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

TL;DR

This study investigates why some early-type galaxies lack emission lines by comparing properties of liny and lineless retired galaxies, revealing differences in stellar populations, dust, and gas origin, and suggesting gas inflow or mergers as sources.

Contribution

It provides the first detailed comparison of liny and lineless early-type galaxies, identifying key differences in their gas content and stellar populations, and constraining the origin of their ionized gas.

Findings

Liny RGs have more intermediate-age stars and higher dust attenuation.

Warm gas mass in liny RGs ranges from 10^5 to 10^8 solar masses.

Gas in liny RGs likely originates from inflow or mergers, not stellar mass-loss.

Abstract

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.