26,904 research outputs found
Probing the cool ISM in galaxies via 21cm HI absorption
Recent targeted studies of associated HI absorption in radio galaxies are
starting to map out the location, and potential cosmological evolution, of the
cold gas in the host galaxies of Active Galactic Nuclei (AGN). The observed 21
cm absorption profiles often show two distinct spectral-line components:
narrow, deep lines arising from cold gas in the extended disc of the galaxy,
and broad, shallow lines from cold gas close to the AGN (e.g. Morganti et al.
2011). Here, we present results from a targeted search for associated HI
absorption in the youngest and most recently-triggered radio AGN in the local
universe (Allison et al. 2012b). So far, by using the recently commissioned
Australia Telescope Compact Array Broadband Backend (CABB; Wilson et al. 2011),
we have detected two new absorbers and one previously-known system. While two
of these show both a broad, shallow component and a narrow, deep component (see
Fig. 1), one of the new detections has only a single broad, shallow component.
Interestingly, the host galaxies of the first two detections are classified as
gas-rich spirals, while the latter is an early-type galaxy. These detections
were obtained using a spectral-line finding method, based on Bayesian
inference, developed for future large-scale absorption surveys (Allison et al.
2012a).Comment: 1 page, 1 figure, published in Proceedings of IAU Symposium No. 29
Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies
We present a statistical method for measuring the average HI spin temperature
in distant galaxies using the expected detection yields from future wide-field
21cm absorption surveys. As a demonstrative case study we consider a simulated
all-southern-sky survey of 2-h per pointing with the Australian Square
Kilometre Array Pathfinder for intervening HI absorbers at intermediate
cosmological redshifts between and . For example, if such a survey
yielded absorbers we would infer a harmonic-mean spin temperature of
K for the population of damped Lyman
(DLAs) absorbers at these redshifts, indicating that more than
per cent of the neutral gas in these systems is in a cold neutral medium (CNM).
Conversely, a lower yield of only 100 detections would imply
K and a CNM fraction less than per
cent. We propose that this method can be used to provide independent
verification of the spin temperature evolution reported in recent 21cm surveys
of known DLAs at high redshift and for measuring the spin temperature at
intermediate redshifts below , where the Lyman- line is
inaccessible using ground-based observatories. Increasingly more sensitive and
larger surveys with the Square Kilometre Array should provide stronger
statistical constraints on the average spin temperature. However, these will
ultimately be limited by the accuracy to which we can determine the HI column
density frequency distribution, the covering factor and the redshift
distribution of the background radio source population.Comment: 11 pages, 9 figures, 1 table. Proof corrected versio
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
Mutual information for examining correlations in DNA
This paper examines two methods for finding whether long-range correlations
exist in DNA: a fractal measure and a mutual information technique. We evaluate
the performance and implications of these methods in detail. In particular we
explore their use comparing DNA sequences from a variety of sources. Using
software for performing in silico mutations, we also consider evolutionary
events leading to long range correlations and analyse these correlations using
the techniques presented. Comparisons are made between these virtual sequences,
randomly generated sequences, and real sequences. We also explore correlations
in chromosomes from different species.Comment: 8 pages, 3 figure
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