Abstract
We quantify the gas-phase abundance of deuterium in cosmological zoom-in
simulations from the Feedback In Realistic Environments project. The cosmic
deuterium fraction decreases with time, because mass lost from stars is
deuterium-free. At low metallicity, our simulations confirm that the deuterium
abundance is very close to the primordial value. The deuterium abundance
decreases towards higher metallicity, with very small scatter between the
deuterium and oxygen abundance. We compare our simulations to existing
high-redshift observations in order to determine a primordial deuterium
fraction of (2.549 +/- 0.033) x 10^-5 and stress that future observations at
higher metallicity can also be used to constrain this value. At fixed
metallicity, the deuterium fraction decreases slightly with decreasing
redshift, due to the increased importance of mass loss from intermediate-mass
stars. We find that the evolution of the average deuterium fraction in a galaxy
correlates with its star formation history. Our simulations are consistent with
observations of the Milky Way's interstellar medium: the deuterium fraction at
the solar circle is 83-92% of the primordial deuterium fraction. We use our
simulations to make predictions for future observations. In particular, the
deuterium abundance is lower at smaller galactocentric radii and in higher mass
galaxies, showing that stellar mass loss is more important for fuelling star
formation in these regimes (and can even dominate). Gas accreting onto galaxies
has a deuterium fraction above that of the galaxies' interstellar medium, but
below the primordial fraction, because it is a mix of gas accreting from the
intergalactic medium and gas previously ejected or stripped from galaxies.
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