Dust-to-metal ratios in Damped Lyman-α absorbers: Fresh clues to the origins of dust and optical extinction towards γ-ray bursts

Motivated by the anomalous dust-to-metal ratios derived in the literature for γ-ray burst (GRB) damped Lyman-α absorbers (DLAs), we measure these ratios using the dust-depletion pattern observed in UV/optical afterglow spectra associated with the interstellar medium (ISM) at the GRB host-galaxy redshifts. Our sample consists of 20 GRB absorbers and a comparison sample of 72 DLAs toward quasars (QSOs) with redshift 1.2 < z < 4.0 and down to Z = 0.002 Z⊙ metallicities. The dust-to-metal ratio in QSO- and GRB-DLAs increases both with metallicity and metal column density, spanning ~10-110% of the Galactic value and pointing to a nonuniversal dust-to-metal ratio. The low values of dust-to-metal ratio suggest that low-metallicity systems have lower dust fractions than typical spiral galaxies and, perhaps, that the dust in these systems is produced inefficiently, i.e. by grain growth in the low-metallicity regime with negligible contribution from supernovae (SNe) and asymptotic giant branch (AGB) stars. On the other hand, some GRB- and QSO-DLAs show high dust-to-metal ratio values out to z ~ 4, requiring rapid dust production, such as in SN ejecta, but also in AGB winds and via grain growth for the highest metallicity systems. GRB-DLAs overall follow the dust-to-metal-ratio properties of QSO-DLAs, GRBs probing larger column and volume densities. For comparison, the dust-to-metal ratio that we derive for the SMC and LMC are ~82-100% and ~98% of the Galactic value, respectively. The literature dust-to-metal ratio of the low-metallicity galaxy I Zw 18 (<37%) is consistent with the distribution that we find. The dust extinction AV increases steeply with the column density of iron in dust, N(Fe)dust, calculated from relative metal abundances, confirming that dust extinction is mostly occurring in the host galaxy ISM. Most GRB-DLAs display log N(Fe)dust > 14.7, above which several QSO-DLAs reveal molecular hydrogen, making GRB-DLAs promising candidates for molecular detection and study.