Abstract
Overdense, metal-rich regions, shielded from stellar radiation might remain
neutral throughout reionization and produce metal as well as 21 cm absorption
lines. Simultaneous absorption from metals and 21 cm can complement each other
as probes of underlying gas properties. We use Aurora, a suite of high
resolution radiation-hydrodynamical simulations of galaxy formation, to
investigate the occurrence of such äligned" absorbers. We calculate absorption
spectra for 21 cm, OI, CII, SiII and FeII. We find velocity windows with
absorption from at least one metal and 21 cm, and classify the aligned
absorbers into two categories: 'aligned and cospatial absorbers' and 'aligned
but not cospatial absorbers'. While 'aligned and cospatial absorbers' originate
from overdense structures and can be used to trace gas properties, 'aligned but
not cospatial absorbers' are due to peculiar velocity effects. The incidence of
absorbers is redshift dependent, as it is dictated by the interplay between
reionization and metal enrichment, and shows a peak at $z 8$ for the
aligned and cospatial absorbers. While aligned but not cospatial absorbers
disappear towards the end of reionization because of the lack of an ambient 21
cm forest, aligned and cospatial absorbers are associated with overdense
pockets of neutral gas which can be found at lower redshift. We produce mock
observations for a set of sightlines for the next generation of telescopes like
the ELT and SKA1-LOW, finding that given a sufficiently bright background
quasar, these telescopes will be able to detect both types of absorbers
throughout reionization.
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