Evolution of dust content in galaxies probed by gamma-ray burst
  afterglows
  Because of their brightness, gamma-ray burst (GRB) afterglows are viable
targets for investigating the dust content in their host galaxies. Simple
intrinsic spectral shapes of GRB afterglows allow us to derive the dust
extinction. Recently, the extinction data of GRB afterglows are compiled up to
redshift $z=6.3$, in combination with hydrogen column densities and
metallicities. This data set enables us to investigate the relation between
dust-to-gas ratio and metallicity out to high redshift for a wide metallicity
range. By applying our evolution models of dust content in galaxies, we find
that the dust-to-gas ratio derived from GRB afterglow extinction data are
excessively high such that they can be explained with a fraction of gas-phase
metals condensed into dust ($f_\mathrm{in}$) $\sim 1$, while theoretical
calculations on dust formation in the wind of asymptotic giant branch stars and
in the ejecta of Type II supernovae suggest a much more moderate condensation
efficiency ($f_\mathrm{in}\sim 0.1$). Efficient dust growth in dense clouds has
difficulty in explaining the excessive dust-to-gas ratio at metallicities
$Z/\mathrm{Z}_\odot <\epsilon$, where $\epsilon$ is the star formation
efficiency of the dense clouds. However, if $\epsilon$ is as small as 0.01, the
dust-to-gas ratio at $Z\sim 10^{-2}$ Z$_\odot$ can be explained with
$n_\mathrm{H}\gtrsim 10^6$ cm$^{-3}$. Therefore, a dense environment hosting
dust growth is required to explain the large fraction of metals condensed into
dust, but such clouds should have low star formation efficiencies to avoid
rapid metal enrichment by stars.
