Diffuse ionized gas and its effects on nebular metallicity estimates of
  star-forming galaxies
  We investigate the impact of the diffuse ionized gas (DIG) on abundance
determinations in star-forming (SF) galaxies. The DIG is characterised using
the H$\alpha$ equivalent width ($W_{\text{H}\alpha}$). From a set of 1,409 SF
galaxies from the Mapping Nearby Galaxies at APO (MaNGA) survey, we calculate
the fractional contribution of the DIG to several emission lines using
high-$S/N$ data from SF spaxels (instead of using noisy emission-lines in
DIG-dominated spaxels). Our method is applicable to spectra with observed
$W_{\text{H}\alpha} \gtrsim 10$ angstroms (which are not dominated by DIG
emission). Since the DIG contribution depends on galactocentric distance, we
provide DIG-correction formulae for both entire galaxies and single aperture
spectra. Applying those to a sample of $\,> 90,000$ SF galaxies from the Sloan
Digital Sky Survey, we find the following. (1) The effect of the DIG on
strong-line abundances depends on the index used. It is negligible for the ([O
III]/H$\beta$)/([N II]/H$\alpha$) index, but reaches $\sim 0.1$ dex at the
high-metallicity end for [N II]/H$\alpha$. (2) This result is based on the
$\sim$kpc MaNGA resolution, so the real effect of the DIG is likely greater.
(3) We revisit the mass-metallicity-star formation rate (SFR) relation by
correcting for the DIG contribution in both abundances and SFR. The effect of
DIG removal is more prominent at higher stellar masses. Using the [N
II]/H$\alpha$ index, O/H increases with SFR at high stellar mass, contrary to
previous claims.
