103 research outputs found
Heated Intracluster Gas and Radio Connections: the Singular case of MKW3s
Similarly to other cluster of galaxies previously classified as cooling flow
systems, the Chandra observation of MKW3s reveals that this object has a
complex X-ray structure hosting both a X-ray cavity and a X-ray filament.
Unlike the other clusters, however, the temperature map of the core of MKW3s
shows the presence of extended regions of gas heated above the radially
averaged gas temperature at any radius.
As the cluster does not show evidences for ongoing major mergers Mazzotta et
al. suggest a connection between the heated gas and the activity of the central
AGN. Nevertheless, due to the lack of high quality radio maps, this
interpretation was controversial.
In this paper we present the results of two new radio observations of MKW3s
at 1.28GHz and 604MHz obtained at the GMRT. Together with the Chandra
observation and a separate VLA observation at 327MHz from Young, we show
unequivocal evidences for a close connection between the heated gas region and
the AGN activity and we briefly summarize possible implications.Comment: To appear in a special issue of the "Journal of the Korean
Astronomical Society" (JKAS). Proceedings of the International conference on
Cosmic Rays and Magnetic Fields in Large Scale Structure, Busan, Korea, 200
Occurrence of radio minihalos in a mass-limited sample of galaxy clusters
We investigate the occurrence of radio minihalos --- diffuse radio sources of
unknown origin observed in the cores of some galaxy clusters --- in a
statistical sample of 58 clusters drawn from the Planck Sunyaev-Zel'dovich
cluster catalog using a mass cut (). We
supplement our statistical sample with a similarly-sized non-statistical sample
mostly consisting of clusters in the ACCEPT X-ray catalog with suitable X-ray
and radio data, which includes lower-mass clusters. Where necessary (for 9
clusters), we reanalyzed the Very Large Array archival radio data to determine
if a mihinalo is present. Our total sample includes all 28 currently known and
recently discovered radio minihalos, including 6 candidates. We classify
clusters as cool-core or non-cool core according to the value of the specific
entropy floor in the cluster center, rederived or newly derived from the
Chandra X-ray density and temperature profiles where necessary (for 27
clusters). Contrary to the common wisdom that minihalos are rare, we find that
almost all cool cores - at least 12 out of 15 (80%) - in our complete sample of
massive clusters exhibit minihalos. The supplementary sample shows that the
occurrence of minihalos may be lower in lower-mass cool-core clusters. No
minihalos are found in non-cool-cores or "warm cores". These findings will help
test theories of the origin of minihalos and provide information on the
physical processes and energetics of the cluster cores.Comment: 34 pages, accepted for publication in ApJ. Added a section
"Definition of a minihalo" and an appendix "Radio size and average surface
brigthtness of minihalos and halos
Low frequency follow up of radio halos and relics in the GMRT Radio Halo Cluster Survey
We performed GMRT low frequency observations of the radio halos, relics and
new candidates belonging to the GMRT Radio Halo Cluster Sample first observed
at 610 MHz. High sensitivity imaging was performed using the GMRT at 325 MHz
and 240 MHz. The properties of the diffuse emission in each cluster were
compared to our 610 MHz images and/or literature information available at other
frequencies, in order to derive the integrated spectra over a wide frequency
range.Beyond the classical radio halos, whose spectral index is in the
range (S), we found sources with
. This result supports the idea that the spectra of the
radiating particles in radio halos is not universal, and that inefficient
mechanisms of particle acceleration are responsible for their origin. We also
found a variety of brightness distributions, i.e. centrally peaked as well as
clumpy halos. Even though the thermal and relativistic plasma tend to occupy
the same cluster volume, in some cases a positional shift between the radio and
X-ray peaks of emission is evident. Our observations also revealed the
existence of diffuse cluster sources which cannot be easily classified either
as halos or relics. New candidate relics were found in A1300 and in A1682, and
in some clusters "bridges" of radio emission have been detected, connecting the
relic and radio halo emission. Combining our new data with literature
information, we derived the LogL-LogP correlation for
radio halos, and investigated the possible trend of the spectral index of radio
halos with the temperature of the intracluster medium.Comment: 19 pages, 10 figures, 5 tables, accepted for publication on A&
