989 research outputs found
On the occurrence of Radio Halos in galaxy clusters - Insight from a mass-selected sample
Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in
a fraction of massive and merging galaxy clusters. An unbiased study of the
statistical properties of RHs is crucial to constrain their origin and
evolution. We aim at investigating the occurrence of RHs and its dependence on
the cluster mass in a SZ-selected sample of galaxy clusters, which is as close
as possible to be a mass-selected sample. Moreover, we analyse the connection
between RHs and merging clusters. We select from the Planck SZ catalogue
(Planck Collaboration XXIX 2014) clusters with
at z=0.08-0.33 and we search for the presence of RHs using the NVSS for z<0.2
and the GMRT RH survey (GRHS, Venturi et al. 2007, 2008) and its extension
(EGRHS, Kale et al. 2013, 2015) for 0.2<z<0.33. We use archival Chandra X-ray
data to derive information on the clusters dynamical status. We confirm that RH
clusters are merging systems while the majority of clusters without RH are
relaxed, thus supporting the idea that mergers play a fundamental role in the
generation of RHs. We find evidence for an increase of the fraction of clusters
with RHs with the cluster mass and this is in line with expectations derived on
the basis of the turbulence re-acceleration scenario. Finally, we discuss the
effect of the incompleteness of our sample on this result.Comment: 11 pages, 7 figures, accepted for publication in A&
New giant radio sources and underluminous radio halos in two galaxy clusters
The aim of this work is to analyse the radio properties of the massive and
dynamical disturbed clusters Abell 1451 and Zwcl 0634.1+4750, especially
focusing on the possible presence of diffuse emission. We present new GMRT 320
MHz and JVLA 1.5 GHz observations of these two clusters. We found that both
Abell 1451 and Zwcl 0634.1+4750 host a radio halo with a typical spectrum
(). Similarly to a few other cases reported in the recent
literature, these radio halos are significantly fainter in radio luminosity
with respect to the current radio power-mass correlations and they are smaller
than classical giant radio halos. These underluminous sources might contribute
to shed light on the complex mechanisms of formation and evolution of radio
halos. Furthermore, we detected a candidate radio relic at large distance from
the cluster center in Abell 1451 and a peculiar head tail radio galaxy in Zwcl
0634.1+4750, which might be interacting with a shock front.Comment: 15 pages, 13 figures, accepted for publication in A&
The Extended GMRT Radio Halo Survey II: Further results and analysis of the full sample
The intra-cluster medium contains cosmic rays and magnetic fields that are
manifested through the large scale synchrotron sources, termed as radio halos,
relics and mini-halos. The Extended Giant Metrewave Radio Telescope (GMRT)
Radio Halo Survey (EGRHS) is an extension of the GMRT Radio Halo Survey (GRHS)
designed to search for radio halos using GMRT 610/235 MHz observations. The
GRHS+EGRHS consists of 64 clusters in the redshift range 0.2 -- 0.4 that have
an X-ray luminosity larger than 5x10^44 erg/s in the 0.1 -- 2.4 keV band and
with declinations > -31 deg in the REFLEX and eBCS X-ray cluster catalogues. In
this second paper in the series, GMRT 610/235 MHz data on the last batch of 11
galaxy clusters and the statistical analysis of the full sample are presented.
