386 research outputs found
Diffuse neutral hydrogen in the HI Parkes All Sky Survey
Observations of neutral hydrogen can provide a wealth of information about
the distribution and kinematics of galaxies. To detect HI beyond the ionisation
edge of galaxy disks, column density sensitivities have to be achieved that
probe the regime of Lyman limit systems. Typically HI observations are limited
to a brightness sensitivity of NHI~10^19 cm-2 but this has to be improved by at
least an order of magnitude. In this paper, reprocessed data is presented that
was originally observed for the HI Parkes All Sky Survey (HIPASS). HIPASS
provides complete coverage of the region that has been observed for the
Westerbork Virgo Filament HI Survey (WVFS), presented in accompanying papers,
and thus is an excellent product for data comparison. The region of interest
extends from 8 to 17 hours in right ascension and from -1 to 10 degrees in
declination. Although the original HIPASS product already has good flux
sensitivity, the sensitivity and noise characteristics can be significantly
improved with a different processing method. The newly processed data has an
1sigma RMS flux sensitivity of ~10 mJy beam-1 over 26 km s-1, corresponding to
a column density sensitivity of ~3\cdot10^17 cm-2. While the RMS sensitivity is
improved by only a modest 20%, the more substantial benefit is in the reduction
of spectral artefacts near bright sources by more than an order of magnitude.
In the reprocessed region we confirm all previously catalogued HIPASS sources
and have identified 29 additional sources of which 14 are completely new HI
detections. Extended emission or companions were sought in the nearby
environment of each discrete detection. With the improved sensitivity after
reprocessing and its large sky coverage, the HIPASS data is a valuable resource
for detection of faint HI emission.(Abridged)Comment: 22 pages plus appendix, 6 figures, appendix will only appear in
online format. Accepted for publication in A&
The WSRT Virgo Hi filament survey II; Cross Correlation Data
The extended environment of galaxies contains a wealth of information about
the formation and life cycle of galaxies which are regulated by accretion and
feedback processes. Observations of neutral hydrogen are routinely used to
image the high brightness disks of galaxies and to study their kinematics.
Deeper observations will give more insight into the distribution of diffuse gas
in the extended halo of the galaxies and the IGM, where numerical simulations
predict a cosmic web of extended structures and gaseous filaments. To observe
the extended environment of galaxies, column density sensitivities have to be
achieved that probe the regime of Lyman limit systems. HI observations are
typically limited to a brightness sensitivity of NHI~10^19 cm-2, but this must
be improved upon by ~2 orders of magnitude. In this paper we present the
interferometric data of the Westerbork Virgo HI Filament Survey (WVFS) - the
total power product of this survey has been published in an earlier paper. By
observing at extreme hour angles, a filled aperture is simulated of 300x25
meters in size, that has the typical collecting power and sensitivity of a
single dish telescope, but the well defined bandpass characteristics of an
interferometer. With the very good surface brightness sensitivity of the data,
we hope to make new HI detections of diffuse systems with moderate angular
resolution. The survey maps 135 degrees in Right Ascension between 8 and 17
hours and 11 degrees in Declination between -1 and 10 degrees, including the
galaxy filament connecting the Local Group with the Virgo Cluster. Only
positive declinations could be completely processed and analysed due to
projection effects. A typical flux sensitivity of 6 mJy beam-1 over 16 km s-1
is achieved, that corresponds to a brightness sensitivity of NHI~10^18 cm-2. In
total, 199 objects have been detected, of which 17 are new HI detections.
(Abridged)Comment: 19 pages plus appendix, 19 figures, appendix will only appear in
online format. Accepted for publication in A&
Using negative detections to estimate source finder reliability
We describe a simple method to determine the reliability of source finders
based on the detection of sources with both positive and negative total flux.
Under the assumption that the noise is symmetric and that real sources have
positive total flux, negative detections can be used to assign to each positive
detection a probability of being real. We discuss this method in the context of
upcoming, interferometric HI surveys.Comment: Accepted for publication on the 2012 PASA source finding special
issu
The Characterised Noise Hi source finder: Detecting Hi galaxies using a novel implementation of matched filtering
The spectral line datacubes obtained from the Square Kilometre Array (SKA)
and its precursors, such as the Australian SKA Pathfinder (ASKAP), will be
sufficiently large to necessitate automated detection and parametrisation of
sources. Matched filtering is widely acknowledged as the best possible method
for the automated detection of sources. This paper presents the Characterised
Noise Hi (CNHI) source finder, which employs a novel implementation of matched
filtering. This implementation is optimised for the 3-D nature of the planned
Wide-field ASKAP Legacy L-band All- sky Blind surveY's (WALLABY) Hi spectral
line observations. The CNHI source finder also employs a novel sparse
representation of 3-D objects, with a high compression rate, to implement Lutz
one-pass algorithm on datacubes that are too large to process in a single pass.
WALLABY will use ASKAP's phenomenal 30 square degree field of view to image
\sim 70% of the sky. It is expected that WALLABY will find 500 000 Hi galaxies
out to z \sim 0.2.Comment: Part of the 2012 PASA Source Finding Special Issue, 10 figure
The process of homelessness:an event history analysis of length of stay for the Groningen shelter accommodation
The process of homelessness:an event history analysis of length of stay for the Groningen shelter accommodation
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
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