The KLEVER survey: Nitrogen abundances at $z\sim$2 and probing the
  existence of a fundamental nitrogen relation
  We present a comparison of the nitrogen-to-oxygen ratio (N/O) in 37
high-redshift galaxies at $z\sim$2 taken from the KMOS Lensed Emission Lines
and VElocity Review (KLEVER) Survey with a comparison sample of local galaxies,
taken from the Sloan Digital Sky Survey (SDSS). The KLEVER sample shows only a
mild enrichment in N/O of $+$0.1 dex when compared to local galaxies at a given
gas-phase metallicity (O/H), but shows a depletion in N/O of $-$0.36 dex when
compared at a fixed stellar mass (M$_*$). We find a strong anti-correlation in
local galaxies between N/O and SFR in the M$_*$-N/O plane, similar to the
anti-correlation between O/H and SFR found in the mass-metallicity relation
(MZR). We use this anti-correlation to construct a fundamental nitrogen
relation (FNR), analogous to the fundamental metallicity relation (FMR). We
find that KLEVER galaxies are consistent with both the FMR and the FNR. This
suggests that the depletion of N/O in high-$z$ galaxies when considered at a
fixed M$_*$ is driven by the redshift-evolution of the mass-metallicity
relation in combination with a near redshift-invariant N/O-O/H relation.
Furthermore, the existence of an fundamental nitrogen relation suggests that
the mechanisms governing the fundamental metallicity relation must be probed by
not only O/H, but also N/O, suggesting pure-pristine gas inflows are not the
primary driver of the FMR, and other properties such as variations in galaxy
age and star formation efficiency must be important.
