315 research outputs found
FIRST-based survey of Compact Steep Spectrum sources I. MERLIN images of arc-second scale objects
Compact Steep Spectrum (CSS) sources are powerful extragalactic radio sources
with angular dimensions of the order of a few arcseconds or less. Such a
compactness is apparently linked to the youth of these objects. The majority of
CSSs investigated so far have been known since the early 1980s. This paper is
the first in a series where we report the results of an observational campaign
targeted on a completely new sample of CSSs which are significantly weaker than
those investigated before. The ultimate goal of that campaign is to find out
how ``weak'' CSSs compare to ``strong'', classical ones, especially with regard
to the morphologies. Here we present an analysis of morphological and physical
properties of five relatively large sources based on MERLIN observations at 1.6
and 5 GHz.Comment: 8 pages, 7 figures, A&A in pres
Interferon beta-related nephropathy and interstitial lung disease: a new association and a long-term warning.
Radio-optically selected clusters of galaxies. I. The radiogalaxy sample
In order to study the status and the possible evolution of clusters of
galaxies at intermediate redshifts (z ~ 0.1 - 0.3), as well as their spatial
correlation and relationship with the local environment, we built a sample of
candidate groups and clusters of galaxies using radiogalaxies as tracers of
dense environments. This technique - complementary to purely optical or X-ray
cluster selection methods - represents an interesting tool for the selection of
clusters in a wide range of richness, so to make it possible to study the
global properties of groups and clusters of galaxies, such as their
morphological content, dynamical status and number density, as well as the
effect of the environment on the radio emission phenomena. In this paper we
describe the compilation of a catalogue of 16000 radio sources in the region of
the South Galactic Pole extracted from the publicly available NRAO VLA Sky
Survey maps, and the optical identification procedure with galaxies brighter
than b_J=20.0 in the EDSGC Catalogue. The radiogalaxy sample, valuable for the
study of radio source populations down to low flux levels, consists of 1288
identifications and has been used to detect candidate groups and clusters
associated to NVSS radio sources. In a companion paper we will discuss the
cluster detection method, the cluster sample as well as first spectroscopic
results.Comment: 15 pages, 6 Postscript and 1 GIF figures. Accepted for publication in
A&
A study of the environments of large radio galaxies using SDSS
The distributions of galaxies in the environments of 16 large radio sources
have been examined using the Sloan Digital Sky Survey. In the giant radio
galaxy J1552+2005 (3C326) which has the highest arm-length ratio, the shorter
arm is found to interact with a group of galaxies which forms part of a
filamentary structure. Although most large sources occur in regions of low
galaxy density, the shorter arm is brighter in most cases suggesting
asymmetries in the intergalactic medium which may not be apparent in the
distribution of galaxies. In two cases with strong and variable cores,
J0313+4120 and J1147+3501, the large flux density asymmetries are possibly also
caused by the effects of relativistic motion.Comment: Accepted for publication in MNRA
Radio-optically selected clusters of galaxies. II. The cluster sample
We present a sample of 171 candidate groups and clusters of galaxies at
intermediate redshift over an area of ~550 square degrees at the South Galactic
Pole selected by using optically identified radio sources from the NRAO VLA Sky
Survey as tracers of dense environment. Out of these 171 candidates, 76 have a
counterpart in the literature while 95 of them are previously unknown clusters.
This paper presents the cluster selection technique, based on the search of
excesses in the optical surface density of galaxies near identified
radiogalaxies, and the first spectroscopic results aimed to confirm the
presence of a cluster. Spectroscopy for 11 candidates led to the detection of 9
clusters at redshift in the range 0.13 - 0.3, with estimated velocity
dispersions ranging from values typical of clusters to those of galaxy groups.