A new mini-halo in RXJ2129.6+0005 and candidate diffuse sources in Z5247, A2552
and Z1953 are discovered. A unique feature of this survey are the upper limits
on the detections of 1 Mpc sized radio halos; 4 new are presented here making a
total of 31 in the survey. Of the sample, 58 clusters that have adequately
sensitive radio information were used to obtain the most accurate occurrence
fractions so far. The occurrence of radio halos in our X-ray selected sample is
~22%, that of mini-halos is 13% and that of relics is ~5%. The radio power -
X-ray luminosity diagrams for the radio halos and mini-halos with the
detections and upper limits are presented. The morphological estimators namely,
centroid shift (w), concentration parameter (c) and power ratios (P_3/P_0)
derived from the Chandra X-ray images are used as proxies for the dynamical
states of the GRHS+EGRHS clusters. The clusters with radio halos and mini-halos
occupy distinct quadrants in the c-w, c-P_3/P_0 and w - P_3/P_0 planes,
corresponding to the more and less morphological disturbance, respectively. The
non-detections span both the quadrants.Comment: 24 pages, 5 tables, 25 figures, accepted for publication in A&
A KAT-7 view of a low-mass sample of galaxy clusters
Radio observations over the last two decades have provided evidence that
diffuse synchrotron emission in the form of megaparsec-scale radio halos in
galaxy clusters is likely tracing regions of the intracluster medium where
relativistic particles are accelerated during cluster mergers. In this paper we
present results of a survey of 14 galaxy clusters carried out with the
7-element Karoo Array Telescope at 1.86 GHz, aimed to extend the current
studies of radio halo occurrence to systems with lower masses (M M). We found upper limits at the Watt Hz level for of the sample, confirming that
bright radio halos in less massive galaxy clusters are statistically rare.Comment: 7 pages, 4 figures. Conference proceeding of "The many facets of
extragalactic radio surveys: towards new scientific challenges", 20-23
October 2105, Bologna, Ital
A High Diffusive Model for Nanomaterials
Considerable attention is today devoted to the engineering of films widely used in photocatalytic, solar energy converters, photochemical and photoelectrochemical cells, dye-sensitized solar cells (DSSCs), to optimize electronic time response following photogeneration. However, the precise nature of transport processes in these systems has remained unresolved. To investigate such aspects of carrier dynamics, we have suggested a model for the calculation of correlation functions, expressed as the Fourier transform of the frequency-dependent complex conductivity σ(ω). Results are presented for the velocity correlation functions, the mean square deviation of position and the diffusion coefficient in systems, like TiO2 and doped Si, of large interest in present devices. Fast diffusion occurs in short time intervals of the order of few collision times. Consequences for efficiency of this fast response are discussed in relation to nanostructured devices
X-ray Properties of the GigaHertz-Peaked and Compact Steep Spectrum Sources
We present {\it Chandra} X-ray Observatory observations of Giga-Hertz Peaked
Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. The {\it
Chandra} sample contains 13 quasars and 3 galaxies with measured 2-10 keV X-ray
luminosity within erg s. We detect all of the
sources, five of which are observed in X-ray for the first time. We study the
X-ray spectral properties of the sample. The measured absorption columns in the
quasars are different than those in the galaxies in the sense that the quasars
show no absorption (with limits ) while the galaxies
have large absorption columns () consistent with
previous findings. The median photon index of the sources with high S/N is
and it is larger than the typical index of radio loud
quasars. The arcsec resolution of {\it Chandra} telescope allows us to
investigate X-ray extended emission, and look for diffuse components and X-ray
jets. We found X-ray jets in two quasars (PKS 1127-145, B2 0738+32), an X-ray
cluster surrounding a CSS quasar (z=1.1, 3C 186), detected a possible binary
structure in 0941-080 galaxy and an extended diffuse emission in galaxy PKS B2
1345+12. We discuss our results in the context of X-ray emission processes and
radio source evolution. We conclude that the X-ray emission in these sources is
most likely unrelated to a relativistic jet, while the sources' radio-loudness
may suggest a high radiative efficiency of the jet power in these sources.Comment: 15 pages, to be published in Ap
RATAN-600 7.6-cm Deep Sky Strip Surveys at the Declination of the SS433 Source During the 1980-1999 Period. Data Reduction and the Catalog of Radio Sources in the Right-Ascension Interval 7h < R.A. < 17h
We use two independent methods to reduce the data of the surveys made with
RATAN-600 radio telescope at 7.6 cm in 1988-1999 at the declination of the
SS433 source. We also reprocess the data of the "Cold" survey (1980-1981). The
resulting RCR (RATAN COLD REFINED) catalog contains the right ascensions and
fluxes of objects identified with those of the NVSS catalog in the
right-ascension interval 7h < R.A. < 17h. We obtain the spectra of the radio
sources and determine their spectral indices at 3.94 and 0.5 GHz. The spectra
are based on the data from all known catalogs available from the CATS, Vizier,
and NED databases, and the flux estimates inferred from the maps of the VLSS
and GB6 surveys. For 245 of the 550 objects of the RCR catalog the fluxes are
known at two frequencies only: 3.94 GHz (RCR) and 1.4 GHz (NVSS). These are
mostly sources with fluxes smaller than 30 mJy. About 65% of these sources have
