5,379 research outputs found
Application of a Bayesian Method to Absorption Spectral-Line Finding in Simulated ASKAP Data
The large spectral bandwidth and wide field of view of the Australian SKA
Pathfinder radio telescope will open up a completely new parameter space for
large extragalactic HI surveys. Here we focus on identifying and parametrising
HI absorption lines which occur in the line of sight towards strong radio
continuum sources. We have developed a method for simultaneously finding and
fitting HI absorption lines in radio data by using multi-nested sampling, a
Bayesian Monte Carlo algorithm. The method is tested on a simulated ASKAP data
cube, and is shown to be reliable at detecting absorption lines in low
signal-to-noise data without the need to smooth or alter the data. Estimation
of the local Bayesian evidence statistic provides a quantitative criterion for
assigning significance to a detection and selecting between competing
analytical line-profile models.Comment: 8 pages, 5 figures and 1 table; accepted for publication in PAS
Is space really expanding? A counterexample
In all Friedman models, the cosmological redshift is widely interpreted as a
consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other
commonly believed consequences of this phenomenon are superluminal recession
velocities of distant galaxies and the distance to the particle horizon greater
than c*t (where t is the age of the Universe), in apparent conflict with
special relativity. Here, we study a particular Friedman model: empty universe.
This model exhibits both cosmological redshift, superluminal velocities and
infinite distance to the horizon. However, we show that the cosmological
redshift is there simply a relativistic Doppler shift. Moreover, apparently
superluminal velocities and `acausal' distance to the horizon are in fact a
direct consequence of special-relativistic phenomenon of time dilation, as well
as of the adopted definition of distance in cosmology. There is no conflict
with special relativity, whatsoever. In particular, INERTIAL recession
velocities are subluminal. Since in the real Universe, sufficiently distant
galaxies recede with relativistic velocities, these special-relativistic
effects must be at least partly responsible for the cosmological redshift and
the aforementioned `superluminalities', commonly attributed to the expansion of
space. Let us finish with a question resembling a Buddhism-Zen `koan': in an
empty universe, what is expanding?Comment: 12 pages, no figures; added Appendix with a calculation of the
cosmological redshift in `private space
On the Existence of Radiation Gauges in Petrov type II spacetimes
The radiation gauges used by Chrzanowski (his IRG/ORG) for metric
reconstruction in the Kerr spacetime seem to be over-specified. Their
specification consists of five conditions: four, which we treat here as valid
gauge conditions, plus an additional condition on the trace of the metric
perturbation. In this work, we utilize a newly developed form of the perturbed
Einstein equations to establish a condition -- on a particular tetrad component
of the stress-energy tensor -- under which the full IRG/ORG can be imposed.
Using gauge freedom, we are able to impose the full IRG for Petrov type II and
type D backgrounds, using a different tetrad for each case. As a specific
example, we work through the process of imposing the IRG in a Schwarzschild
background, using a more traditional approach. Implications for metric
reconstruction using the Teukolsky curvature perturbations in type D spacetimes
are briefly discussed.Comment: 21 pages, uses iop style files. v2: proved a stronger result for type
II backgrounds, added a subsection on remaining gauge freedom in the full IRG
and improved calrity and readability throughout due to insightful referee
comments; published as Class. Quantum Grav. 24 (2007) 2367-238
A fourth HI 21-cm absorption system in the sight-line of MG J0414+0534: a record for intervening absorbers
We report the detection of a strong HI 21-cm absorption system at z=0.5344,
as well as a candidate system at z=0.3389, in the sight-line towards the z=2.64
quasar MG J0414+0534. This, in addition to the absorption at the host redshift
and the other two intervening absorbers, takes the total to four (possibly
five). The previous maximum number of 21-cm absorbers detected along a single
sight-line is two and so we suspect that this number of gas-rich absorbers is
in some way related to the very red colour of the background source. Despite
this, no molecular gas (through OH absorption) has yet been detected at any of
the 21-cm redshifts, although, from the population of 21-cm absorbers as a
whole, there is evidence for a weak correlation between the atomic line
strength and the optical--near-infrared colour. In either case, the fact that
so many gas-rich galaxies (likely to be damped Lyman-alpha absorption systems)
have been found along a single sight-line towards a highly obscured source may
have far reaching implications for the population of faint galaxies not
detected in optical surveys, a possibility which could be addressed through
future wide-field absorption line surveys with the Square Kilometre Array.Comment: Accepted by ApJ Letter
A survey for redshifted molecular and atomic absorption lines - II. Associated HI, OH and millimetre lines in the z >~ 3 Parkes quarter-Jansky flat-spectrum sample
We present the results of a z>2.9 survey for HI 21-cm and molecular
absorption in the hosts of radio quasars using the GMRT and the Tidbinbilla
70-m telescope. Previously published searches, which are overwhelmingly at
redshifts of z<1, exhibit a 42% detection rate (31 out of 73 sources), and the
inclusion of our survey yields a 17% detection rate (2 out of 12 sources) at
z>2.5. We therefore believe that our high redshift selection is responsible for
our exclusive non-detections, and find that at ultra-violet luminosities of
>10e23 W/Hz, 21-cm absorption has never been detected. We also find this to not
only apply to our targets, but also those at low redshift exhibiting similar
luminosities, giving zero detections out of a total of 16 sources over z=0.24
to 3.8. This is in contrast to the < 10e23 W/Hz sources where there is a near
50% detection rate of 21-cm absorption.
The mix of 21-cm detections and non-detections is currently attributed to
orientation effects, where according to unified schemes of active galactic
nuclei, 21-cm absorption is more likely to occur in sources designated as radio
galaxies (type-2 objects, where the nucleus is viewed through dense obscuring
circumnuclear gas) than in quasars(type-1 objects, where we have a direct view
to the nucleus). However, due to the exclusively high ultra-violet luminosities
of our targets it is not clear whether orientation effects alone can wholly
account for the distribution, although there exists the possibility that the
large luminosities are indicative of a changing demographic of galaxy types. We
also find that below luminosities of ~10e23 W/Hz, both type-1 and type-2
objects have a 50% likelihood of exhibiting 21-cm absorption.Comment: 21 pages, accepted by MNRA
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