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[33.04] HI Tracers of the Local Group Mass Distribution

R. Braun (Astron), V. De Heij, W.B. Burton (Leiden Univ.)

Recent all-sky surveys of the HI sky have revealed a population of objects which are isolated in position and velocity down to column densities below 1.5\times10¹⁸cm^-2. While not selected on the basis of angular size, these objects turn out to have a median angular size of only 1^\circ. A representative sample has now been studied with high angular resolution (sub-arcmin) on the one hand, and extremely high column density sensitivity (<10¹⁷cm^-2) on the other. Indirect distance estimates to individual objects range from 150 to 850 kpc. The observable properties of the all-sky sample of such objects have been modeled as the baryonic counterparts of a distribution of dark matter halos (with N(M)~M^-2), with kinematics consistent with evolution in the Local Group potential. Tidal and ram pressure disruption have been considered, as well as realistic accounting for the effects of Galactic foreground obscuration and the finite resolution and sensitivity of the survey data. The best-fitting models have a somewhat flatter HI mass distribution, N(M_HI)~M_HI^-1.7, which extends from M_HI=10^5.5--10⁷~M_\odot. The lower mass limit corresponds to the minimum required to survive ionization and disruption; the upper to avoid internal star formation. The spatial distributions are concentrated around M31 and the Galaxy with a Gaussian dispersion of 150--200~kpc. The disrupted objects from the low mass tails of these distributions are predicted to have added a few times 10⁹~M_\odot of gas to the halos of both M31 and the Galaxy. Sensitive new data have revealed such isolated HI clouds which are apparently undergoing mergers with both M33 and M31.

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.