10,617 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
Second-chance punitivism and the contractual governance of crime and incivility: New Labour, old Hobbes
The growing application of mechanisms of contractual governance to behaviour that breaches social norms, rather than the criminal law, appears to represent an ethopolitical concern with delinquent self-reform through the activation of technologies of the self. In fact, there is little empirical evidence that the contractual governance of incivility leads to such self-reform. Beneath the ideology of contractual agreement to observe social norms lies what this paper calls a ‘second-chance punitivism’ which operates to crystallise behavioural elements of the Hobbesian social contract, after breach, into a more specific form. The responsibilising and individualising properties of this form of contractual governance set the moral-ideological platform for a retributive punitivism, when the rational agents it creates fail to live up to their image, and are taken to have wasted their ‘second chance’
A human factors evaluation of an electronic display system for an aircraft flight deck
This thesis describes a two and a half year study of the human
factors associated with the introduction of electronic displays,
i.e. cathode ray tubes, and their associated controls onto a civil
transport aircraft flight deck.
A simulator was constructed for the evaluation which was
managed jointly by the British Aircraft Corporation Ltd. and
Hawker Siddeley Aviation Ltd. (now British Aerospace) under a
Ministry of Defence contract.[Continues.
Digital computation of downstream modes generated by the interaction of a shock wave with an upstream flow containing the three disturbance modes /vorticity, entropy and sound/
Digital computation of downstream modes from shock wave interaction with upstream flow containing vorticity, entropy, and sound disturbance
A multivariate interpolation technique to produce three-dimensional surface plots of vibrating structures
Multivariate interpolation technique for three dimensional surface plots of vibrating structure
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
Understanding the Geometry of Astrophysical Magnetic Fields
Faraday rotation measurements have provided an invaluable technique with
which to measure the properties of astrophysical magnetized plasmas.
Unfortunately, typical observations provide information only about the
density-weighted average of the magnetic field component parallel to the line
of sight. As a result, the magnetic field geometry along the line of sight, and
in many cases even the location of the rotating material, is poorly
constrained. Frequently, interpretations of Faraday rotation observations are
dependent upon underlying models of the magnetic field being probed (e.g.,
uniform, turbulent, equipartition). However, we show that at sufficiently low
frequencies, specifically below roughly 13 (RM/rad m^-2)^(1/4) (B/G)^(1/2) MHz,
the character of Faraday rotation changes, entering what we term the
``super-adiabatic regime'' in which the rotation measure is proportional to the
integrated absolute value of the line-of-sight component of the field. As a
consequence, comparing rotation measures at high frequencies with those in this
new regime provides direct information about the geometry of the magnetic field
along the line of sight. Furthermore, the frequency defining the transition to
this new regime, nu_SA, depends directly upon the local electron density and
magnetic field strength where the magnetic field is perpendicular to the line
of sight, allowing the unambiguous distinction between Faraday rotation within
and in front of the emission region. Typical values of nu_SA range from 10 kHz
to 10 GHz, depending upon the details of the Faraday rotating environment. In
particular, for resolved AGN, including the black holes at the center of the
Milky Way (Sgr A*) and M81, nu_SA ranges from roughly 10 MHz to 10 GHz, and
thus can be probed via existing and up-coming ground-based radio observatories.Comment: 13 pages, 5 figures, submitted to Ap
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
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