222 research outputs found
A morphological comparison between giant radio halos and radio mini--halos in galaxy clusters
In this letter we present a morphological comparison between giant radio
halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity,
P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report
evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for
mini--halos: mini--halo clusters share the same region of giant halo clusters
in the (P_{1.4},L_X) plane, whereas they are clearly separated in the
(P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be
more than 50 times larger than that of giant halos, implying a very efficient
process for their origins. By assuming a scenario of sporadical turbulent
particle re-acceleration for both giant and mini halos, we discuss basic
physical differences between these sources. Regardless of the origin of the
turbulence, a more efficient source of injection of particles, which eventually
takes part in the re-acceleration process, is required in mini-halos, and this
may result from the central radio galaxy or from proton-proton collisions in
the dense cool core regions.Comment: 4 pages, 3 figures, A&A Letter in pres
Evidence for AGN Feedback in Galaxy Clusters and Groups
The current generation of flagship X-ray missions, Chandra and XMM-Newton,
has changed our understanding of the so-called "cool core" galaxy clusters and
groups. Instead of the initial idea that the thermal gas is cooling and flowing
toward the center, the new picture envisages a complex dynamical evolution of
the intra-cluster medium (ICM) regulated by the radiative cooling and the
nongravitational heating from the active galactic nucleus (AGN). Understanding
the physics of the hot gas and its interplay with the relativistic plasma
ejected by the AGN is key for understanding the growth and evolution of
galaxies and their central black holes, the history of star formation, and the
formation of large-scale structures. It has thus become clear that the feedback
from the central black hole must be taken into account in any model of galaxy
evolution. In this paper, we draw a qualitative picture of the current
knowledge of the effects of the AGN feedback on the ICM by summarizing the
recent results in this field.Comment: Accepted for publication in Advances in Astronomy, 30 pages, 6
figures. Tutorial Review to appear in the Special Issue "Seeking for the
Leading Actor on the Cosmic Stage: Galaxies versus Supermassive Black Holes
A radio minihalo in the extreme cool-core galaxy cluster RXCJ1504.1-0248
Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a
massive galaxy cluster that has an extremely luminous cool core. To date, only
9 radio minihalos are known, thus the discovery of a new one, in one of the
most luminous cool-core clusters, provides important information on this
peculiar class of sources and sheds light on their origin. Methods. The diffuse
radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA
data at 1.46 GHz. The minihalo has a radius of 140 kpc. A Chandra gas
temperature map shows that the minihalo emission fills the cluster cool core
and has some morphological similarities to it, as has been previously observed
for other minihalos. Results. The Chandra data reveal two subtle cold fronts in
the cool core, likely created by sloshing of the core gas, as observed in most
cool-core clusters. Following previous work, we speculate that the origin of
the minihalo is related to sloshing. Sloshing may result in particle
acceleration by generating turbulence and/or amplifying the magnetic field in
the cool core, leading to the formation of a minihalo.Comment: 4 pages, 1 table, 3 color figures. Accepted for publication in A&A
Letter
A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst
We present the results of a Chandra study of the Hydra A galaxy cluster,
where a powerful AGN outburst created a large-scale cocoon shock. We
investigated possible azimuthal variations in shock strength and shape, finding
indications for a weak shock with a Mach number in the range ~1.2-1.3. We
measured the temperature change across the shock front. However, the detection
of a temperature rise in the regions immediately inside of the front is
complicated by the underlying temperature profile of the cluster atmosphere. We
measured the global temperature profile of the cluster up to 700 kpc, which
represents the farthest measurement obtained with Chandra for this cluster. A
"plateau" in the temperature profile in the range ~70-150 kpc indicates the
presence of cool gas, which is likely the result of uplift of material by the
AGN outburst. After masking the cool filaments visible in the hardness ratio
map, the plateau disappears and the temperature profile recovers a typical
