2,900 research outputs found
The Diverse Properties of GPS Sources
We discuss the morphology and kinematics of five gigahertz-peaked spectrum
(GPS) sources that have been observed with the VLBA. We find a wide range of
observed properties including core-jet structure, superluminal motion,
variability, extended structure, and polarization, all of which appear to
deviate from commonly-accepted GPS paradigms (e.g., O'Dea 1998). We suggest
that the observed low frequency cutoff in GPS sources may be primarily due to
free-free absorption rather than synchrotron self-absorption.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. (4
pages, 5 figures, needs evn2002.cls style file
Space VLBI Observations Show in the Quasar NRAO 530
We present here space-based VLBI observations with VSOP and a southern
hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz.
The brightness temperature of the core at 1.6 GHz is K. The
size is near the minimum observable value in the direction of NRAO~530 due to
interstellar scattering. The 5 GHz data show a single component with a
brightness temperature of K, significantly in excess of
the inverse Compton limit and of the equipartition brightness temperature limit
(Readhead 1994). This is strong evidence for relativistic motion in a jet
requiring model-dependent Doppler boosting factors in the range 6 to 60. We
show that a simple homogeneous sphere probably does not model the emission
region accurately. We favor instead an inhomogeneous jet model with a Doppler
boosting factor of 15.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter
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
The under-explored radio-loudness of quasars and the possibility of radio-source--environment interactions
I demonstrate that radio observations in the literature to date of
optically-selected quasars are largely inadequate to reveal the full extent of
their jet-activity. I discuss a recent example of an optically-powerful quasar,
which is radio-quiet according to all the standard classifications, which
Blundell & Rawlings discovered to have a >100 kpc jet, and show that other than
being the first FRI quasar to be identified, there is no reason to presume it
is exceptional. I also discuss a possible new probe of accounting for the
interactions of radio sources with their environments. This tool could help to
avoid over-estimating magnetic fields strengths within cluster gas. I briefly
describe recent analyses by Rudnick & Blundell which confront claims in the
literature of cluster gas B-fields > 10 micro-G.Comment: invited talk at "The Physics of Relativistic Jets in the CHANDRA and
XMM Era", proceedings edited by G. Brunetti, D.E. Harris, R.M. Sambruna, and
G. Setti, to be published in New Astronomy Review
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
MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration
We discuss acceleration measurements for a large sample of extragalactic
radio jets from the MOJAVE program which studies the parsec-scale jet structure
and kinematics of a complete, flux-density-limited sample of Active Galactic
Nuclei (AGN). Accelerations are measured from the apparent motion of individual
jet features or "components" which may represent patterns in the jet flow. We
find that significant accelerations are common both parallel and perpendicular
to the observed component velocities. Parallel accelerations, representing
changes in apparent speed, are generally larger than perpendicular acceleration
that represent changes in apparent direction. The trend for larger parallel
accelerations indicates that a significant fraction of these changes in
apparent speed are due to changes in intrinsic speed of the component rather
than changes in direction to the line of sight. We find an overall tendency for
components with increasing apparent speed to be closer to the base of their
jets than components with decreasing apparent speed. This suggests a link
between the observed pattern motions and the underlying flow which, in some
cases, may increase in speed close to the base and decrease in speed further
out; however, common hydro-dynamical processes for propagating shocks may also
play a role. About half of the components show "non-radial" motion, or a
misalignment between the component's structural position angle and its velocity
direction, and these misalignments generally better align the component motion
with the downstream emission. Perpendicular accelerations are closely linked
with non-radial motion. When observed together, perpendicular accelerations are
usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa
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