Diffuse Gas in Retired Galaxies: Nebular Emission Templates and
  Constraints on the Sources of Ionization
  We present emission line templates for passively evolving ("retired")
galaxies, useful for investigation of the evolution of the ISM in these
galaxies, and characterization of their high-temperature source populations.
The templates are based on high signal-to-noise ($>800$) co-added spectra
($3700-6800$\AA) of $\sim11500$ gas-rich Sloan Digital Sky Survey galaxies
devoid of star-formation and active galactic nuclei. Stacked spectra are
provided for the entire sample and sub-samples binned by mean stellar age. In
Johansson~et al (2014), these spectra provided the first measurements of the He
II 4686\AA\ line in passively-evolving galaxies, and the observed He
II/H$\beta$ ratio constrained the contribution of accreting white dwarfs (the
"single-degenerate" scenario) to the type Ia supernova rate. In this paper, the
full range of unambiguously detected emission lines are presented. Comparison
of the observed [O I] 6300\AA/H$\alpha$ ratio with photoionization models
further constrains any high-temperature single-degenerate scenario for type Ia
supernovae (with 1.5 $\lesssim$ T/$10^{5}K$ $\lesssim$ 10) to $\lesssim$3-6\%
of the observed rate in the youngest age bin (i.e. highest SN Ia rate). Hence,
for the same temperatures, in the presence of an ambient population of post-AGB
stars, we exclude additional high-temperature sources with a combined ionizing
luminosity of $\approx 1.35\times 10^{30} L_{\odot}/M_{\odot,*}$ for stellar
populations with mean ages of 1 - 4 Gyrs. Furthermore, we investigate the
extinction affecting both the stellar and nebular continuum. The latter shows
about five times higher values. This contradicts isotropically distributed dust
and gas that renders similar extinction values for both cases.
