MgII Absorbers: Halos or Disks?

Abstract

We challenge the conventional view that the majority of MgII/Lyman limit absorbers are extended halos of galaxies comprised of ``clouds'' with near-unity covering factor. The gaseous disks of spiral galaxies are known to extend to large radii and likely contribute a significant cross-section for absorption. We perform a Monte-Carlo survey of QSO fields in which the model galaxies have MgII absorbing ``clouds'' in a spherical halo or in a randomly oriented disk. For both geometries, models that recover the observed properties of MgII absorbers have only a 70-80% covering factor. Therefore, regardless of absorber geometry, a survey of randomly selected QSO fields should yield a non-negligible number of non-absorbing galaxies at small impact parameters. Very few have been observed (Steidel (1995)). However, selection effects are important, and once the observational procedures are applied to our model fields we find that this result is expected. Since both spherical halo and disk models can be made consistent with survey results, we present tests for discerning the geometric distribution of absorbing gas from Hubble Space Telescope images and high resolution spectra.