Nitrogen-to-Oxygen Abundance Ratio Variation In Quiescent Galaxies
  For the first time, we establish a gas phase abundance pattern calibration
for quiescent galaxies using optical emission lines. Quiescent galaxies have
warm ionized gas showing line ratios similar to low-ionization nuclear emission
line regions (LINER). The ionization mechanism for the gas is still an
unsettled puzzle. Despite the uncertainty in the ionization mechanism, we argue
that we can still infer certain gas phase abundance pattern from first
principles. We show that the relative trend in N/O abundance can still be
reliably measured based on [NII]6548,6583/[OII]3726,3729 and a direct
measurement of the electron temperature. We construct a composite direct
temperature tracer that is independent of extinction correction, by combining
[OII] 3726,3729/[OII] 7320,7330 and [SII] 6716,6730/[SII] 4068,4076 and
canceling out the effect of extinction, as these involve the easiest-to-detect
auroral lines in quiescent galaxies. With theoretical modeling, we establish
the calibration for N/O based on [NII]/[OII] and a temperature tracer. We apply
this technique to quiescent galaxies in the nearby Universe and find they span
a range of 0.35 dex in N/O ratio from 17-percentile to 83-percentiles of the
whole distribution. These measurements can shed light on the chemical
enrichment history of the warm ionized gas in quiescent galaxies.
