1,520 research outputs found
Star Formation and Gas Accretion in Nearby Galaxies
In order to quantify the relationship between gas accretion and star
formation, we analyse a sample of 29 nearby galaxies from the WHISP survey
which contains galaxies with and without evidence for recent gas accretion. We
compare combined radial profiles of FUV (GALEX) and IR 24 {\mu}m (Spitzer)
characterizing distributions of recent star formation with radial profiles of
CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas
contents to examine star formation efficiencies in symmetric (quiescent),
asymmetric (accreting), and interacting (tidally disturbed) galaxies. In
addition, we investigate the relationship between star formation rate and HI in
the outer discs for the three groups of galaxies. We confirm the general
relationship between gas surface density and star formation surface density,
but do not find a significant difference between the three groups of galaxies.Comment: Accepted for publication in MNRAS, 18 pages, 9 figures, 1 tabl
Star formation and the interstellar medium in low surface brightness galaxies. II. Deep CO observations of low surface brightness disk galaxies
We present deep, pointed CO() observations of three late-type
LSB galaxies. The beam-size was small enough that we could probe different
environments (\HI maximum, \HI mininum, star forming region) in these galaxies.
No CO was found at any of the positions observed. We argue that the implied
lack of molecular gas is real and not caused by conversion factor effects. The
virtual absence of a molecular phase may explain the very low star formation
rates in these galaxies.Comment: 9 pages, 3 figure, uses aa.cls. Typos in Tables and text correcte
First Detection of Molecular Gas in the Shells of CenA
Shells are faint arc-like stellar structures, which have been observed around
early type galaxies and are thought to be the result of an interaction. HI gas
has recently been detected in shells, a surprising result in view of the
theoretical predictions that most of the gas should decouple from stars and
fall into the nucleus in such interactions. Here we report the first detection
of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128
(CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The
ratio between CO and HI emission in the shells is the same as that found in the
central regions, which is unexpected given the metallicity gradient usually
observed in galaxies. We propose that the dynamics of the gas can be understood
within the standard picture of shell formation if one takes into account that
the interstellar medium is clumpy and hence not highly dissipative. The
observed metal enrichment could be due to star formation induced by the AGN jet
in the shells. Furthermore our observations provide evidence that molecular gas
in mergers may be spread out far from the nuclear regions.Comment: Accepted for publication in Astronomy & Astrophysics Letters, (Vol.
356), 4 pages + 1 color figur
Redundancy Calibration of Phased Array Stations
Our aim is to assess the benefits and limitations of using the redundant
visibility information in regular phased array systems for improving the
calibration.
Regular arrays offer the possibility to use redundant visibility information
to constrain the calibration of the array independent of a sky model and a beam
models of the station elements. It requires a regular arrangement in the
configuration of array elements and identical beam patterns.
We revised a calibration method for phased array stations using the redundant
visibility information in the system and applied it successfully to a LOFAR
station. The performance and limitations of the method were demonstrated by
comparing its use on real and simulated data. The main limitation is the mutual
coupling between the station elements, which leads to non-identical beams and
stronger baseline dependent noise. Comparing the variance of the estimated
complex gains with the Cramer-Rao Bound (CRB) indicates that redundancy is a
stable and optimum method for calibrating the complex gains of the system.
Our study shows that the use of the redundant visibility does improve the
quality of the calibration in phased array systems. In addition it provides a
powerful tool for system diagnostics. Our results demonstrate that designing
redundancy in both the station layout and the array configuration of future
aperture arrays is strongly recommended. In particular in the case of the
Square Kilometre Array with its dynamic range requirement which surpasses any
existing array by an order of magnitude.Comment: 16 pages, 15 figures, accepted for publication in the A&A in Section
13, acceptance date: 1st May 2012. NOTE: Please contact the first author for
high resolution figure
Finding faint HI structure in and around galaxies: scraping the barrel
Soon to be operational HI survey instruments such as APERTIF and ASKAP will
produce large datasets. These surveys will provide information about the HI in
and around hundreds of galaxies with a typical signal-to-noise ratio of
10 in the inner regions and 1 in the outer regions. In addition, such
surveys will make it possible to probe faint HI structures, typically located
in the vicinity of galaxies, such as extra-planar-gas, tails and filaments.
