4,615 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
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
Reconstruction of Black Hole Metric Perturbations from Weyl Curvature II: The Regge-Wheeler gauge
Perturbation theory of rotating black holes is described in terms of the Weyl
scalars and ; each satisfying the Teukolsky's complex master
wave equation with spin , and respectively representing outgoing and
ingoing radiation. We explicitly construct the metric perturbations out of
these Weyl scalars in the Regge-Wheeler gauge in the nonrotating limit. We
propose a generalization of the Regge-Wheeler gauge for Kerr background in the
Newman-Penrose language, and discuss the approach for building up the perturbed
spacetime of a rotating black hole. We also provide both-way relationships
between waveforms defined in the metric and curvature approaches in the time
domain, also known as the (inverse-) Chandrasekhar transformations, generalized
to include matter.Comment: 22 pages, no figure
Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
We report the results of a survey for HI 21-cm absorption at redshifts of z >
2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave
Radio Telescopes. From a total of 25 targets, we report zero detections in the
16 for which optical depth limits could be obtained. Based upon the detection
rate for z > 0.1 associated absorption, we would expect approximately four
detections. Of the 11 which have previously not been searched, there is
sufficient source-frame optical/ultra-violet photometry to determine the
ionising photon rate for four. Adding these to the literature, the hypothesis
that there is a critical rate of logQ = 56 ionising photons per second is now
significant at ~7 sigma. This reaffirms our assertion that searching z > 3
active galaxies for which optical redshifts are available selects sources in
which the ultra-violet luminosity is sufficient to ionise all of the neutral
gas in the host galaxy.Comment: Accepted by MNRA
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
Unravelling Active Galactic Nuclei
A complete flat-spectrum radio-loud sample of AGN includes a significant
fraction of Seyfert-like AGN including a NLS1. Analysis of their optical
spectra suggests that the reddest continuum colours are either associated with
AGN in nearby resolved galaxies, or distant quasars showing relatively narrow
permitted emission lines.Comment: Poster contribution presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Estimating Waterfowl Densities in a Flooded Forest: a Comparison of Methods
During winter, aerial surveys are used to estimate densities of ducks that occupy open-water habitats. However, such surveys are ineffective for sampling forest-dwelling species, especially Aix sponsa (Wood Ducks), Anas platyrhynchos (Mallards), and Lophodytes cucullatus (Hooded Mergansers). We evaluated fixed-radius plot (FRP) and Reynolds and Goodrum variable-radius plot (VRP) methods for estimating waterfowl densities in a flooded hardwood bottomland. We constructed 15 elevated blinds on the Angelina River flood plain in eastern Texas and established a 1-ha FRP around each blind; color-coded markers were placed at fixed intervals from each blind. Observers surveyed waterfowl from blinds for 21 mornings during January–March, 1990. For FRPs, species, sex, and time a bird entered and exited the plot were recorded. For VRPs, similar data and estimated observer-to-bird distance were recorded. Data were arranged in a randomized block design and tested using 1-way analyses of variances. Wood Ducks, Mallards, and Hooded Mergansers comprised 68, 18, and 10% of the birds recorded, respectively. Wood Duck density estimates (per ha) for FRP, Reynolds VRP, and Goodrum VRP methods were 0.65, 0.49, and 1.00 (P \u3c 0.001), respectively; for Mallards, estimates were 0.27, 0.20, and 0.33 (P \u3c 0.001), respectively; and estimates were 0.09, 0.13, and 0.15 (P = 0.003) for Hooded Mergansers, respectively. Based on ease of implementation, complexity of data analyses, and precision of density estimates, the FRP and Goodrum VRP methods are recommended for sampling waterfowl in flooded forests
Black Hole Mass Estimates of Radio Selected Quasars
The black hole (BH) mass in the centre of AGN has been estimated for a sample
of radio-selected flat-spectrum quasars to investigate the relationship between
BH mass and radio properties of quasars. We have used the virial assumption
with measurements of the H FWHM and luminosity to estimate the central
BH mass. In contrast to previous studies we find no correlation between BH mass
and radio power in these AGN. We find a range in BH mass similar to that seen
in radio-quiet quasars from previous studies. We believe the reason that the
low BH mass radio-loud quasars have not been measured in previous studies is
due to optical selection effects which tend to miss the less optically luminous
radio-loud sources.Comment: 22 pages, 7 figures, accepted for publication in Ap
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