9,061 research outputs found
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
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets
We discuss the jet kinematics of a complete flux-density-limited sample of
135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to
investigate the structure and evolution of parsec-scale jet phenomena. Our
analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained
between 2002 and 2007, but includes our previously published observations made
at the same wavelength, and is supplemented by VLBA archive data. In all, we
have used 2424 images spanning the years 1994-2007 to study and determine the
motions of 526 separate jet features in 127 jets. The data quality and temporal
coverage (a median of 15 epochs per source) of this complete AGN jet sample
represents a significant advance over previous kinematics surveys. In all but
five AGNs, the jets appear one-sided, most likely the result of differential
Doppler boosting. In general the observed motions are directed along the jet
ridge line, outward from the optically thick core feature. We directly observe
changes in speed and/or direction in one third of the well-sampled jet
components in our survey. While there is some spread in the apparent speeds of
separate features within an individual jet, the dispersion is about three times
smaller than the overall dispersion of speeds among all jets. This supports the
idea that there is a characteristic flow that describes each jet, which we have
characterized by the fastest observed component speed. The observed maximum
speed distribution is peaked at ~10c, with a tail that extends out to ~50c.
This requires a distribution of intrinsic Lorentz factors in the parent
population that range up to ~50. We also note the presence of some rare
low-pattern speeds or even stationary features in otherwise rapidly flowing
jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal;
online only material is available from
http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi
An Extended Radio Counterpart of TeV J2032+4130?
We carried out a 5-pointing mosaic observation of TeV J2032+4130 at 1.4 and
4.8 GHz with the VLA in April of 2003. The analysis of the 4.8GHz data indicate
weak wispy shell-like radio structure(s) which are at least partially
non-thermal. The radio data is compatible with one or more young supernova
remnants or perhaps the signature of large scale cluster shocks in this region
induced by the violent action of the many massive stars in Cyg OB2.Comment: Proc. 1st GLAST Symp. Feb 5-8, 2007, Stanford C
What do loose groups tell us about galaxy formation?
We present the results of a Parkes Multibeam HI survey of six loose groups of
galaxies analogous to the Local Group. This survey was sensitive to HI-rich
objects in these groups to below 10^7 M(sun) of HI, and was designed to search
for low mass, gas-rich satellite galaxies and potential analogs to the
high-velocity clouds seen around the Milky Way. This survey detected a total of
79 HI-rich objects associated with the six groups, half of which were new
detections. While the survey identified a small number of dwarf galaxies, no
star-free HI clouds were discovered. The HI mass function of the six groups
appears to be roughly flat as is that of the Local Group. The cumulative
velocity distribution function (CVDF) of the HI-rich halos in the six groups is
identical to that of the Local Group. Both of these facts imply that these
groups are true analogs to the Local Group and that the Local Group is not
unique in its lack of low-mass dwarf galaxies as compared to the predictions of
cold dark matter models of galaxy formation. This survey also constrains the
distance to and HI masses of the compact high-velocity clouds (CHVCs) around
the Milky Way. The lack of CHVC analog detections implies that they are
distributed within <160 kpc of the Milky Way and have average HI masses of
<4x10^5 M(sun). The spatial distribution of CHVCs is consistent with the
predictions of simulations for dark matter halos. Furthermore the CVDF of Local
Group galaxies plus CHVCs matches the predicted CVDF of cold dark matter
simulations of galaxy formation. This provides circumstantial evidence that
CHVCs may be associated with low-mass dark matter halos.Comment: 4 pages, to appear in the proceedings of "Baryons in Dark Matter
Halos" Eds R-J., Dettmar, U. Klein, P. Salucci, PoS, SISSA,
http://pos.sissa.i
Buoyancy-driven plumes in a layered porous medium
Thin, roughly horizontal low-permeability layers are a common form of large-scale
heterogeneity in geological porous formations. In this paper, the dynamics of a buoyancydriven plume in a two-dimensional layered porous medium is studied theoretically, with
the aid of high-resolution numerical simulations. The medium is uniform apart from
a thin, horizontal layer of a much lower permeability, located a dimensionless distance
L 1 below the dense plume source. If the dimensionless thickness 2εL and permeability
Î of the low-permeability layer are small, the effect of the layer is found to be well
parameterized by its impedance Ω = 2εL/Π. Five different regimes of flow are identified
and characterized. For Ω L
1/3
, the layer has no effect on the plume, but as Ω is
increased the plume widens and spreads over the layer as a gravity current. For still
larger Ω, the flow becomes destabilized by convective instabilities both below and above
the layer, until, for Ω L, the spread of the plume is dominated by convective mixing
and buoyancy is transported across the layer by diffusion alone. Analytical models for
the spread of the plume over the layer in the various different regimes are presented
An HI census of Loose Groups of Galaxies
We present results from our Parkes Multibeam HI survey of 3 loose groups of
galaxies that are analogous to the Local Group. This is a survey of groups
containing only spiral galaxies with mean separations of a few hundred kpc, and
total areas of approximately 1 sq. Mpc; groups similar to our own Local Group.
