219 research outputs found
Diffuse radio emission in the merging cluster MACS J0717.5+3745: the discovery of the most powerful radio halo
Hierarchical models of structure formation predict that galaxy clusters grow
via mergers of smaller clusters and galaxy groups, as well as through
continuous accretion of gas. MACS J0717.5+3745 is an X-ray luminous and complex
merging cluster, located at a redshift of 0.55. Here we present Giant Metrewave
Radio Telescope (GMRT) radio observations at 610 MHz of this cluster. The main
aim of the observations is to search for diffuse radio emission within the
galaxy cluster MACS J0717.5+3745 related to the ongoing merger. These GMRT
observations are complemented by Very Large Array (VLA) archival observations
at 1.4, 4.9 and 8.5 GHz. We have discovered a radio halo in the cluster MACS
J0717.5+3745 with a size of about 1.2 Mpc. The radio power P_1.4 GHz is 5 x
10^25 W/Hz, which makes it the most powerful radio halo known till date. A 700
kpc radio structure, which we classify as a radio relic, is located in between
the merging substructures of the system. The location of this relic roughly
coincides with regions of the intra-cluster medium (ICM) that have a
significant enhancement in temperature as shown by Chandra. The major axis of
the relic is also roughly perpendicular to the merger axis. This shows that the
relic might be the result of a merger-related shock wave, where particles are
accelerated via the diffuse shock acceleration (DSA) mechanism. Alternatively,
the relic might trace an accretion shock of a large-scale galaxy filament to
the south-west. The global spectral index of radio emission within the cluster
is found to be -1.24 +/-0.05 between 4.9 GHz and 610 MHz. We derive a value of
5.8 microGauss for the equipartition magnetic field strength at the location of
the radio halo. [abridged].Comment: 8 pages, 9 figures, accepted for publication in A&A on August 3, 200
A vestige low metallicity gas shell surrounding the radio galaxy 0943-242 at z=2.92
Observations are presented showing the doublet CIV 1550 absorption lines
superimposed on the CIV emission in the radio galaxy 0943-242. Within the
errors, the redshift of the absorption system that has a column density of
N_CIV = 10^{14.5 +- 0.1} cm-2 coincides with that of the deep Ly-alpha
absorption trough observed by Rottgering et al. (1995). The gas seen in
absorption has a resolved spatial extent of at least 13 kpc (the size of the
extended emission line region). We first model the absorption and emission gas
as co-spatial components with the same metallicity and degree of excitation.
Using the information provided by the emission and absorption line ratios of
CIV and Ly-alpha, we find that the observed quantities are incompatible with
photoionization or collisional ionization of cloudlets with uniform properties.
We therefore reject the possibility that the absorption and emission phases are
co-spatial and favour the explanation that the absorption gas has low
metallicity and is located further away from the host galaxy (than the emission
line gas). The estimated low metallicity for the absorption gas in 0943-242 (Z
\~ 1% solar) and its proposed location -outer halo outside the radio cocoon-
suggest that its existence preceeds the observed AGN phase and is a vestige of
the initial starburst at the onset of formation of the parent galaxy.Comment: 11 pages,5 figures, A&A accepte
The radio source B 1834+620: A double-double radio galaxy with interesting properties
We present a study of the peculiar radio galaxy B 1834+620. It is
characterised by the presence of a 420-kpc large edge-brightened radio source
which is situated within, and well aligned with, a larger (1.66 Mpc) radio
source. Both sources apparently originate in the same host galaxy, which has a
R_s-magnitude of 19.7 and a redshift of 0.5194, as determined from the strong
emission-lines in the spectrum. We have determined the rotation measures
towards this source, as well as the radio spectral energy distribution of its
components. The radio spectrum of the large outer source is steeper than that
of the smaller inner source. The radio core has a spectrum that peaks at a
frequency of a few GHz. The rotation measures towards the four main components
are quite similar, within rad m of 58 rad m. They are
probably largely galactic in origin. We have used the presence of a bright
hotspot in the northern outer lobe to constrain the advance velocity of the
inner radio lobes to the range between 0.19c and 0.29c, depending on the
orientation of the source. This corresponds to an age of this structure in the
range between 2.6 and 5.8 Myr. We estimate a density of the ambient medium of
the inner lobes of \la 1.6 \times 10^{-30} gr\,cm (particle density
\la 8 \times 10^{-7} cm). A low ambient density is further supported
by the discrepancy between the large optical emission-line luminosity of the
host galaxy and the relatively low radio power of the inner lobes.Comment: Accepted for publication in MNRA
Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster
Galaxy clusters form through a sequence of mergers of smaller galaxy clusters
and groups. Models of diffusive shock acceleration (DSA) suggest that in shocks
that occur during cluster mergers, particles are accelerated to relativistic
energies, similar to supernova remnants. Together with magnetic fields these
particles emit synchrotron radiation and may form so-called radio relics. Here
we report the detection of a radio relic for which we find highly aligned
magnetic fields, a strong spectral index gradient, and a narrow relic width,
giving a measure of the magnetic field in an unexplored site of the universe.
