  Observations have revealed unexpectedly large amounts of dust in
high-redshift galaxies and its origin is still much debated. Valiante et al.
(2009, MNRAS, 397, 1661) suggested the net stellar dust production of the
quasar host galaxy SDSS J1148+5251 may be sufficient to explain the large dust
mass detected in this galaxy, albeit under some very special assumptions (e.g.,
'closed box' evolution and a rather high gas mass). Here it is shown that since
accretion of essentially pristine material may lower the efficiency of dust
formation significantly, and the observationally derived dust-to-gas ratios for
these high-redshift galaxies are remarkably high, stellar dust production is
likely insufficient. A model including metallicity-dependent, non-stellar dust
formation ('secondary dust') is presented. The required contribution from this
non-stellar dust component appears too large, however. If all observational
constraints are to be met, the resultant dust-to-metals ratio is close to
unity, which means that almost all interstellar metals exist in the form dust.
This is a very unlikely situation and suggests the large dust-to-gas ratios at
high-redshifts may be due to observational uncertainties and/or or incorrect
calibration of conversion factors for gas and dust tracers.

Cosmology and Nongalactic Astrophysics