4,410 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
Multi-object spectroscopy of the field surrounding PKS 2126-158: Discovery of a z=0.66 galaxy group
The high-redshift radio-loud quasar PKS 2126-158 is found to have a large
number of red galaxies in close apparent proximity. We use the Gemini
Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for
a large fraction of these sources. We show that there is a group of galaxies at
, coincident with a metal-line absorption system seen in the
quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us
to measure redshifts of many foreground galaxies in the field surrounding the
quasar.
The galaxy group has five confirmed members, and a further four fainter
galaxies are possibly associated. All confirmed members exhibit early-type
galaxy spectra, a rare situation for a Mg II absorbing system. We discuss the
relationship of this group to the absorbing gas, and the possibility of
gravitational lensing of the quasar due to the intervening galaxies.Comment: Monthly Notices of the Royal Astronomical Society, in press. 10
pages, 8 figure
A New Galaxy in the Local Group: the Antlia Dwarf Galaxy
We report the discovery of new member of the Local Group in the constellation
of Antlia. Optically the system appears to be a typical dwarf spheroidal galaxy
of type dE3.5 with no apparent young blue stars or unusual features. A
color-magnitude diagram in I, V-I shows the tip of the red giant branch, giving
a distance modulus of 25.3 +/- 0.2 (1.15 Mpc +/- 0.1) and a metallicity of -1.6
+/- 0.3. Although Antlia is in a relatively isolated part of the Local Group it
is only 1.2 degrees away on the sky from the Local Group dwarf NGC3109, and may
be an associated system.Comment: AJ in press, 15 pages, 7 figures, figure 2 in b/w for space saving,
full postscript version available at
http://www.ast.cam.ac.uk/~gkth/antlia-pp.htm
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
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
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