1,953 research outputs found
Radio faint AGN: a tale of two populations
We study the Extended Chandra Deep Field South (E-CDFS) Very Large Array
sample, which reaches a flux density limit at 1.4 GHz of 32.5 microJy at the
field centre and redshift ~ 4, and covers ~ 0.3 deg^2. Number counts are
presented for the whole sample while the evolutionary properties and luminosity
functions are derived for active galactic nuclei (AGN). The faint radio sky
contains two totally distinct AGN populations, characterised by very different
evolutions, luminosity functions, and Eddington ratios: radio-quiet
(RQ)/radiative-mode, and radio-loud/jet-mode AGN. The radio power of RQ AGN
evolves ~ (1+z)^2.5, similarly to star-forming galaxies, while the number
density of radio-loud ones has a peak at ~ 0.5 and then declines at higher
redshifts. The number density of radio-selected RQ AGN is consistent with that
of X-ray selected AGN, which shows that we are sampling the same population.
The unbiased fraction of radiative-mode RL AGN, derived from our own and
previously published data, is a strong function of radio power, decreasing from
~ 0.5 at P_1.4GHz ~ 10^24 W/Hz to ~ 0.04$ at P_1.4GHz ~ 10^22 W/Hz. Thanks to
our enlarged sample, which now includes ~ 700 radio sources, we also confirm
and strengthen our previous results on the source population of the faint radio
sky: star-forming galaxies start to dominate the radio sky only below ~ 0.1
mJy, which is also where radio-quiet AGN overtake radio-loud ones.Comment: 19 pages, 13 figures, accepted for publication in MNRA
Multi-frequency investigation of the parsec- and kilo-parsec-scale radio structures in high-redshift quasar PKS 1402+044
We investigate the frequency-dependent radio properties of the jet of the
luminous high-redshift (z = 3.2) radio quasar PKS 1402+044 (J1405+0415) by
means of radio interferometric observations. The observational data were
obtained with the VLBI Space Observatory Programme (VSOP) at 1.6 and 5 GHz,
supplemented by other multi-frequency observations with the Very Long Baseline
Array (VLBA; 2.3, 8.4, and 15 GHz) and the Very Large Array (VLA; 1.4, 5, 15,
and 43 GHz). The observations span a period of 7 years. We find that the
luminous high-redshift quasar PKS 1402+044 has a pronounced "core-jet"
morphology from the parsec to the kilo-parsec scales. The jet shows a steeper
spectral index and lower brightness temperature with increasing distance from
the jet core. The variation of brightness temperature agrees well with the
shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic
field, we estimate the mass of the central object as ~10^9 M_sun. The upper
limit of the jet proper motion of PKS 1402+044 is 0.03 mas/yr (~3c) in the
east-west direction.Comment: 9 pages, 6 figures
The micro-Jy Radio Source Population: the VLA-CDFS View
We analyse the 267 radio sources from our deep (flux limit of 42 microJy at
the field center at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey.
The radio population is studied by using a wealth of multi-wavelength
information, including morphology and spectral types, in the radio, optical,
and X-ray bands. The availability of redshifts for ~ 70% of our sources allows
us to derive reliable luminosity estimates for the majority of the objects.
Contrary to some previous results, we find that star-forming galaxies make up
only a minority (~ 1/3) of sub-mJy sources, the bulk of which are faint radio
galaxies, mostly of the Fanaroff-Riley I type.Comment: 6 pages, 3 figures, to appear in the proceedings of "At the Edge of
the Universe", Sintra, Portugal, Oct. 9 - 13, 200
The Parkes quarter-Jansky flat-spectrum sample 3. Space density and evolution of QSOs
We analyze the Parkes quarter-Jansky flat-spectrum sample of QSOs in terms of
space density, including the redshift distribution, the radio luminosity
function, and the evidence for a redshift cutoff. With regard to the luminosity
function, we note the strong evolution in space density from the present day to
epochs corresponding to redshifts ~ 1. We draw attention to a selection effect
due to spread in spectral shape that may have misled other investigators to
consider the apparent similarities in shape of luminosity functions in
different redshift shells as evidence for luminosity evolution. To examine the
evolution at redshifts beyond 3, we develop a model-independent method based on
the V_max test using each object to predict expectation densities beyond z=3.