Mapping the particle acceleration in the cool core of the galaxy cluster RX J1720.1+2638
We present new deep, high-resolution radio images of the diffuse minihalo in
the cool core of the galaxy cluster RX J1720.1+2638. The images have been
obtained with the Giant Metrewave Radio Telescope at 317, 617 and 1280 MHz and
with the Very Large Array at 1.5, 4.9 and 8.4 GHz, with angular resolutions
ranging from 1" to 10". This represents the best radio spectral and imaging
dataset for any minihalo. Most of the radio flux of the minihalo arises from a
bright central component with a maximum radius of ~80 kpc. A fainter tail of
emission extends out from the central component to form a spiral-shaped
structure with a length of ~230 kpc, seen at frequencies 1.5 GHz and below. We
find indication of a possible steepening of the total radio spectrum of the
minihalo at high frequencies. Furthermore, a spectral index image shows that
the spectrum of the diffuse emission steepens with the increasing distance
along the tail. A striking spatial correlation is observed between the minihalo
emission and two cold fronts visible in the Chandra X-ray image of this cool
core. These cold fronts confine the minihalo, as also seen in numerical
simulations of minihalo formation by sloshing-induced turbulence. All these
observations favor the hypothesis that the radio emitting electrons in cluster
cool cores are produced by turbulent reacceleration.Comment: 16 pages, 11 figures, accepted for publication in The Astrophysical
Journa
Discovery of a radio relic in the low mass, merging galaxy cluster PLCK G200.9-28.2
Radio relics at the peripheries of galaxy clusters are tracers of the elusive
cluster merger shocks. We report the discovery of a single radio relic in the
galaxy cluster PLCK G200.9-28.2 (, ) using the Giant Metrewave Radio Telescope at 235 and 610 MHz and
the Karl G. Jansky Very Large Array at 1500 MHz. The relic has a size of Mpc, an arc-like morphology and is located at 0.9 Mpc from the
X-ray brightness peak in the cluster. The integrated spectral index of the
relic is . The spectral index map between 235 and 610 MHz shows
steepening from the outer to the inner edge of the relic in line with the
expectation from a cluster merger shock. Under the assumption of diffusive
shock acceleration, the radio spectral index implies a Mach number of
for the shock. The analysis of archival XMM Newton data shows that
PLCK G200.9-28.2 consists of a northern brighter sub-cluster, and a southern
sub-cluster in a state of merger. This cluster has the lowest mass among the
clusters hosting single radio relics. The position of the Planck Sunyaev
Ze'ldovich effect in this cluster is offset by 700 kpc from the X-ray peak in
the direction of the radio relic, suggests a physical origin for the offset.
Such large offsets in low mass clusters can be a useful tool to select
disturbed clusters and to study the state of merger.Comment: 10 pages, 7 figures, 4 tables. Accepted for publication in MNRA
Are radio minihalos confined by cold fronts in galaxy clusters? Minihalos and large-scale sloshing in A3444 and MS 1455.0+2232
We present radio and X-ray studies of A3444 and MS1455.0+2232, two galaxy
clusters with radio minihalos in their cool cores. A3444 is imaged using the
Giant Metrewave Radio Telescope (GMRT) at 333, 607 and 1300 MHz and the Very
Large Array at 1435 MHz. Most of the minihalo is contained within r<120 kpc,
but a fainter extension, stretching out to 380 kpc South-West of the center, is
detected at 607 MHz. Using Chandra, we detect four X-ray sloshing cold fronts:
three in the cool core at r=60, 120 and 230 kpc, and a fourth one at r=400 kpc
- in the region of the southwestern radio extension - suggesting that the
intracluster medium (ICM) is sloshing on a cluster-wide scale. The radio
emission is contained within the envelope defined by these fronts. We also
analyzed archival 383 MHz GMRT and Chandra observations of MS1455.0+2232, which
exhibits a known minihalo with its bright part delineated by cold fronts inside
the cool core, but with a faint extension beyond the core. Similarly to A3444,
we find a cold front at r~425 kpc, containing the radio emission. Thus the
entire diffuse radio emission seen in these clusters appears to be related to
large-scale sloshing of the ICM. The radio spectrum of the A3444 minihalo is a
power law with a steep index . The spectrum steepens with
increasing distance from the center, as expected if the minihalo originates
from re-acceleration of relativistic particles by the sloshing-induced
turbulence in the ICM.Comment: 29 pages, 15 figures, accepted for publication in Ap
A shock front in the merging galaxy cluster Abell 754: X-ray and radio observations
We present new Chandra X-ray and Giant Meterwave Radio Telescope (GMRT) radio
observations of the nearby merging galaxy cluster Abell 754. Our X-ray data
