Issue |
A&A
Volume 392, Number 2, September III 2002
|
|
---|---|---|
Page(s) | 417 - 451 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20020908 | |
Published online | 30 August 2002 |
An all–sky study of compact, isolated 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:
25
February
2002
Accepted:
14
June
2002
We combine the catalog of compact high–velocity H i clouds
extracted by de Heij et al. ([CITE]) from the
Leiden/Dwingeloo Survey in the northern hemisphere with the catalog
extracted by Putman et al. ([CITE]) from the Parkes HIPASS
data in the southern hemisphere, and analyze the all–sky properties of
the ensemble. Compact high–velocity clouds are a subclass of the
general high–velocity cloud phenomenon which are isolated in position
and velocity from the extended high–velocity Complexes and Streams
down to column densities below cm-2. Objects
satisfying these criteria for isolation are found to have a median
angular size of less than one degree. We discuss selection effects
relevant to the two surveys; in particular the crucial role played by
obscuration due to Galactic H i. Five principal observables are
defined for the CHVC population: (1) the spatial deployment of the
objects on the sky, (2) the kinematic distribution, (3) the number
distribution of observed H i column densities, (4) the number
distribution of angular sizes, and (5) the number distribution of H i
linewidth. Two classes of models are considered to reproduce the
observed properties. The agreement of models with the data is judged
by extracting these same observables from simulations, in a manner
consistent with the sensitivities of the observations and explicitly
taking account of Galactic obscuration. We show that models in which
the CHVCs are the H i counterparts of dark–matter halos evolving in
the Local Group potential provide a good match to the observables. The
best–fitting populations have a maximum HI mass of
,
a power-law slope of the HI mass distribution in the range -1.7
to -1.8, and a Gaussian dispersion for their spatial distributions of
between 150 and 200 kpc centered on both the Milky Way and M 31. Given
its greater mean distance, only a small fraction of the M 31
sub–population is predicted to have been detected in present
surveys. An empirical model for an extended Galactic halo distribution
for the CHVCs is also considered. While reproducing some aspects of the
population, this class of models does not account for some key
systematic features of the population.
Key words: ISM: atoms / ISM: clouds / Galaxy: evolution / Galaxy: formation / galaxies: dwarf / galaxies: Local Group
© ESO, 2002
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