Evolution of dust content in galaxies probed by gamma-ray burst afterglows

International audienceBecause 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 ratios 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(in)) similar to 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(in) similar to 0.1). Efficient dust growth in dense clouds has difficulty in explaining the excessive dust-to-gas ratio at metallicities Z/Z(circle dot) \textless 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 similar to 10(-2) Z(circle dot) can be explained with n(H) greater than or similar to 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