flat or inverse spectra (alpha > -0.5). We analyze the reliability of the
results obtained for the entire list of objects and construct the histograms of
the spectral indices and fluxes of the sources. Our main conclusion is that all
10-15 mJy objects found in the considered right-ascension interval were already
included in the decimeter-wave catalogs.Comment: 26 pages, 18 figure
A MeerKAT view on galaxy clusters: a radio-optical study of Abell 1300 and MACS J1931.8--2634
In this paper we present results from a radio-optical study of the galaxy
populations of the galaxy clusters Abell 1300 and MACS J1931.82634, a merger
and a relaxed system respectively both located at , aimed at
finding evidence of merger-induced radio emission. Radio observations are taken
at 1.28 GHz with the MeerKAT interferometer during its early-stage
commissioning phase, and combined with archive optical data. We generated
catalogues containing 107 and 162 radio sources in the A1300 and MACS
J1931.8--2634 cluster fields respectively, above a 0.2 mJy threshold and within
a 30~arcmin radius from the cluster centre (corresponding to 8.1 and 8.8 Mpc
respectively). By cross-correlating the radio and optical catalogues, and
including spectroscopic information, 9 and 6 sources were found to be cluster
members and used to construct the radio luminosity functions respectively for
both clusters. The comparison of the radio source catalogues between the two
cluster fields leads to a marginal difference, with a statistical
significance. We derived the radio luminosity function at 1.28 GHz in both
clusters, in the power range ,
and obtained that in A 1300 the radio luminosity function averaged over the
full radio power interval is only times higher than the MACS
J1931.8--2634 one, suggesting no statistical difference in their probability to
host nuclear radio emission. We conclude that, at least for the two clusters
studied here, the role of cluster mergers in affecting the statistical
properties of the radio galaxy population is negligible.Comment: 18 pages, 8 figures, MNRAS accepte
Radio halos in a mass-selected sample of 75 galaxy clusters: I. Sample selection and data analysis
Context. Radio halos are synchrotron diffiuse sources at the centre of a fraction of galaxy clusters. The study of large samples of clusters with adequate radio and X-ray data is necessary to investigate the origin of radio halos and their connection with the cluster dynamics and formation history. Aims. The aim of this paper is to compile a well-selected sample of galaxy clusters with deep radio observations to perform an unbiased statistical study of the properties of radio halos. Methods. We selected 75 clusters with M ≤ 6 × 1014M⊙at z = 0.08-0.33 from the Planck Sunyaev-Zel'dovich catalogue. Clusters without suitable radio data were observed with the Giant Metrewave Radio Telescope and/or the Jansky Very Large Array to complete the information about the possible presence of diffiuse emission. We used archival Chandra X-ray data to derive information on the clusters' dynamical states. Results. This observational campaign led to the detection of several cluster-scale diffiuse radio sources and candidates that deserve future follow-up observations. Here we summarise their properties and add information resulting from our new observations. For the clusters where we did not detect any hint of diffiuse emission, we derived new upper limits to their diffiuse flux. Conclusions.We have built the largest mass-selected (>80% complete in mass) sample of galaxy clusters with deep radio observations available to date. The statistical analysis of the sample, which includes the connection between radio halos and cluster mergers, the radio power - mass correlation, and the occurrence of radio halos as a function of the cluster mass, will be presented in Paper II
The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223
Radio halos and relics are Mpc-scale diffuse radio sources in galaxy
clusters, with a steep spectral index (). It
has been proposed that they arise from particle acceleration induced by
turbulence and weak shocks, injected in the intracluster medium (ICM) during
mergers. MACS J1149.5+2223 (MACS J1149) is a high redshift () galaxy
cluster possibly hosting a radio halo and a relic. We analysed LOFAR, GMRT, and
JVLA radio data at 144, 323, 1500 MHz, and Chandra X-ray data to characterise
the thermal and non-thermal properties of the cluster. We obtained radio images
at different frequencies to investigate the spectral properties of the radio
halo. We used Chandra X-ray images to constrain the thermal properties of the
cluster. We measured a steep spectrum of the halo, with
between 144 and 1500 MHz. The radio surface brightness distribution across the
halo is found to correlate with the X-ray brightness of the ICM, with a
sub-linear slope in the range 0.4 to 0.6. We also report two possible cold
fronts in north-east and north-west, but deeper X-ray observations are required
to firmly constrain the properties of the upstream emission. We show that the
combination of high redshift, steep radio spectrum, and sub-linear radio-X
scaling of the halo rules out hadronic models. An old ( Gyr ago) major
merger likely induced the formation of the halo through stochastic
re-acceleration of relativistic electrons. We suggest that the two possible
X-ray discontinuities may actually be part of the same cold front. In this
case, the coolest gas pushed towards the north-west might be associated with
the cool core of a sub-cluster involved in the major merger. The peculiar
orientation of the south-east relic might indicate a different nature of this
source and requires further investigation.Comment: 15 pages, accepted for publication in A&
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