nebula is the most luminous around any observed BCG.
The number of ionizing stars in the BCG is barely sufficient to ionize and heat
the nebula, and the line ratios indicate an additional heat source is needed.
This heat source can contribute to the H\alpha-deduced star formation rates
(SFRs) in BCGs and therefore the derived SFRs should only be considered upper
limits. AGN feedback can slow down the cooling flow to the observed mass
deposition rate if the black hole accretion rate is of the order of 0.5 Msun/yr
at 10% energy output efficiency. The average turbulent velocity of the nebula
is vturb ~325 km/s which, if shared by the hot gas, limits the ratio of
turbulent to thermal energy of the intracluster medium to less than 6%.Comment: 15 pages, 11 figures, MNRAS in press. Corrected typo in abstract
Abell 1033: birth of a radio phoenix
Extended steep-spectrum radio emission in a galaxy cluster is usually
associated with a recent merger. However, given the complex scenario of galaxy
cluster mergers, many of the discovered sources hardly fit into the strict
boundaries of a precise taxonomy. This is especially true for radio phoenixes
that do not have very well defined observational criteria. Radio phoenixes are
aged radio galaxy lobes whose emission is reactivated by compression or other
mechanisms. Here, we present the detection of a radio phoenix close to the
moment of its formation. The source is located in Abell 1033, a peculiar galaxy
cluster which underwent a recent merger. To support our claim, we present
unpublished Westerbork Synthesis Radio Telescope and Chandra observations
together with archival data from the Very Large Array and the Sloan Digital Sky
Survey. We discover the presence of two sub-clusters displaced along the N-S
direction. The two sub-clusters probably underwent a recent merger which is the
cause of a moderately perturbed X-ray brightness distribution. A steep-spectrum
extended radio source very close to an AGN is proposed to be a newly born radio
phoenix: the AGN lobes have been displaced/compressed by shocks formed during
the merger event. This scenario explains the source location, morphology,
spectral index, and brightness. Finally, we show evidence of a density
discontinuity close to the radio phoenix and discuss the consequences of its
presence.Comment: accepted MNRA
Multiple density discontinuities in the merging galaxy cluster CIZA J2242.8+5301
CIZA J2242.8+5301, a merging galaxy cluster at z=0.19, hosts a double-relic
system and a faint radio halo. Radio observations at frequencies ranging from a
few MHz to several GHz have shown that the radio spectral index at the outer
edge of the N relic corresponds to a shock of Mach number 4.6+/-1.1, under the
assumptions of diffusive shock acceleration of thermal particles in the test
particle regime. Here, we present results from new Chandra observations of the
cluster. The Chandra surface brightness profile across the N relic only hints
to a surface brightness discontinuity (<2-sigma detection). Nevertheless, our
reanalysis of archival Suzaku data indicates a temperature discontinuity across
the relic that is consistent with a Mach number of 2.5+/-0.5, in agreement with
previously published results. This confirms that the Mach number at the shock
traced by the N relic is much weaker than predicted from the radio. Puzzlingly,
in the Chandra data we also identify additional inner small density
discontinuities both on and off the merger axis. Temperature measurements on
both sides of the discontinuities do not allow us to undoubtedly determine
their nature, although a shock front interpretation seems more likely. We
speculate that if the inner density discontinuities are indeed shock fronts,
then they are the consequence of violent relaxation of the dark matter cores of
the clusters involved in the merger.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
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