Abstract
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.
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