shape with a peak around 190 kpc, just inside the shock front. However, it is
unlikely that such a temperature feature is produced by the shock as it is
consistent with the general shape of the temperature profiles observed for
relaxed galaxy clusters. We studied the spectral properties of the cool
filaments finding evidence that ~10^11 M_sun of low-entropy material has been
dredged up by the rising lobes from the central 30 kpc to the observed current
position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x
10^60 erg, which is comparable to the work required to inflate the cavities and
is ~25% of the total energy of the large-scale shock. Our results show that the
AGN feedback in Hydra A is acting not only by directly heating the gas, but
also by removing a substantial amount of potential fuel for the SMBH.Comment: 11 pages, 9 figures, accepted for publication in ApJ (version with
full resolution figures available at
http://www.bo.astro.it/~myriam/files/papers/gitti-hydra.pdf
Mass distribution in the most X-ray-luminous galaxy cluster RX J1347.5-1145 studied with XMM-Newton
We report on the analysis of XMM-Newton observations of RX J1347.5-1145
(z=0.451), the most X-ray-luminous galaxy cluster. We present a detailed total
and gas mass determination up to large distances (~1.7 Mpc), study the scaling
properties of the cluster, and explore the role of AGN heating in the cluster
cool core. By means of spatially resolved spectroscopy we derive density,
temperature, entropy, and cooling time profiles of the intra-cluster medium. We
compute the total mass profile of the cluster in the assumption of hydrostatic
equilibrium. If the disturbed south-east region of the cluster is excluded from
the analysis, our results on shape, normalization, scaling properties of
density, temperature, entropy, and cooling time profiles are fully consistent
with those of relaxed, cool core clusters. We compare our total and gas mass
estimates with previous X-ray, lensing, dynamical, and SZ studies. We find good
agreement with other X-ray results, dynamical mass measurements, weak lensing
masses and SZ results. We confirm a discrepancy of a factor ~2 between strong
lensing and X-ray mass determinations and find a gross mismatch between our
total mass estimate and the mass reconstructed through the combination of both
strong and weak lensing. We explore the effervescent heating scenario in the
core of RX J1347.5-1145 and find support to the picture that AGN outflows and
heat conduction are able to quenching radiative cooling.Comment: 12 pages, 6 figures, accepted for publication in A&
A Chandra - VLA Investigation of the X-ray Cavity System and Radio Mini-Halo in the Galaxy Cluster RBS 797
We present a study of the cavity system in the galaxy cluster RBS 797 based
on Chandra and VLA data. RBS 797 (z = 0.35), is one of the most distant galaxy
clusters in which two pronounced X-ray cavities have been discovered. The
Chandra data confirm the presence of a cool core and indicate an higher
metallicity along the cavity directions. This is likely due to the AGN
outburst, which lifts cool metal-rich gas from the center along the cavities,
as seen in other systems. We find indications that the cavities are hotter than
the surrounding gas. Moreover, the new Chandra images show bright rims
contrasting with the deep, X-ray deficient cavities. The likely cause is that
the expanding 1.4 GHz radio lobes have displaced the gas, compressing it into a
shell that appears as bright cool arms. Finally we show that the large-scale
radio emission detected with our VLA observations may be classified as a radio
mini-halo, powered by the cooling flow (CF), as it nicely follows the trend
P_radio vs. P_CF predicted by the re-acceleration model.Comment: Accepted for publication in ApJ, 14 pages, 13 figure
Diffuse steep-spectrum sources from the 74 MHz VLSS survey
Galaxy clusters grow by a sequence of mergers with other clusters and galaxy
groups. During these mergers, shocks and/or turbulence are created within the
intracluster medium (ICM). In this process, particles could be accelerated to
highly relativistic energies. The synchrotron radiation from these particles is
observed in the form of radio relics and halos that are generally characterized
by a steep radio spectral index. Shocks can also revive fossil radio plasma
from a previous episode of AGN activity, creating a so-called radio phoenix.
Here we present multi-frequency radio observations of diffuse steep-spectrum
radio sources selected from the 74 MHz VLSS survey. Previous GMRT observations
showed that some of these sources had filamentary and elongated morphologies,
which are expected for radio relics.
We carried out radio continuum observations at 325 MHz with the GMRT.