With this we show that a diminution in space density at z > 3 is present at a
significance level >4 sigma. We identify a severe bias in such determinations
from using flux-density measurements at epochs significantly later than that of
the finding survey. The form of the diminution is estimated, and is shown to be
very similar to that found for QSOs selected in X-ray and optical wavebands.
The diminution is also compared with the current estimates of star-formation
evolution, with less conclusive results. In summary we suggest that the
reionization epoch is little influenced by powerful flat-spectrum QSOs, and
that dust obscuration does not play a major role in our view of the QSO
population selected at radio, optical or X-ray wavelengths.Comment: 18 pages, 11 figures, accepted 18 Dec 2004, Astron. & Astrophys. The
accepted version is expanded to include an analysis of the form of the
decline in radio-QSO space density at high redshifts. This is compared with
the forms of epoch dependence derived for optically-selected QSOs, for
X-ray-selected QSOs, and for star formation rat
A Global 86GHz VLBI Survey of Compact Radio Sources
We present results from a large 86GHz global VLBI survey of compact radio
sources. The main goal of the survey is to increase by factors of 3--5 the
total number of objects accessible for future 3-mm VLBI imaging. The survey
observations reach the baseline sensitivity of 0.1Jy and image sensitivity of
better than 10 mJy/beam. The total of 127 compact radio sources have been
observed. The observations have yielded images for 109 sources, extending the
database of the sources imaged at 86GHz with VLBI observation by a factor of 5,
and only 6 sources have not been detected. The remaining 12 objects have been
detected but could not be imaged due to insufficient closure phase information.
Radio galaxies are less compact than quasars and BL Lacs on sub-milliarcsecond
scale. Flux densities and sizes of core and jet components of all imaged
sources have been estimated using Gaussian model fitting. From these
measurements, brightness temperatures have been calculated, taking into account
resolution limits of the data. The cores of 70% of the imaged sources are
resolved. The core brightness temperatures of the sources peak at K and only 1% have brightness temperatures higher than K.
Cores of Intraday Variable (IDV) sources are smaller in angular size than
non-IDV sources, and so yield higher brightness temperatures.Comment: 72 pages, 12 figures, accepted for publication in the Astronomical
Journa
The VLA 1.4GHz Survey of the Extended Chandra Deep Field South: Second Data Release
Deep radio observations at 1.4GHz for the Extended Chandra Deep Field South
were performed in June through September of 2007 and presented in a first data
release (Miller et al. 2008). The survey was made using six separate pointings
of the Very Large Array (VLA) with over 40 hours of observation per pointing.
In the current paper, we improve on the data reduction to produce a second data
release (DR2) mosaic image. This DR2 image covers an area of about a third of a
square degree and reaches a best rms sensitivity of 6 uJy and has a typical
sensitivity of 7.4 uJy per 2.8" by 1.6" beam. We also present a more
comprehensive catalog, including sources down to peak flux densities of five or
more times the local rms noise along with information on source sizes and
relevant pointing data. We discuss in some detail the consideration of whether
sources are resolved under the complication of a radio image created as a
mosaic of separate pointings each suffering some degree of bandwidth smearing,
and the accurate evaluation of the flux densities of such sources. Finally, the
radio morphologies and optical/near-IR counterpart identifications (Bonzini et
al. 2012) are used to identify 17 likely multiple-component sources and arrive
at a catalog of 883 radio sources, which is roughly double the number of
sources contained in the first data release.Comment: to appear in the Astrophysical Journal Supplement Series; 41 page
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