2,143 research outputs found
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
Variability and Velocity of Superluminal Sources
We investigate the relation between the Doppler factor determined from
variations in total flux at 22 and 37 GHz, and the apparent transverse velocity
determined from VLBA observations at 2 cm. The data are consistent with the
relativistic beaming theory for compact radio sources, in that the distribution
of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte
Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close
to the "equipartition value" calculated by Readhead (1994). We deduce the
distribution of Lorentz factors for a group of 48 sources; the values range up
to about gamma=40.Comment: To be published in "Radio Astronomy at the Fringe", ASP Conf. Ser.
Vol. 300, J. A. Zensus, M. H. Cohen, & E. Ros (eds.), 8 pages, 3 figures,
needs rafringe.st
Kinematics of parsec-scale structures in AGN: the 2cm VLBA Survey
We are investigating the kinematics of jets in active galactic nuclei on
parsec scales by studying a representative population of sources. This study is
being carried out using the Very Long Baseline Array at 15 GHz, with more than
800 images taken since 1994. In this contribution we present an overview of the
diversity of kinematics for a complete sample of sources.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. 2
pages, 3 figures, needs evn2002.cls style fil
Intrinsic Brightness Temperatures of AGN Jets
We present a new method for studying the intrinsic brightness temperatures of
the parsec-scale jet cores of Active Galactic Nuclei (AGN). Our method uses
observed superluminal motions and observed brightness temperatures for a large
sample of AGN to constrain the characteristic intrinsic brightness temperature
of the sample as a whole. To study changes in intrinsic brightness temperature,
we assume that the Doppler factors of individual jets are constant in time as
justified by their relatively small changes in observed flux density. We find
that in their median-low brightness temperature state, the sources in our
sample have a narrow range of intrinsic brightness temperatures centered on a
characteristic temperature, T_int = 3 x 10^10 K, which is close to the value
expected for equipartition, when the energy in the radiating particles equals
the energy stored in the magnetic fields. However, in their maximum brightness
state, we find that sources in our sample have a characteristic intrinsic
brightness temperature greater than 2 x 10^11 K, which is well in excess of the
equipartition temperature. In this state, we estimate the energy in radiating
particles exceeds the energy in the magnetic field by a factor of ~ 10^5. We
suggest that the excess of particle energy when sources are in their maximum
brightness state is due to injection or acceleration of particles at the base
of the jet. Our results suggest that the common method of estimating jet
Doppler factors by using a single measurement of observed brightness
temperature and/or the assumption of equipartition may lead to large scatter or
systematic errors in the derived values.Comment: 4 pages, 2 figures, Accepted to Appear in ApJ Letter
The Shroud Around the Twin Radio Jets in NGC 1052
(Abridged) We discuss multiple VLBI continuum and spectral line observations
and WSRT spectroscopy of NGC 1052. Sub-parsec scale features move outward at
approximately 0.26c in bi-symmetric jets, most likely oriented near the plane
of the sky. Absorption and emission signatures reveal ionised, atomic, and
molecular components of the surrounding medium.
Seven-frequency (1.4 to 43 GHz) VLBA observations show free-free absorption
in the inner parsec, probably together with synchrotron self-absorption. There
is apparently a geometrically thick but patchy structure oriented roughly
orthogonal to the jets. The western jet is receding: it is covered more deeply
and extensively. HI spectral line VLBI reveals atomic gas in front of both
jets. There appear to be three velocity systems. The deepest, at "high
velocities" (receding by 125 to 200 km/s), seems restricted to a shell 1 to 2
pc away from the core, within which this gas might be largely ionised. WSRT
spectroscopy has revealed 1667 and 1665 MHz OH absorption with their line ratio
varying roughly from 1:1 to 2:1 between -35 and 200 km/s. In the high velocity
system the OH profiles are similar to HI, suggesting co-location of that atomic
and molecular gas, and leaving unclear the connection to the H2O masing gas
seen elsewhere. We have also detected both 18cm OH satellite lines in the high
velocity system. They have conjugate profiles: 1612 MHz is in absorption, and
1720 MHz in emission.Comment: 16 pages, 14 figures, LaTeX, includes aa.cls, accepted for
publication in Astronomy and Astrophysic
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