52 research outputs found
Relics of structure formation: extra-planar gas and high-velocity clouds around the Andromeda Galaxy
Using the 100-m radio telescope at Effelsberg, we mapped a large area around
the Andromeda Galaxy in the 21-cm line emission of neutral hydrogen to search
for high-velocity clouds (HVCs) out to large projected distances in excess of
100 kpc. Our 3-sigma HI mass sensitivity for the warm neutral medium is 8x10^4
solar masses. We can confirm the existence of a population of HVCs with typical
HI masses of a few times 10^5 solar masses near the disc of M31. However, we
did not detect any HVCs beyond a projected distance of about 50 kpc from M31,
suggesting that HVCs are generally found in proximity of large spiral galaxies
at typical distances of a few 10 kpc. Comparison with CDM-based models and
simulations suggests that only a few of the detected HVCs could be associated
with primordial dark-matter satellites, whereas others are most likely the
result of tidal stripping. The lack of clouds beyond a projected distance of 50
kpc from M31 is also in conflict with the predictions of recent CDM structure
formation simulations. A possible solution to this problem could be ionisation
of the HVCs as a result of decreasing pressure of the ambient coronal gas at
larger distances from M31. A consequence of this scenario would be the presence
of hundreds of mainly ionised or pure dark-matter satellites around large
spiral galaxies like the Milky Way and M31.Comment: 21 pages, 15 figures, accepted for publication in MNRA
HI observations of three compact high-velocity clouds around the Milky Way
We present deep HI observations of three compact high-velocity clouds
(CHVCs). The main goal is to study their diffuse warm gas and compact cold
cores. We use both low- and high-resolution data obtained with the 100 m
Effelsberg telescope and the Westerbork Synthesis Radio Telescope (WSRT). The
combination is essential in order to study the morphological properties of the
clouds since the single-dish telescope lacks a sufficient angular resolution
while the interferometer misses a large portion of the diffuse gas. Here
single-dish and interferometer data are combined in the image domain with a new
combination pipeline. The combination makes it possible to examine interactions
between the clouds and their surrounding environment in great detail. The
apparent difference between single-dish and radio interferometer total flux
densities shows that the CHVCs contain a considerable amount of diffuse gas
with low brightness temperatures. A Gaussian decomposition indicates that the
clouds consist predominantly of warm gas.Comment: 11 pages, 7 figures, accepted for publication by A&
Physical properties of two compact high-velocity clouds possibly associated with the Leading Arm of the Magellanic System
We observed two compact high-velocity clouds HVC 291+26+195 and HVC
297+09+253 to analyse their structure, dynamics, and physical parameters. In
both cases there is evidence for an association with the Leading Arm of the
Magellanic Clouds. The goal of our study is to learn more about the origin of
the two CHVCs and to use them as probes for the structure and evolution of the
Leading Arm. We have used the Parkes 64 m radio telescope and the Australia
Telescope Compact Array (ATCA) to study the two CHVCs in the 21 cm line
emission of neutral hydrogen. We present a method to estimate the distance of
the two CHVCs. The investigation of the line profiles of HVC 297+09+253 reveals
the presence of two line components in the spectra which can be identified with
a cold and a warm gas phase. In addition, we find a distinct head-tail
structure in combination with a radial velocity gradient along the tail,
suggesting a ram-pressure interaction of this cloud with an ambient medium. HVC
291+26+195 has only a cold gas phase and no head-tail structure. The ATCA data
show several cold, compact clumps in both clouds which, in the case of HVC
297+09+253, are embedded in the warm, diffuse envelope. All these clumps have
very narrow HI lines with typical line widths between 2 and 4 km/s FWHM,
yielding an upper limit for the kinetic temperature of the gas of T_max = 300
K. We obtain distance estimates for both CHVCs of the order of 10 to 60 kpc,
providing additional evidence for an association of the clouds with the Leading
Arm.Comment: 11 pages, 6 Postscript figures, 2 tables; accepted for publication in
Astronomy & Astrophysic
SoFiA: a flexible source finder for 3D spectral line data
We introduce SoFiA, a flexible software application for the detection and
parameterization of sources in 3D spectral-line datasets. SoFiA combines for
the first time in a single piece of software a set of new source-finding and
parameterization algorithms developed on the way to future HI surveys with
ASKAP (WALLABY, DINGO) and APERTIF. It is designed to enable the general use of
these new algorithms by the community on a broad range of datasets. The key
advantages of SoFiA are the ability to: search for line emission on multiple
scales to detect 3D sources in a complete and reliable way, taking into account
noise level variations and the presence of artefacts in a data cube; estimate
the reliability of individual detections; look for signal in arbitrarily large
data cubes using a catalogue of 3D coordinates as a prior; provide a wide range
of source parameters and output products which facilitate further analysis by
the user. We highlight the modularity of SoFiA, which makes it a flexible
package allowing users to select and apply only the algorithms useful for their
data and science questions. This modularity makes it also possible to easily
expand SoFiA in order to include additional methods as they become available.
The full SoFiA distribution, including a dedicated graphical user interface, is
publicly available for download.Comment: MNRAS, accepted. SoFiA is registered at the Astrophysics Source Code
Library with ID ascl:1412.001. Download SoFiA at
https://github.com/SoFiA-Admin/SoFi
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