Empirical Constraints on the Nucleosynthesis of Nitrogen
  We derive empirical constraints on the nucleosynthetic yields of nitrogen by
incorporating N enrichment into our previously developed and empirically tuned
multi-zone galactic chemical evolution model. We adopt a
metallicity-independent ("primary") N yield from massive stars and a
metallicity-dependent ("secondary") N yield from AGB stars. In our model,
galactic radial zones do not evolve along the observed [N/O]-[O/H] relation,
but first increase in [O/H] at roughly constant [N/O], then move upward in
[N/O] via secondary N production. By $t\approx5$ Gyr, the model approaches an
equilibrium [N/O]-[O/H] relation, which traces the radial oxygen gradient. We
find good agreement with the [N/O]-[O/H] trend observed in extra-galactic
systems if we adopt an IMF-averaged massive star yield
$y_\text{N}^\text{CC}=3.6\times10^{-4}$, consistent with predictions for
rapidly rotating progenitors, and a fractional AGB yield that is linear in mass
and metallicity
$y_\text{N}^\text{AGB}=(9\times10^{-4})(M_*/M_\odot)(Z_*/Z_\odot)$. This model
reproduces the [N/O]-[O/H] relation found for Milky Way stars in the APOGEE
survey, and it reproduces (though imperfectly) the trends of stellar [N/O] with
age and [O/Fe]. The metallicity-dependent yield plays the dominant role in
shaping the gas-phase [N/O]-[O/H] relation, but the AGB time-delay is required
to match the APOGEE stellar age and [O/Fe] trends. If we add $\sim$40\%
oscillations to the star formation rate, the model reproduces the scatter in
gas-phase [N/O] vs. [O/H] observed in external galaxies by MaNGA. We also
construct models using published AGB yields and examine their empirical
successes and shortcomings. For all AGB yields we consider, simple stellar
populations release half their N after only $\sim$250 Myr.
