Chapter

STUDYING GALAXY FORMATION IN LOOSE GALAXY GROUPS

DOI: 10.1007/978-1-4020-5573-7_55

ABSTRACT We present the results of our Hi survey of six loose groups of galaxies analogous to the Local Group. The survey was conducted
using the Parkes telescope and the Australia Telescope Compact Array to produce a census of all the gas-rich galaxies and
analogs to the high-velocity clouds (HVCs) within these groups down to MHI< 107 as a test of models of galaxy formation. We present the Hi mass function and halo mass function of the loose groups and show
that they are consistent with those of the Local Group. We discuss the possible role of HVCs in solving the “missing satellite”
problem and discuss the implications of our observations for models of galaxy formation.

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    ABSTRACT: We combine the catalogs of compact high-velocity HI clouds extracted from the LDS and HIPASS surveys and analyze the all-sky properties of the ensemble. 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 HI column densities, (4) the number distribution of angular sizes, and (5) the number distribution of HI 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 HI 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 10^7 M_Sun 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 M31. Given its greater mean distance, only a small fraction of the M31 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. Comment: 39 pages, 29 (low res.) png figs, accepted for pub. in A&A
    Astronomy and Astrophysics 06/2002; · 5.08 Impact Factor

David G. Barnes