40 research outputs found
A search for high redshift clusters associated with radio galaxies at 2 < z < 4
High redshift radio galaxies are amongst the most massive galaxies in the
early Universe and have properties expected from central galaxies in forming
clusters. We are carrying out an observational programme on the VLT to find and
study galaxy proto clusters around radio galaxies at redshifts 2 < z < 4.
First, we use narrow band imaging to select candidate galaxies which show
excess Lyman alpha emission at redshifts similar to the central radio galaxy.
Then, we use multi object spectroscopy to confirm the redshifts of these
candidates and measure the velocity dispersion of the cluster members. Our goal
is to observe a sample of about 10 targets and investigate galaxy overdensities
as a function of redshift. Here, we report on the current progress of the
programme and show some preliminary results which include the discovery of a
structure of galaxies at redshift 4.1.Comment: 4 pages, 3 figures, Sesto conference proceeding 'Probing cosmic
evolution with galaxy clusters
WNB 0313+683: Analysis of a Newly Discovered Giant Radio Galaxy
We present the results of a detailed analysis of the newly discovered 15
arcmin large FRII-type radio galaxy WNB 0313+683. It has a redshift of 0.0901
+/- 0.0002 and a projected linear size of 2.0 Mpc. Using multi-frequency radio
data from the WENSS and NVSS surveys, the VLA, the WSRT and the 100m Effelsberg
telescope, we have derived the age of the source (1.4 +/- 0.1 x 10^8 yrs) and
the particle densities around the two radio lobes (>1.6 x 10^(-6) per cubic cm
for the southern lobe and >5.8 x 10^(-7) per cubic cm for the northern). From
optical spectroscopy we find that the H emission line has a broad
component, and that the extinction is large (with colour index E(B-V) = 0.98
+/- 0.10 mag), but is probably mostly galactic in origin (b= +9.8 degrees).
Further, the radio core has an inverted spectrum with a spectral index of +0.42
+/- 0.03, suggesting a very compact structure. At 10.5 GHz, it contributes 25%
of the total flux density of the source, which is extremely high for radio
galaxies. WNB 0313+683 has a very large optical emission line flux with respect
to its estimated jet power, when compared with the correlation between these
two properties found by Rawlings & Saunders (1991). We argue that this,
together with the relatively high radio power and the inverted radio spectrum
of the radio core, is suggestive of a new phase of radio activity in this
source.Comment: 26 pages, LaTeX (l-aa30 style, including 5 Tables and 30 figures),
Accepted for publication in A&A Main Journa
Radio Observations of the Hubble Deep Field South Region III: The 2.5, 5.2 and 8.7 GHz Catalogues and Radio Source Properties
Deep radio observations of a wide region centred on the Hubble Deep Field
South have been performed, providing one of the most sensitive set of radio
observations acquired on the Australia Telescope Compact Array to date. A
central rms of ~10 microJy is reached at four frequencies (1.4, 2.5, 5.2 and
8.7 GHz). In this paper the full source catalogues from the 2.5, 5.2 and 8.7
GHz observations are presented to complement Paper II, along with a detailed
analysis of image quality and noise. We produce a consolidated catalogue by
matching sources across all four frequencies of our survey. Radio spectral
indices are used to investigate the nature of the radio sources and identify a
number of sources with flat or inverted radio spectra, which indicates AGN
activity. We also find several other interesting sources, including a broadline
emitting radio galaxy, a giant radio galaxy and three Gigahertz Peaked Spectrum
sources.Comment: Accepted by AJ. 13 figures and 13 table
The Nascent Red Sequence at z~2
We present new constraints on the evolution of the early-type galaxy
color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy
protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space
Telescope. This field contains a spectroscopically confirmed space-overdensity
of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio
galaxy MRC 1138-262. Using these NICMOS data we identify a significant
surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram
(when compared with deep NICMOS imaging from the HDF-N and UDF). The
optical-NIR colors of these prospective red-sequence galaxies indicate the
presence of on-going dust-obscured star-formation or recently formed (<~ 1.5
Gyr)stellar populations in a majority of the red galaxies. We measure the slope
and intrinsic scatter of the CMR for three different red galaxy samples
selected by a wide color cut, and using photometric redshifts both with and
without restrictions on rest-frame optical morphology. In all three cases both
the rest-frame slope and intrinsic color scatter are considerably higher
than corresponding values for lower redshift galaxy clusters. These results
suggest that while some relatively quiescent galaxies do exist in this
protocluster both the majority of the galaxy population and hence the
color-magnitude relation are still in the process of forming, as expected.Comment: 8 pages, 7 figures, accepted for publication in ApJ (to appear June
1, 2008, v679n2
The Planck clusters in the LOFAR sky: IV: LoTSS-DR2: statistics of radio halos and re-acceleration models
Diffuse cluster-scale synchrotron radio emission is discovered in an
