Self-consistent Predictions for LIER-like Emission Lines from Post-AGB Stars

Early-type galaxies (ETGs) frequently show emission from warm ionized gas. These low-ionization emission regions (LIERs) were originally attributed to a central, low-luminosity active galactic nucleus. However, the recent discovery of spatially extended LIER emission suggests ionization by both a central source and an extended component that follows a stellar-like radial distribution. For passively evolving galaxies with old stellar populations, hot post-asymptotic giant branch (AGB) stars are the only viable extended source of ionizing photons. In this work, we present the first prediction of LIER-like emission from post-AGB stars that is based on fully self-consistent models of stellar evolution and photoionization. We show that models where post-AGB stars are the dominant source of ionizing photons reproduce the signatures of nebular emission observed in ETGs, including LIER-like emission line ratios in standard optical diagnostic diagrams and {{H}}α equivalent widths of the order of 0.1-3 {{\mathringA }}. We test the sensitivity of LIER-like emission to the details of post-AGB models, including the mass loss efficiency and convective mixing efficiency, and show that line strengths are relatively insensitive to variations in post-AGB timescale. Finally, we examine the UV-optical colors of the models and the stellar populations responsible for the UV excess observed in some ETGs. We find that allowing as little as 3% of the horizontal branch population to be uniformly distributed to very high temperatures (30,000 K) produces realistic UV colors for old, quiescent ETGs.