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

Abstract

Abstract 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 equivalent widths of the order of 0.1–3 . 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.