1305.1153 (A. De Cia et al.)

Dust-to-metal ratios in Damped Lyman-alpha absorbers: Fresh clues to the origins of dust and optical extinction towards gamma-ray bursts    [PDF]

A. De Cia, C. Ledoux, S. Savaglio, P. Schady, P. M. Vreeswijk

Motivated by the anomalous dust-to-metal ratios (DTM) derived in the literature for gamma-ray burst (GRB) damped Ly-alpha 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 QSO-DLAs, overall at redshift 1.2 < z < 4.0 and down to Z = 0.002 Zsol metallicities. The DTM in QSO- and GRB-DLAs increases both with metallicity and metal column density, spanning ~10--110% the Galactic value and pointing to a non-universal dust-to-metal ratio. The low values of DTM indicate 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 formation in asymptotic giant branch (AGB) winds and by grain growth in the low-metallicity regime, with little contribution from supernovae (SNe). On the other hand, some GRB- and QSO-DLAs show high DTM 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 DTM properties of QSO-DLAs, GRBs probing up to higher column and volume densities. For comparison, the DTM that we derive for the SMC and LMC are ~82--100% and ~98% of the Galactic value, respectively. The literature DTM value of the low-metallicity galaxy I Zw 18 (< 37%) is consistent with the DTM 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 H2, making GRB-DLAs promising candidates for molecular research.

View original: http://arxiv.org/abs/1305.1153