Our observations prove that DSA also operates on scales much larger than in
supernova remnants and that shocks in galaxy clusters are capable of producing
extremely energetic cosmic rays.Comment: Published in Science Express on 23 September 2010, 6 figures,
Supporting Online Material included. This is the author's version of the
work. It is posted here by permission of the AAAS for personal use, not for
redistribution. The definitive version was published in Science, volume 330,
15 October 201
VLA radio continuum observations of a new sample of high redshift radio galaxies
We present new deep multi-frequency radio-polarimetric images of a sample of
high redshift radio galaxies (HzRGs), having redshift between 1.7 and 4.1. The
radio data at 4.7 and 8.2 GHz were taken with the Very Large Array in the A
configuration and provide a highest angular resolution of 0.2''. Maps of total
intensity, radio spectral index, radio polarization and internal magnetic field
are presented for each source. The morphology of most objects is that of
standard FRII double radio sources, but several contain multiple hot-spots in
one or both lobes. Compared to similar samples of HzRGs previously imaged,
there is a higher fraction (29%) of compact steep spectrum sources (i.e.
sources with a projected linear size less than 20 kpc). Radio cores are
identified in about half of the sample and tend to have relatively steep
spectra (alpha < -1). Polarization is detected in all but 4 sources, with
typical polarization at 8.2 GHz of around 10-20%. The Faraday rotation can be
measured in most of the radio galaxies: the observed rotation measure (RM) of 8
radio sources exceeds 100 rad m^{-2} in at least one of the lobes, with large
gradients between the two lobes. We find no dependence of Faraday rotation with
other properties of the radio sources. If the origin of the Faraday rotation is
local to the sources, as we believe, then the intrinsic RM is more than a 1000
rad m^{-2}. Because low redshift radio galaxies residing at the center of
clusters usually show extreme RMs, we suggest that the high-z large RM sources
also lie in very dense environments. Finally, we find that the fraction of
powerful radio galaxies with extreme Faraday rotation increases with redshift,
as would be expected if their average environment tends to become denser with
decreasing cosmic epoch.Comment: Accepted for publication in A&A Supplemen
The Lockman Hole Project: new constraints on the sub-mJy source counts from a wide-area 1.4 GHz mosaic
This paper is part of a series discussing the results obtained in the
framework of a wide international collaboration - the Lockman Hole Project -
aimed at improving the extensive multiband coverage available in the Lockman
Hole region, through novel deep, wide-area, multifrequency (60, 150, 350 MHz,
and 1.4 GHz) radio surveys. This multifrequency, multi-band information will be
exploited to get a comprehensive view of star formation and active galactic
nucleus activities in the high-redshift Universe from a radio perspective. In
this paper, we present novel 1.4 GHz mosaic observations obtained with the
Westerbork Synthesis Radio Telescope. With an area coverage of 6.6 deg2, this
is the largest survey reaching an rms noise of 11 uJy/beam. In this paper, we
present the source catalogue (~6000 sources with flux densities S>55 uJy
(5sigma), and we discuss the 1.4 GHz source counts derived from it. Our source
counts provide very robust statistics in the flux range 0.1<S<1 mJy, and are in
excellent agreement with other robust determinations obtained at lower and
higher flux densities. A clear excess is found with respect to the counts
predicted by the semi-empirical radio sky simulations developed in the
framework of the Square Kilometre Array Simulated Skies project. A preliminary
analysis of the identified (and classified) sources suggests this excess is to
be ascribed to star-forming galaxies, which seem to show a steeper evolution
than predicted.Comment: accepted for publication on MNRAS. New version that corrects latex
errors and contain the correct version of figure 1
Faint Gigahertz peaked spectrum sources and the evolution of young radio sources
GPS sources are the objects of choice to study the initial evolution of
extragalactic radio sources, since it is most likely that they are the young
counterparts of large scale radio sources. Correlations found between their
peak frequency, peak flux density and angular size provide strong evidence that
synchrotron self absorption is the cause of the spectral turnovers, and
indicate that young radio sources evolve in a self-similar way. The difference
in redshift distribution between young and old radio sources must be due to a
difference in slope of their luminosity functions, and we argue that this slope
is strongly affected by the luminosity evolution of the individual sources. A
luminosity evolution scenario is proposed in which GPS sources increase in
luminosity and large scale radio sources decrease in luminosity with time. It
is shown that such a scenario agrees with the local luminosity function of GPS
galaxies.Comment: Late, 6 pages, 2 figs. To appear in the proceedings of EVN/JIVE
Symposium No 4, New Astronomy Reviews (eds. Garrett et al.). For related
papers, see http://www.ast.cam.ac.uk/~snelle
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