These results show that this technique represents a powerful tool for the
selection of homogeneous samples of intermediate redshift clusters over a wide
range of richness.Comment: 13 pages, 3 Postscript and 2 GIF figures. Accepted for publication in
A&
Measurements of the reaction of antiproton annihilation at rest at three hydrogen target densities
The proton-antiproton annihilation at rest into the final state
was measured for three different target densities: liquid hydrogen, gaseous
hydrogen at NTP and at a low pressure of 5 mbar. The yield of this reaction in
the liquid hydrogen target is smaller than in the low-pressure gas target. The
branching ratios of the channel were calculated on the basis of
simultaneous analysis of the three data samples. The branching ratio for
annihilation into from the protonium state turns out to be
about ten times smaller as compared to the one from the state.Comment: 10 pages, 3 Postscript figures. Accepted by Physics Letters
New data on OZI rule violation in bar{p}p annihilation at rest
The results of a measurement of the ratio R = Y(phi pi+ pi-) / Y(omega pi+
pi-) for antiproton annihilation at rest in a gaseous and in a liquid hydrogen
target are presented. It was found that the value of this ratio increases with
the decreasing of the dipion mass, which demonstrates the difference in the phi
and omega production mechanisms. An indication on the momentum transfer
dependence of the apparent OZI rule violation for phi production from the 3S1
initial state was found.Comment: 11 pages, 3 PostScript figures, submitted to Physics Letter
Decline of the space density of quasars between z=2 and z=4
We define a new complete sample of 13 optically-luminous radio quasars
M_AB(1450 Angstrom) 25.7 with redshift
3.8 < z < 4.5, obtained by cross-correlating the FIRST radio survey and the APM
catalogue of POSS-I. We measure the space density to be 1.0 +/- 0.3 /Gpc^3, a
factor 1.9 +/- 0.7 smaller than the space density of similar quasars at z=2.
Using a new measurement of the radio-loud fraction of quasars we find that at
z=4 the total space density of quasars with M_AB(1450 Angstrom) < -26.9 is 7.4
+/- 2.6/Gpc^3. This is a factor 1.8 +/- 0.8 less than the space density at z=2,
found by the 2dF quasar survey. This (z=2)/(z=4) ratio, consistent with that of
the radio-loud quasars, is significantly different from the ratio of about 10
found for samples including lower-luminosity quasars. This suggests that the
decline of the space density beyond z=2 is slower for optically-luminous
quasars than for less-luminous ones.Comment: 13 pages, 6 postscript figures, to be published in Astrophys.
Journal, July 2003 issu
FIRST-based survey of Compact Steep Spectrum sources, II. MERLIN and VLA observations of Medium-sized Symmetric Objects
A new sample of candidate Compact Steep Spectrum (CSS) sources that are much
weaker than the CSS source prototypes has been selected from the VLA FIRST
catalogue. MERLIN `snapshot' observations of the sources at 5 GHz indicate that
six of them have an FR II-like morphology, but are not edge-brightened as is
normal for Medium-sized Symmetric Objects (MSOs) and FR IIs. Further
observations of these six sources with the VLA at 4.9 GHz and MERLIN at 1.7
GHz, as well as subsequent full-track observations with MERLIN at 5 GHz of what
appeared to be the two sources of greatest interest are presented. The results
are discussed with reference to the established evolutionary model of CSS
sources being young but in which not all of them evolve to become old objects
with extended radio structures. A lack of stable fuelling in some of them may
result in an early transition to a so-called coasting phase so that they fade
away instead of growing to become large-scale objects. It is possible that one
of the six sources (1542+323) could be labelled as a prematurely `dying' MSO or
a `fader'.Comment: 13 pages, matches the version printed in Astronomy & Astrophysic
A multifrequency study of the large radio galaxies 3C46 and 3C452
We present low-frequency observations starting from ~150 MHz with the Giant
Metrewave Radio Telescope (GMRT), and high-frequency observations with the Very
Large Array (VLA) of two large radio galaxies 3C46 and 3C452. These
observations were made with the objectives of estimating their spectral ages
and examining any evidence of diffuse extended emission at low radio
frequencies due to an earlier cycle of activity. While no evidence of extended
emission due to an earlier cycle of activity has been found, the spectral ages
have been estimated to be ~15 and 27 Myr for the oldest relativistic plasma
seen in the regions close to the cores for 3C46 and 3C452 respectively. The
spectra in the vicinity of the hotspots are consistent with a straight spectrum
with injection spectral indices of ~1.0 and 0.78 respectively, somewhat steeper
than theoretical expectations.Comment: Accepted for publication in MNRAS, 10 pages, 8 figures and 4 table
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