Distribution of Dust around Galaxies: An Analytic Model
  We develop an analytic halo model for the distribution of dust around
galaxies. The model results are compared with the observed surface dust density
profile measured through reddening of background quasars in the Sloan Digital
Sky Survey (SDSS) reported by Menard et al.(2010). We assume that the dust
distribution around a galaxy is described by a simple power law, similarly to
the mass distribution, but with a sharp cut-off at $\alpha R_{\rm vir}$ where
$R_{\rm vir}$ is the galaxy's virial radius and $\alpha$ is a model parameter.
Our model reproduces the observed dust distribution profile very well over a
wide range of radial distance of $10 - 10^{4} h^{-1}$kpc. For the
characteristic galaxy halo mass of $2\times 10^{12} h^{-1}M_{\odot}$ estimated
for the SDSS galaxies, the best fit model is obtained if $\alpha$ is greater
than unity, which suggests that dust is distributed to over a few hundred
kilo-parsecs from the galaxies. The observed large-scale dust distribution
profile is reproduced if we assume the total amount of dust is equal to that
estimated from the integrated stellar evolution over the cosmic time.
