4,825 research outputs found
Extragalactic radio source evolution under the dual-population unification scheme
We show that a dual-population unification scheme provides a successful
paradigm with which to describe the evolution and beaming of all bright
extragalactic radio sources. The paradigm consists of two intrinsic
radio-source populations, based on the two distinct radio-galaxy morphologies
of Fanaroff-Riley classes I and II. These represent the `unbeamed' or `side-on'
parent populations of steep radio spectra; the `beamed' source types including
flat-spectrum quasars and BL Lac objects, arise through the random alignment of
their radio-axis to our line-of-sight where Doppler-beaming of the relativistic
radio jets produces highly anisotropic radio emission.Comment: 18 pages & 18 postscript figures, accepted for publication in MNRA
The correlation function of radio sources
We investigate the large-scale clustering of radio sources in the Green Bank
and Parkes-MIT-NRAO 4.85 GHz surveys by measuring the angular two-point
correlation function w(\theta). Excluding contaminated areas, the two surveys
together cover 70 per cent of the whole sky. We find both surveys to be
reasonably complete above 50 mJy. On the basis of previous studies, the radio
sources are galaxies and radio-loud quasars lying at redshifts up to z \sim 4,
with a median redshift z \sim 1. This provides the opportunity to probe
large-scale structures in a volume far larger than that within the reach of
present optical and infrared surveys. We detect a clustering signal w(\theta)
\approx 0.01 for \theta = 1\degr. By assuming an evolving power-law spatial
correlation function in comoving coordinates \xi(r_c,z) = ( r_c / r_0
)^{-\gamma} (1+z)^{\gamma-(3+\epsilon)}, where \gamma = 1.8, and the redshift
distribution N(z) of the radio galaxies, we constrain the r_0--\epsilon
parameter space. For `stable clustering' (\epsilon = 0), we find the
correlation length r_0 \approx 18 Mpc/h, larger than the value for nearby
normal galaxies and comparable to the cluster-cluster correlation length.Comment: 8 pages, 7 ps figures included, LaTeX (mn,sty). Accepted by MNRA
Variance and Skewness in the FIRST survey
We investigate the large-scale clustering of radio sources in the FIRST
1.4-GHz survey by analysing the distribution function (counts in cells). We
select a reliable sample from the the FIRST catalogue, paying particular
attention to the problem of how to define single radio sources from the
multiple components listed. We also consider the incompleteness of the
catalogue. We estimate the angular two-point correlation function ,
the variance , and skewness of the distribution for the
various sub-samples chosen on different criteria. Both and
show power-law behaviour with an amplitude corresponding a spatial correlation
length of Mpc. We detect significant skewness in the
distribution, the first such detection in radio surveys. This skewness is found
to be related to the variance through , with
, consistent with the non-linear gravitational growth of
perturbations from primordial Gaussian initial conditions. We show that the
amplitude of variance and skewness are consistent with realistic models of
galaxy clustering.Comment: 13 pages, 21 inline figures, to appear in MNRA
The Deep Diffuse Extragalactic Radio Sky at 1.75 GHz
We present a study of diffuse extragalactic radio emission at GHz
from part of the ELAIS-S1 field using the Australia Telescope Compact Array.
The resulting mosaic is deg, with a roughly constant noise region
of deg used for analysis. The image has a beam size of arcsec and instrumental Jy beam. Using point-source models from the ATLAS survey, we
subtract the discrete emission in this field for Jy
beam. Comparison of the source-subtracted probability distribution, or
\pd, with the predicted distribution from unsubtracted discrete emission and
noise, yields an excess of Jy beam. Taking this as
an upper limit on any extended emission we constrain several models of extended
source counts, assuming arcmin. The best-fitting
models yield temperatures of the radio background from extended emission of
mK, giving an upper limit on the total temperature at
GHz of mK. Further modelling shows that our data are
inconsistent with the reported excess temperature of ARCADE2 to a source-count
limit of Jy. Our new data close a loop-hole in the previous
constraints, because of the possibility of extended emission being resolved out
at higher resolution. Additionally, we look at a model of cluster halo emission
and two WIMP dark matter annihilation source-count models, and discuss general
constraints on any predicted counts from such sources. Finally, we report the
derived integral count at GHz using the deepest discrete count plus our
new extended-emission limits, providing numbers that can be used for planning
future ultra-deep surveys.Comment: 18 pages, 15 figures, 7 tables, Accepted by MNRA
Searching for Large Scale Structure in Deep Radio Surveys
(Abridged Abstract) We calculate the expected amplitude of the dipole and
higher spherical harmonics in the angular distribution of radio galaxies. The
median redshift of radio sources in existing catalogues is z=1, which allows us
to study large scale structure on scales between those accessible to present
optical and infrared surveys, and that of the Cosmic Microwave Background
(CMB). The dipole is due to 2 effects which turn out to be of comparable
magnitude: (i) our motion with respect to the CMB, and (ii) large scale
structure, parameterised here by a family of Cold Dark Matter power-spectra. We
make specific predictions for the Green Bank (87GB) and Parkes-MIT-NRAO (PMN)
catalogues. For these relatively sparse catalogues both the motion and large
scale structure dipole effects are expected to be smaller than the Poisson
shot-noise. However, we detect dipole and higher harmonics in the combined
87GB-PMN catalogue which are far larger than expected. We attribute this to a 2
% flux mismatch between the two catalogues. We also investigate the existence
and extent of the Supergalactic Plane in the above catalogues. In a strip of +-
10 deg of the standard Supergalactic equator, we find a 3-sigma detection in
PMN, but only 1-sigma in 87GB.Comment: 15 pages, 5 ps figures, Latex, Submitted to MNRA
The Redshift Distribution of FIRST Radio Sources at 1 mJy
We present spectra for a sample of radio sources from the FIRST survey, and
use them to define the form of the redshift distribution of radio sources at
mJy levels.We targeted 365 sources and obtained 46 redshifts (13 per cent of
the sample). We find that our sample is complete in redshift measurement to R
, corresponding to . Early-type galaxies represent the
largest subset (45 per cent) of the sample and have redshifts 0.15\la z \la
0.5 ; late-type galaxies make up 15 per cent of the sample and have redshifts
0.05\la z \la 0.2; starbursting galaxies are a small fraction ( per
cent), and are very nearby (z\la 0.05). Some 9 per cent of the population
have Seyfert1/quasar-type spectra, all at z\ga 0.8, and there are 4 per cent
are Seyfert2 type galaxies at intermediate redshifts (). Using our
measurements and data from the Phoenix survey, we obtain an estimate for
at mJy and compare this with model predictions. At
variance with previous conclusions, we find that the population of starbursting
objects makes up \la 5 per cent of the radio population at S mJy.Comment: 20 pages, sumbitted to MNRA
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