Observations with the VLA and WSRT were taken at 1.4 GHz in full polarization
mode. Optical images around the radio sources were taken with the WHT and INT
telescopes. Most of the sources in our sample consist of old radio plasma from
AGNs located in small galaxy clusters. The sources can be classified as AGN
relics or radio phoenices. The spectral indices across most of the radio
sources display large variations. We conclude that diffuse steep-spectrum radio
sources are not only found in massive X-ray luminous galaxy clusters but also
in smaller systems. Future low-frequency surveys will uncover large numbers of
steep-spectrum radio relics related to previous episodes of AGN activity.
[abridged]Comment: 15 pages, 15 figures, accepted for publication in A&A on December 9,
201
Cosmological Effects of Powerful AGN Outbursts in Galaxy Clusters: Insights from an XMM-Newton Observation of MS0735+7421
We report on the results of an analysis of XMM-Newton observations of
MS0735+7421, the galaxy cluster which hosts the most energetic AGN outburst
currently known. The previous Chandra image shows twin giant X-ray cavities
(~200 kpc diameter) filled with radio emission and surrounded by a weak shock
front. XMM data are consistent with these findings. The total energy in
cavities and shock (~6 \times 10^{61} erg) is enough to quench the cooling flow
and, since most of the energy is deposited outside the cooling region (~100
kpc), to heat the gas within 1 Mpc by ~1/4 keV per particle. The cluster
exhibits an upward departure (factor ~2) from the mean L-T relation. The boost
in emissivity produced by the ICM compression in the bright shells due to the
cavity expansion may contribute to explain the high luminosity and high central
gas mass fraction that we measure. The scaled temperature and metallicity
profiles are in general agreement with those observed in relaxed clusters.
Also, the quantities we measure are consistent with the observed M-T relation.
We conclude that violent outbursts such as the one in MS0735+7421 do not cause
dramatic instantaneous departures from cluster scaling relations (other than
the L-T relation). However, if they are relatively common they may play a role
in creating the global cluster properties.Comment: 69 pages, 30 figures, accepted for publication in ApJ Main Journa
Prognostic role of pneumonia in supracricoid and supraglottic laryngectomies.
The goal of this study was to identify host and tumour factors associated with postoperative pneumonia (PP) in a selected population of laryngeal cancer patients, treated by partial laryngectomy in 20 years at our Institution and to assess its potential prognostic impact. Clinical records of 416 consecutive patients were retrospectively reviewed. Tobacco consumption, body mass index (BMI), previous pulmonary disease, age, sex, preoperative blood gas analysis values, tumour stage and type of surgery were tested as potential risk factors for PP. Finally, the prognostic impact of these variables, including PP, in terms of disease-free and actuarial survival by Kaplan-Meier and Cox analyses were evaluated. PP developed in 73 patients (16.8\%). We identified two groups of patients: 26 patients experienced an early PP within the first 7-9 days after surgery, whilst 44 experienced an ab ingestis PP following attempts of oral food intake restoration, three patients died for PP related sepsis. At multivariate Cox analysis, age older than 60 years and BMI greater than 30 were statistically associated with early PP; whereas male gender and laryngectomy with neck dissection were statistically related to a higher risk of ab ingestis PP. Interestingly, the occurrence of early PP was a negative independent prognostic factor for 5-years disease-free and actuarial survival (p=0.049 and p=0.001, respectively). The occurrence of early-onset pneumonia in laryngeal cancer patients selected for conservative laryngectomies is predictable and associated with poor clinical outcome
Non-thermal X-rays, a high abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxies
Using a deep Chandra observation of the Perseus cluster of galaxies, we find
a high-abundance shell 250 arcsec (93 kpc) from the central nucleus. This ridge
lies at the edge of the Perseus radio mini-halo. In addition we identify two
Halpha filaments pointing towards this shell. We hypothesise that this ridge is
the edge of a fossil radio bubble, formed by entrained enriched material lifted
from the core of the cluster. There is a temperature jump outside the shell,
but the pressure is continuous indicating a cold front. A non-thermal component
is mapped over the core of the cluster with a morphology similar to the
mini-halo. Its total luminosity is 4.8x10^43 erg/s, extending in radius to ~75
kpc. Assuming the non-thermal emission is the result of inverse Compton
scattering of the CMB and infrared emission from NGC 1275, we map the magnetic
field over the core of the cluster.Comment: 8 pages, colour, accepted by MNRA
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