We present a census of the HI-rich objects in these groups down to an M(HI),
1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these
detections from follow-up Compact Array observations. We found 7 new HI-rich
members in the 3 groups, all of which have stellar counterparts and are,
therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich
galaxies in these groups is less than in the Local Group meaning that the
``missing satellite'' problem is not unique. No high-velocity cloud analogs
were found in any of the groups. If HVCs in these groups are the same as in the
Local Group, this implies that HVCs must be located within ~300-400 kpc of the
Milky Way.Comment: 6 pages, to appear in the ASP proceedings of IAU Symposium 217,
"Recycling intergalactic and interstellar matter", eds. Pierre-Alain Duc,
Jonathan Braine, Elias Brink
Neutral Hydrogen in the Ringed Barred Galaxies NGC 1433 and NGC 6300
We have made observations of the \ion{H}{1} in the southern ringed barred
spiral galaxies NGC~1433 and NGC~6300 with the Australia Telescope Compact
Array (ATCA), the main goal being to test the resonance theory for the origin
of these rings. NGC~1433 is the prototypical ringed barred spiral, and displays
distinct \ion{H}{1}~counterparts to its nuclear ring, inner ring, outer
pseudoring, and plume-like features. The and regions at
corotation, as well as the bar itself, are relatively devoid of neutral atomic
hydrogen. By associating the inner ring of NGC~1433 with the inner second
harmonic resonance, and its outer pseudoring with the outer Lindblad resonance,
we are able to infer a bar pattern speed for NGC~1433 of
~km~s~kpc. By way of contrast, NGC~6300 possesses a much
more extended \ion{H}{1}~disk than NGC~1433. There is a gas ring underlying the
inner pseudoring, but it is both broader and slightly larger in diameter than
the optical feature. By again linking this inner ring feature to the inner
second harmonic resonance, we derive a bar pattern speed for NGC~6300 of
~km~s~kpc, but in this case, neither the outer pseudoring
nor the nuclear ring predicted by the resonance-ring theory can be identified
in NGC~6300. Although it may be the case that the ring in NGC~6300 is not
related to a resonance with the bar at all, we postulate instead that NGC~6300
is merely a less well-developed example of a resonance-ring galaxy than is
NGC~1433.Comment: 21 pages, aas2pp4 LaTeX, no figures included. Accepted for April 1
1996 ApJ. Full paper (with figures) available from
http://www.phys.unsw.edu.au/~sdr/prep.htm
Faraday rotation in the MOJAVE blazars: 3C 273 a case study
Radio polarimetric observations of Active Galactic Nuclei can reveal the
magnetic field structure in the parsec-scale jets of these sources. We have
observed the gamma-ray blazar 3C 273 as part of our multi-frequency survey with
the Very Long Baseline Array to study Faraday rotation in a large sample of
jets. Our observations re-confirm the transverse rotation measure gradient in
3C 273. For the first time the gradient is seen to cross zero which is further
indication for a helical magnetic field and spine-sheath structure in the jet.
We believe the difference to previous epochs is due to a different part of the
jet being illuminated in our observations.Comment: 6 pages, 3 figures. To appear in the proceedings of "Beamed and
Unbeamed Gamma-rays from Galaxies", held in Muonio, Finland, April 11-15,
2011. Journal of Physics: Conference Serie
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