confirm the presence of a shock front by obtaining the first direct measurement
of a gas temperature jump across the X-ray brightness edge previously seen in
the imaging data. A754 is only the fourth galaxy cluster with confirmed merger
shock fronts, and it has the weakest shock of those, with a Mach number
M=1.57+0.16-0.12. In our new GMRT observation at 330 MHz, we find that the
previously-known centrally located radio halo extends eastward to the position
of the shock. The X-ray shock front also coincides with the position of a radio
relic previously observed at 74 MHz. The radio spectrum of the post-shock
region, using our radio data and the earlier results at 74 MHz and 1.4 GHz, is
very steep. We argue that acceleration of electrons at the shock front directly
from thermal to ultrarelativistic energies is problematic due to energy
arguments, while reacceleration of preexisting relativistic electrons is more
plausible.Comment: 10 pages, 8 figures, "emulateapj" format. Submitted to Ap
The SKA view of cool-core clusters: evolution of radio mini-halos and AGN feedback
In about 70% of the population of relaxed, cool-core galaxy clusters, the
brightest cluster galaxy (BCG) is radio loud, showing non-thermal radio jets
and lobes ejected by the central active galactic nucleus (AGN). In recent years
such relativistic plasma has been unambiguously shown to interact with the
surrounding thermal intra-cluster medium (ICM) thanks to spectacular images
where the lobe radio emission is observed to fill the cavities in the
X-ray-emitting gas. This `radio feedback' phenomenon is widespread and is
critical to understand the physics of the inner regions of galaxy clusters and
the properties of the central BCG. At the same time, mechanically-powerful AGN
are likely to drive turbulence in the central ICM which may also play a role
for the origin of non-thermal emission on cluster-scales. Diffuse non-thermal
emission has been observed in a number of cool-core clusters in the form of a
radio mini-halo surrounding the radio-loud BCG on scales comparable to that of
the cooling region. Large mini-halo samples are necessary to establish their
origin and connection with the cluster thermal properties and dynamics,
especially in light of future X-ray characterization of the cluster cores as it
is expected by Athena-XIFU. We show that All-Sky reference survey at Band 2
with SKA1 at confusion limit (rms ~2 {\mu}Jy per beam) has the potential to
detect up to ~620 mini-halos at redshift z<0.6, whereas Deep Tier reference
surveys at Band 1/2 with SKA1 at sub-arcsec resolution (rms ~0.2 {\mu}Jy per
beam) will allow a complete census of the radio-loud BCGs at any redshift down
to a 1.4 GHz power of 10^{22} W/Hz. We further anticipate that SKA2 might
detect up to ~1900 new mini-halos at redshift z<0.6 and characterize the
radio-mode AGN feedback in every cluster and group up to redshift z ~1.7 (the
highest-z where virialized clusters are currently detected).Comment: 17 pages, 5 figures. To appear as part of 'Continuum Science' in
Proceedings of "Advancing Astrophysics with the SKA (AASKA14)",
PoS(AASKA14)07
Recurrent radio outbursts at the center of the NGC1407 galaxy group
We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at
240, 330 and 610 MHz of the complex radio source at the center of the NGC1407
galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse
emission enclosing the central twin-jet radio galaxy. This has been interpreted
as an indication of two possible radio outbursts occurring at different times.
Both the inner double and diffuse component are detected in the new GMRT images
at high levels of significance. Combining the GMRT observations with archival
Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both
components. The inner double has a spectral index \alpha=0.7, typical for
active, extended radio galaxies, whereas the spectrum of the large-scale
emission is very steep, with \alpha=1.8 between 240 MHz and 1.4 GHz. The
radiative age of the large-scale component is very long, ~300 Myr, compared to
~30 Myr estimated for the central double, confirming that the diffuse component
was generated during a former cycle of activity of the central galaxy. The
current activity have so far released an energy which is nearly one order of
magnitude lower than that associated with the former outburst. The group X-ray
emission in the Chandra and XMM-Newton images and extended radio emission show
a similar swept-back morphology. We speculate that the two structures are both
affected by the motion of the group core, perhaps due to the core sloshing in
response to a recent encounter with the nearby elliptical galaxy NGC1400.Comment: 15 pages, 12 figures and 5 tables. Accepted for publication in Ap
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