Why do many early-type galaxies lack emission lines? I. Fossil clues
  Early-type retired galaxies (RGs, i.e. galaxies which no longer form stars)
can be divided into two classes: those with no emission lines, here dubbed
lineless RGs, and those with emission lines, dubbed liny RGs. Both types of
galaxies contain hot low-mass evolved stars (HOLMES) which emit ionizing
photons. The difference must thus lie in the presence or absence of a reservoir
of ionizable gas. From a volume-limited sample of 38\,038 elliptical galaxies,
we explore differences in physical properties between liny and lineless using
data from the SDSS, WISE and GALEX catalogues. To avoid biases in the
comparison, we pair-match liny and lineless in stellar-mass, redshift and
half-light Petrosian radius. We detect marginal differences in their optical
stellar ages and NUV luminosities, indicating that liny RGs have an excess of
intermediate-age (0.1--5 Gyr) stellar populations. Liny RGs show higher dust
attenuation and $W3$ luminosities than their lineless counterparts. We also
find that the amount of warm gas needed to explain the observed \Ha luminosity
in liny RGs is $10^5$--$10^8$\msun, and that their \nii/\oii emission-line
ratios are typical of those of the most massive star-forming galaxies. Taken
together, these results rules out the following sources for the warm gas in
liny RGs: mass-loss from intermediate-mass stars, mergers with metal-poor
galaxies and intergalactic streams. They imply instead an inflow of enriched
gas previously expelled from the galaxy or a merger with a metal-rich galaxy.
The ionization source and the origin of the gas producing the emission lines
are thus disconnected.
