1,282 research outputs found
CARMA observations of massive Planck-discovered cluster candidates at z>0.5 associated with WISE overdensities: Breaking the size-flux degeneracy
We use a Bayesian software package to analyze CARMA-8 data towards 19
unconfirmed Planck SZ-cluster candidates from Rodriguez-Gonzalvez et al.
(2015), that are associated with significant overdensities in WISE. We used two
cluster parameterizations, one based on a (fixed shape) generalized-NFW
pressure profile and another based on a beta-gas-density profile (with varying
shape parameters) to obtain parameter estimates for the nine CARMA-8
SZ-detected clusters. We find our sample is comprised of massive,
Y_{500}=0.0010 \pm 0.0015 arcmin^2, relatively compact, theta_{500}= 3.9 \pm
2.0 arcmin systems. Results from the beta model show that our cluster
candidates exhibit a heterogeneous set of brightness-temperature profiles.
Comparison of Planck and CARMA-8 measurements showed good agreement in Y_{500}
and an absence of obvious biases. We estimated the total cluster mass M_{500}
as a function of z for one of the systems; at the preferred photometric
redshift of 0.5, the derived mass, M_{500} \approx 0.8 \pm 0.2 \times 10^{15}
Msun. Spectroscopic Keck/MOSFIRE data confirmed a galaxy member of one of our
cluster candidates to be at z=0.565. Applying a Planck prior in Y_{500} to the
CARMA-8 results reduces uncertainties for both parameters by a factor >4,
relative to the independent Planck or CARMA-8 measurements. We here demonstrate
a powerful technique to find massive clusters at intermediate z \gtrsim 0.5
redshifts using a cross-correlation between Planck and WISE data, with
high-resolution follow-up with CARMA-8. We also use the combined capabilities
of Planck and CARMA-8 to obtain a dramatic reduction by a factor of several, in
parameter uncertainties.Comment: 26 pages, 8 figures, appearing in MNRAS (responded to referee report
The two-component giant radio halo in the galaxy cluster Abell 2142
We report on a spectral study at radio frequencies of the giant radio halo in
A2142 (z=0.0909), which we performed to explore its nature and origin. A2142 is
not a major merger and the presence of a giant radio halo is somewhat
surprising. We performed deep radio observations with the GMRT at 608 MHz, 322
MHz, and 234 MHz and with the VLA in the 1-2 GHz band. We obtained high-quality
images at all frequencies in a wide range of resolutions. The radio halo is
well detected at all frequencies and extends out to the most distant cold front
in A2142. We studied the spectral index in two regions: the central part of the
halo and a second region in the direction of the most distant south-eastern
cold front, selected to follow the bright part of the halo and X-ray emission.
We complemented our observations with a preliminary LOFAR image at 118 MHz and
with the re-analysis of archival VLA data at 1.4 GHz. The two components of the
radio halo show different observational properties. The central brightest part
has higher surface brightess and a spectrum whose steepness is similar to those
of the known radio halos, i.e. . The ridge, which fades into the larger scale emission, is broader in
size and has considerably lower surface brightess and a moderately steeper
spectrum, i.e. . We propose that
the brightest part of the radio halo is powered by the central sloshing in
A2142, similar to what has been suggested for mini-halos, or by secondary
electrons generated by hadronic collisions in the ICM. On the other hand, the
steeper ridge may probe particle re-acceleration by turbulence generated either
by stirring the gas and magnetic fields on a larger scale or by less energetic
mechanisms, such as continuous infall of galaxy groups or an off-axis merger.Comment: 18 pages, 10 figures, 4 tables - A&A, accepte
Deep LOFAR 150 MHz imaging of the Bo\"otes field: Unveiling the faint low-frequency sky
We have conducted a deep survey (with a central rms of )
with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bo\"otes field, with
an angular resolution of , and obtained a sample of
10091 radio sources ( limit) over an area of .
The astrometry and flux scale accuracy of our source catalog is investigated.
