On the Halo Occupation of Dark Baryons

We introduce a new technique that adopts the halo occupation framework for understanding the origin of QSO absorption-line systems. Our initial study focuses specifically on Mg II absorbers. We construct a model of the gaseous content in which the absorption equivalent width Wr is determined by the the amount of cold gas, in the form of discrete clouds, along a sightline through a halo. The two quantities that we specify per halo in the model are (1) the mean absorption strength per unit surface mass density AW (M), and (2) the mean covering factor κg(M) of the gaseous clouds. These parameters determine the conditional probability distribution of Wr as a function of halo mass, P(Wr|M). Two empirical measurements are applied to constrain the model: (i) the absorber frequency distribution function and (ii) the Wr-dependent clustering amplitude. We find that the data demand a rapid transition in the gas content of halos at ∼ 1011.5 h−1 M⊙, below which halos contain predominantly cold gas and beyond which gas becomes predominantly hot. In order to reproduce the observed overall strong clustering of the absorbers and the anti-correlation between Wr and halo mass M, roughly 5% of gas in halos up to 1014 h−1 M⊙ is required to be cold. The gas covering factor is near unity over a wide range of halo mass, supporting that Mg II systems probe an unbiased sample of typical galaxies. We discuss the implications of our study in the contexts of mass assembly of distant galaxies and the origin of QSO absorption line systems. Subject headings: Cosmology: theory — dark matter halos — quasars: absorption lines — galaxies: evolution