SDSS-IV MaNGA: Exploring the local scaling relations for N/O
  We present, for the first time, the relationship between local stellar mass
surface density, $\mathrm{\Sigma_{*}}$, and N/O derived from SDSS-IV MaNGA
data, using a sample of $792765$ high signal-to-noise ratio star-forming
spaxels. Using a combination of phenomenological modelling and partial
correlation analysis, we find that $\mathrm{\Sigma_{*}}$ alone is insufficient
to predict the N/O in MaNGA spaxels, and that there is an additional dependence
on the local star formation rate surface density, $\mathrm{\Sigma_{SFR}}$. This
effect is a factor of $3$ stronger than the dependence of 12+log(O/H) on
$\mathrm{\Sigma_{SFR}}$. Surprisingly, we find that the local N/O scaling
relations also depend on the total galaxy stellar mass at fixed $\Sigma_{*}$ as
well as the galaxy size at fixed stellar mass. We find that more compact
galaxies are more nitrogen rich, even when $\mathrm{\Sigma_{*}}$ and
$\mathrm{\Sigma_{SFR}}$ are controlled for. We show that $\sim50\%$ of the
variance of N/O is explained by the total stellar mass and size. Thus, the
evolution of nitrogen in galaxies is set by more than just local effects and
does not simply track the build up of oxygen in galaxies. The precise form of
the N/O-O/H relation is therefore sensitive to the sample of galaxies from
which it is derived. This result casts doubt on the universal applicability of
nitrogen-based strong-line metallicity indicators derived in the local
universe.
