Dust-to-metal ratios in damped Lyman-α absorbers

Fresh clues to the origins of dust and optical extinction towards γ-ray bursts

¹ Department of Particle Physics and Astrophysics, Faculty of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel
e-mail: annalisa.de-cia@weizmann.ac.il
² Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
³ European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago 19, Chile
⁴ Max-Planck Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany

Received: 5 May 2013
Accepted: 18 September 2013

Abstract

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 A_V 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.