These structures are crucial for understanding galaxy evolution, particularly
when they are studied in relation to the local environment. Our aim is to find
optimized kernels for the discovery of faint and morphologically complex HI
structures. Therefore, using HI data from a variety of galaxies, we explore
state-of-the-art filtering algorithms. We show that the intensity-driven
gradient filter, due to its adaptive characteristics, is the optimal choice. In
fact, this filter requires only minimal tuning of the input parameters to
enhance the signal-to-noise ratio of faint components. In addition, it does not
degrade the resolution of the high signal-to-noise component of a source. The
filtering process must be fast and be embedded in an interactive visualization
tool in order to support fast inspection of a large number of sources. To
achieve such interactive exploration, we implemented a multi-core CPU (OpenMP)
and a GPU (OpenGL) version of this filter in a 3D visualization environment
().Comment: 17 pages, 9 figures, 4 tables. Astronomy and Computing, accepte
3-D interactive visualisation tools for HI spectral line imaging
Upcoming HI surveys will deliver such large datasets that automated
processing using the full 3-D information to find and characterize HI objects
is unavoidable. Full 3-D visualization is an essential tool for enabling
qualitative and quantitative inspection and analysis of the 3-D data, which is
often complex in nature. Here we present , an open-source
extension of 3DSlicer, a multi-platform open source software package for
visualization and medical image processing, which we developed for the
inspection and analysis of HI spectral line data. We describe its initial
capabilities, including 3-D filtering, 3-D selection and comparative modelling
Distribution and kinematics of atomic and molecular gas inside the Solar circle
The detailed distribution and kinematics of the atomic and the CO-bright
molecular hydrogen in the disc of the Milky Way inside the Solar circle are
derived under the assumptions of axisymmetry and pure circular motions. We
divide the Galactic disc into a series of rings, and assume that the gas in
each ring is described by four parameters: its rotation velocity, velocity
dispersion, midplane density and its scale height. We fit these parameters to
the Galactic HI and CO (J=1-0) data by producing artificial HI and CO
line-profiles and comparing them with the observations. Our approach allows us
to fit all parameters to the data simultaneously without assuming a-priori a
radial profile for one of the parameters. We present the distribution and
kinematics of the HI and H2 in both the approaching (QIV) and the receding (QI)
regions of the Galaxy. Our best-fit models reproduces remarkably well the
observed HI and CO longitude-velocity diagrams up to a few degrees of distance
from the midplane. With the exception of the innermost 2.5 kpc, QI and QIV show
very similar kinematics. The rotation curves traced by the HI and H2 follow
closely each other, flattening beyond R=6.5 kpc. Both the HI and the H2 surface
densities show a) a deep depression at 0.5<R<2.5 kpc, analogous to that shown
by some nearby barred galaxies, b) local overdensities that can be interpreted
in terms of spiral arms or ring-like features in the disk. The HI (H2)
properties are fairly constant in the region outside the depression, with
typical velocity dispersion of 8.9+/-1.1 (4.4+/-1.2) km/s, density of
0.43+/-0.11 (0.42+/-0.22) cm-3 and HWHM scale height of 202+/-28 (64+/-12) pc.
We also show that the HI opacity in the LAB data can be accounted for by using
an `effective' spin temperature of about 150 K: assuming an optically thin
regime leads to underestimate the HI mass by about 30%.Comment: 23 pages, 24 figures. Accepted by A&
An Interacting Galaxy System Along a Filament in a Void
Cosmological voids provide a unique environment for the study of galaxy
formation and evolution. The galaxy population in their interior have
significantly different properties than average field galaxies. As part of our
Void Galaxy Survey (VGS), we have found a system of three interacting galaxies
(VGS_31) inside a large void. VGS_31 is a small elongated group whose members
are embedded in a common HI envelope. The HI picture suggests a filamentary
structure with accretion of intergalactic cold gas from the filament onto the
galaxies. We present deep optical and narrow band H_alpha data, optical
spectroscopy, near-UV and far-UV GALEX and CO(1-0) data. We find that one of
the galaxies, a Markarian object, has a ring-like structure and a tail evident
both in optical and HI. While all three galaxies form stars in their central
parts, the tail and the ring of the Markarian object are devoid of star
formation. We discuss these findings in terms of a gravitational interaction
and ongoing growth of galaxies out of a filament. VGS_31 is one of the first
observed examples of a filamentary structure in a void. It is an important
prototype for understanding the formation of substructure in a void. This
system also shows that the galaxy evolution in voids can be as dynamic as in
high density environments.Comment: 17 pages, 8 figures, accepted for publication in A
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