The resolution bias, incompleteness and other systematic effects that could
affect our source counts are discussed and accounted for. The derived 150 MHz
source counts present a flattening below sub-mJy flux densities, that is in
agreement with previous results from high- and low- frequency surveys. This
flattening has been argued to be due to an increasing contribution of
star-forming galaxies and faint active galactic nuclei. Additionally, we use
our observations to evaluate the contribution of cosmic variance to the scatter
in source counts measurements. The latter is achieved by dividing our Bo\"otes
mosaic into 10 non-overlapping circular sectors, each one with an approximate
area of The counts in each sector are computed in the
same way as done for the entire mosaic. By comparing the induced scatter with
that of counts obtained from depth observations scaled to 150MHz, we find that
the scatter due to cosmic variance is larger than the Poissonian
errors of the source counts, and it may explain the dispersion from previously
reported depth source counts at flux densities . This work
demonstrates the feasibility of achieving deep radio imaging at low-frequencies
with LOFAR.Comment: A\&A in press. 15 pages, 16 figure
A blind detection of a large, complex, SunyaevâZelâdovich structure
We present an interesting SunyaevâZelâdovich (SZ) detection in the first of the Arcminute Microkelvin Imager (AMI) âblindâ, degree-square fields to have been observed down to our target sensitivity of 100”Jy beam^(-1). In follow-up deep pointed observations the SZ effect is detected with a maximum peak decrement greater than eight times the thermal noise. No corresponding emission is visible in the ROSAT all-sky X-ray survey and no cluster is evident in the Palomar all-sky optical survey. Compared with existing SZ images of distant clusters, the extent is large (â10 arcmin) and complex; our analysis favours a model containing two clusters rather than a single cluster. Our Bayesian analysis is currently limited to modelling each cluster with an ellipsoidal or spherical ÎČ model, which does not do justice to this decrement. Fitting an ellipsoid to the deeper candidate we find the following. (a) Assuming that the Evrard et al. approximation to Press & Schechter correctly gives the number density of clusters as a function of mass and redshift, then, in the search area, the formal Bayesian probability ratio of the AMI detection of this cluster is 7.9 Ă 10^4:1; alternatively assuming Jenkins et al. as the true prior, the formal Bayesian probability ratio of detection is 2.1 Ă 10^5:1. (b) The cluster mass is M_(T,200) = 5.5_(-1.3)^(+1.2) x 10^(14)h^(-1)_(70) M_â. (c) Abandoning a physical model with number density prior and instead simply modelling the SZ decrement using a phenomenological ÎČ model of temperature decrement as a function of angular distance, we find a central SZ temperature decrement of -295_(-15)^(+36) ”K â this allows for cosmic microwave background primary anisotropies, receiver noise and radio sources. We are unsure if the cluster system we observe is a merging system or two separate clusters
Untangling cosmic magnetic fields: Faraday tomography at metre wavelengths with LOFAR
14 pages, 6 figures. Accepted for publication in "The Power of Faraday Tomography" special issue of GalaxiesThe technique of Faraday tomography is a key tool for the study ofmagnetised plasmas in the new era of broadband radio-polarisation observations. In particular, observations at metre wavelengths provide significantly better Faraday depth accuracies compared to traditional centimetre-wavelength observations. However, the effect of Faraday depolarisationmakes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN (Active Galactic Nucleus). The majority of the sources (~64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of 4 Ă 10 26 W Hz -1 (with ~13% of all sources having a linear size > 1 Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of ~20'. One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least eight blazars. Most sources display simple Faraday spectra, but we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function.Peer reviewe
Revived Fossil Plasma Sources in Galaxy Clusters
© 2020 ESO.It is well established that particle acceleration by shocks and turbulence in the intra-cluster medium can produce cluster-scale synchrotron emitting sources. However, the detailed physics of these particle acceleration processes is still not well understood. One of the main open questions is the role of fossil relativistic electrons that have been deposited in the intracluster medium (ICM) by radio galaxies. These synchrotron-emitting electrons are very difficult to study as their radiative lifetime is only tens of Myr at gigahertz frequencies, and they are therefore a relatively unexplored population. Despite the typical steep radio spectrum due to synchrotron losses, these fossil electrons are barely visible even at radio frequencies well below the gigahertz level. However, when a pocket of fossil radio plasma is compressed, it boosts the visibility at sub-gigahertz frequencies, creating what are known as radio phoenices. This compression can be the result of bulk motion and shocks in the ICM due to merger activity. In this paper we demonstrate the discovery potential of low-frequency radio sky surveys to find and study revived fossil plasma sources in galaxy clusters. We used the 150 MHz TIFR GMRT Sky Survey and the 1.4 GHz NVSS sky survey to identify candidate radio phoenices. A subset of three candidates was studied in detail using deep multi-band radio observations (LOFAR and GMRT), X-ray obserations (Chandra or XMM-Newton), and archival optical observations. Two of the three sources are new discoveries. Using these observations, we identified common observational properties (radio morphology, ultra-steep spectrum, X-ray luminosity, dynamical state) that will enable us to identify this class of sources more easily, and will help us to understand the physical origin of these sources.Peer reviewedFinal Accepted Versio
Filaments in the southern giant lobe of Centaurus A : Constraints on nature and origin from modelling and GMRT observations
Date of acceptance: 22/05/2014We present results from imaging of the radio filaments in the southern giant lobe of CentaurusA using data from Giant Metrewave Radio Telescope observations at 325 and 235 MHz, and outcomes from filament modelling. The observations reveal a rich filamentary structure, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to the jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressuredwith respect to the global lobe plasma showing no evidence of largescale efficient Fermi I-type particle acceleration, and persist for ~2-3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318-444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabilities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic magnetohydrodynamical turbulence or from a moderately recent jet activity of the parent AGN, or an interplay of both.Peer reviewe
New constraints on the magnetic field in cosmic web filaments
Strong accretion shocks are expected to illuminate the warm hot intergalactic medium encompassed by the filaments of the cosmic web, through synchrotron radio emission. Given their high sensitivity, large low-frequency radio facilities may already be able to detect signatures of this extended radio emission from the region between two close and massive galaxy clusters. In this work we exploit the non-detection of such diffuse emission by deep observations of two pairs of relatively close ('10 Mpc) and massive (M500 = 1014 M ) galaxy clusters using the LOw-Frequency ARray. By combining the results from the two putative inter-cluster filaments, we derive new independent constraints on the median strength of intergalactic magnetic fields: B0 Mpc < 2:5 Ă 102 nG (95% confidence level). Based on cosmological simulations and assuming a primordial origin of the B-fields, these estimates can be used to limit the amplitude of primordial seed magnetic fields: B0 = 10 nG. We recommend the observation of similar cluster pairs as a powerful tool to set tight constraints on the amplitude of extragalactic magnetic fields
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