Issue |
A&A
Volume 391, Number 1, August III 2002
|
|
---|---|---|
Page(s) | 67 - 81 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20020640 | |
Published online | 29 July 2002 |
High–resolution imaging of compact high–velocity clouds
1
Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
2
Netherlands Foundation for Research in Astronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands
3
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22903, USA
Corresponding author: R. Braun, rbraun@astron.nl
Received:
14
March
2002
Accepted:
24
April
2002
We have imaged five compact high–velocity clouds in H i
with arcmin angular resolution and km s-1 spectral resolution using the
Westerbork Synthesis Radio Telescope. These CHVCs have a
characteristic morphology, consisting of one or more quiescent,
low–dispersion compact cores embedded in a diffuse warm halo. The
compact cores can be unambiguously identified with the cool neutral
medium of condensed atomic hydrogen, since their linewidths are
significantly narrower than the thermal linewidth of the warm neutral
medium. Because of the limited sensitivity to diffuse emission inherent
to interferometric data, the warm medium is not directly detected in
the WSRT observations. Supplementary total–power data, which is fully
sensitive to both the cool and warm components of H i, is
available for comparison for all the sources, albeit with angular
resolutions that vary from 3′ to 36′. The fractional
H i flux in compact CNM components varies from 4% to 16% in our
sample. All objects have at least one local peak in the CNM column
density which exceeds about when observed with
arcmin resolution. It is plausible that a peak column density of
1–2
is a prerequisite for the long–term
survival of these sources. One object in our sample,
CHVC 120-20-443 (Davies' cloud), lies in close projected proximity
to the disk of M 31. This object is characterized by exceptionally
broad linewidths in its CNM concentrations, more than 5 times greater
than the median value found in the 13 CHVCs studied to date at
comparable resolution. These CNM concentrations lie in an arc on the
edge of the source facing the M 31 disk. The diffuse H i component of
this source, seen in total–power data from the NRAO 140–foot
telescope, has a positional offset in the direction of the M 31
disk. All of these attributes suggest that CHVC 120-20-443 is in a
different evolutionary state than most of the other CHVCs which have
been studied. Similarly broad CNM linewidths have only been detected in
one other cloud, CHVC 110.6-07.0-466 (Wakker & Schwarz
[CITE]) which also lies in the Local Group barycenter
direction and has the most extreme radial velocity known. A distinct
possibility for Davies' cloud seems to be physical interaction of some
type with M 31. The most likely form of this interaction might be the
ram–pressure or tidal–stripping by either one of M 31's visible
dwarf companions, M 32 or NGC 205, or else by a dark companion with
an associated H i condensation. The compact objects located in the
direction of the Local Group barycenter have an important role to play
in constraining the Local Group hypothesis for the deployment of CHVCs.
Key words: ISM: atoms / ISM: clouds / Galaxy: evolution / Galaxy: formation / galaxies: dwarf / galaxies: Local Group
© ESO, 2002
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