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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 dubbedlineless RGs, and those with emission lines, dubbed liny RGs. Both types ofgalaxies contain hot low-mass evolved stars (HOLMES) which emit ionizingphotons. The difference must thus lie in the presence or absence of a reservoirof ionizable gas. From a volume-limited sample of 38\,038 elliptical galaxies,we explore differences in physical properties between liny and lineless usingdata from the SDSS, WISE and GALEX catalogues. To avoid biases in thecomparison, we pair-match liny and lineless in stellar-mass, redshift andhalf-light Petrosian radius. We detect marginal differences in their opticalstellar ages and NUV luminosities, indicating that liny RGs have an excess ofintermediate-age (0.1--5 Gyr) stellar populations. Liny RGs show higher dustattenuation and $W3$ luminosities than their lineless counterparts. We alsofind that the amount of warm gas needed to explain the observed \Ha luminosityin liny RGs is $10^5$--$10^8$\msun, and that their \nii/\oii emission-lineratios are typical of those of the most massive star-forming galaxies. Takentogether, these results rules out the following sources for the warm gas inliny RGs: mass-loss from intermediate-mass stars, mergers with metal-poorgalaxies and intergalactic streams. They imply instead an inflow of enrichedgas 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 linesare thus disconnected.

Why do many early-type galaxies lack emission lines? I. Fossil clues