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The Australia Telescope National Facility Colloquium
11:00-12:00 Fri 11 Mar 2005

ATNF Marsfield Lecture Theatre

Dr. D. J. Pisano

(Naval Research Laboratory, Washington DC)

What do loose groups tell us about galaxy formation? - Dr. D. J. Pisano Colloquium

I present the results of a Parkes Multibeam HI survey of six
loose groups of galaxies analogous to the Local Group. This survey was
sensitive to HI-rich objects in these groups to below 10^7 M(sun) of HI,
and was designed to search for low mass, gas-rich satellite galaxies and
potential analogs to the high-velocity clouds seen around the Milky Way.
This survey detected a total of 79 HI-rich objects associated with the six
groups, half of which were new detections. While the survey identified a
small number of dwarf galaxies, no star-free HI clouds were discovered.
The HI mass function and the mass function of the luminous halos for the
six groups are consistent with those for the Local Group. This implies
that these groups are true analogs to the Local Group and that the Local
Group is not unique in its lack of low-mass dwarf galaxies as compared to
the predictions of cold dark matter models of galaxy formation. This
survey also constrains the distance to and HI masses of the compact
high-velocity clouds (CHVCs) around the Milky Way. The lack of CHVC
analog detections implies that they are distributed within <160 kpc of the
Milky Way and have average HI masses of <4x10^5 M(sun). The spatial
distribution of CHVCs is consistent with the predictions of simulations
for dark matter halos. Furthermore the CVDF of Local Group galaxies plus
CHVCs matches the predicted CVDF of cold dark matter simulations of galaxy
formation. This provides circumstantial evidence that CHVCs may be
associated with low-mass dark matter halos.

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