increasing number of galaxy clusters in the form of radio halos (RHs), probing
the presence of relativistic electrons and magnetic fields in the intra-cluster
medium. The favoured scenario to explain their origin is that they trace
turbulent regions generated during cluster mergers where particles are
re-accelerated. In this framework, RHs are expected to probe cluster dynamics
and are predicted to be more frequent in massive systems. Statistical studies
are important to study the connection of RHs with cluster dynamics and to
constrain theoretical models. Furthermore, low-frequency surveys can shed light
on the existence of RHs with very steep radio-spectra, a key prediction of
turbulent models. We study the properties of RHs from clusters of the second
catalog of Planck Sunyaev Zel'dovich detected sources that lie within the 5634
deg^2 covered by the second Data Release (DR2) of the LOFAR Two-meter Sky
Survey. We find that the number of observed RHs, their radio flux density and
redshift distributions are in line with what is expected in the framework of
the re-acceleration scenario. In addition, the fraction of clusters with RHs
increases with the cluster mass, confirming the leading role of the
gravitational process of cluster formation in the generation of RHs. These
models predict a large fraction of RHs with very steep spectrum in the DR2
Planck sample, this will be tested in future studies, yet a comparison of the
occurrence of halos in GMRT and LOFAR samples indeed shows a larger occurrence
of RHs at lower frequencies suggesting the presence of a number of very steep
spectrum RH that is preferentially detected by LOFAR. Using morphological
information we confirm that RHs are preferentially located in merging systems
and that the fraction of newly LOFAR discovered RHs is larger in less disturbed
systems.Comment: 12 pages, 10 figures, accepted for publication in A&
Optimized Trigger for Ultra-High-Energy Cosmic-Ray and Neutrino Observations with the Low Frequency Radio Array
When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an efficient trigger
implementation for LOFAR optimized for the observation of short radio pulses.Comment: Submitted to Nuclear Instruments and Methods in Physics Research
Section
Obscured and powerful AGN and starburst activities at z~3.5
We report the discovery of two sources at z=3.867 and z=3.427 that exhibit
powerful starburst and AGN activities. They benefit from data from radio to X
rays from the CFHTLS-D1/SWIRE/XMDS surveys. Follow-up optical and near-infrared
spectroscopy, and millimeter IRAM/MAMBO observations are also available. We
performed an analysis of their spectral energy distributions to understand the
origin of their emission and constrain their luminosities. A comparison with
other composite systems at similar redshifts from the literature is also
presented. The AGN and starburst bolometric luminosities are ~10^13 Lsun. The
AGN emission dominates at X ray, optical, mid-infrared wavelengths, and
probably in the radio. The starburst emission dominates in the far-infrared.
The estimated star formation rates range from 500 to 3000Msun/yr. The AGN
near-infrared and X ray emissions are heavily obscured in both sources with an
estimated dust extinction Av>4, and Compton-thick gas column densities. The two
sources are the most obscured and most luminous AGNs detected at millimeter
wavelengths currently known. The sources presented in this work are heavily
obscured QSOs, but their properties are not fully explained by the standard AGN
unification model. In one source, the ultraviolet and optical spectra suggest
the presence of outflowing gas and shocks, and both sources show emission from
hot dust, most likely in the vicinity of the nucleus. Evidence of moderate
AGN-driven radio activity is found in both sources. The two sources lie on the
local M_BH-M_bulge relation. To remain on this relation, their star formation
rate has to decrease. Our results support evolutionary models that invoke radio
feedback as star formation quenching mechanism, and suggest that such a
mechanism might play a major role also in powerful AGNs.Comment: Accepted for publication in Astronomy & Astrophysics (12 pages; 6
figures); replaced version includes minor language editing and revised
reference
Multiwavelength characterization of faint ultra steep spectrum radio sources: a search for high-redshift radio galaxies
Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts.
Aims. Using our 325 MHz GMRT observations (5Ï âŒ 800 ÎŒJy) and 1.4 GHz VLA observations (5Ï âŒ 80â100 ÎŒJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature.
Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Results. Redshift estimates are available for 86/116 (âŒ74%) USS sources in the VLA-VVDS field and for 39/44 (âŒ87%) USS sources in the SXDF fields with median values (zmedian) âŒ1.18 and âŒ1.57, respectively, which are higher than estimates for non-USS radio sources (zmedian nonâUSS ⌠0.99 and âŒ0.96), in the two subfields. The MIR colorâcolor diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z > 0.5) being AGN. The flux ratio of radio to mid-IR (S 1.4 GHz/S 3.6 ÎŒm) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ⌠0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (âŒ26% in the VLA-VVDS and âŒ13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates.
Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments