Аннотация
Context: Several issues regarding the nature of dust at high redshift remain
unresolved: its composition, its production and growth mechanisms, and its
effect on background sources. Aims: This paper aims to provide a more accurate
relation between dust depletion levels and dust-to-metals ratio (DTM), and to
use the DTM to investigate the origin and evolution of dust in the high
redshift Universe via GRB-DLAs. Methods: We use absorption-line measured metal
column densities for a total of 19 GRB-DLAs, including five new GRB afterglow
spectra from VLT/X-shooter. We use the latest linear models to calculate the
dust depletion strength factor in each DLA. Using this we calculate total dust
and metal column densities to determine a DTM. We explore the evolution of DTM
with metallicity, and compare this to previous trends in DTM measured with
different methods. Results: We find significant dust depletion in 16 of our 19
GRB-DLAs, yet 18 of the 19 have a DTM significantly lower than the Milky Way.
We find that DTM is positively correlated with metallicity, which supports a
dominant ISM-grain-growth mode of dust formation. We find a substantial
discrepancy between the dust content measured from depletion and that derived
from the total $V$-band extinction, $A_V$, measured by fitting the afterglow
SED. We advise against using a measurement from one method to estimate that
from the other, until the discrepancy can be resolved.
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