Abundance patterns in early-type galaxies: is there a 'knee' in the Fe/H vs. alpha/Fe relation?

摘要

Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in accordance with their old ages and short formation timescales. In this contribution we aim to resolve the enrichment histories of ETGs. This means we study the abundance of Fe (Fe/H) and the alpha-element groups (alpha/Fe) separately for stars older than 9.5 Gyr (Fe/H(o), alpha/Fe(o)) and for stars between 1.5 and 9.5 Gyr (Fe/H(i), alpha/Fe i). Through extensive simulation we show that we can indeed recover the enrichment history per galaxy. We then analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS selected to be ETGs. We separate out those galaxies for which the abundance of iron in stars grows throughout the lifetime of the galaxy, i.e. in which Fe/H(o) < Fe/H(i). We call those consistent with self-enrichment, while the others must have experienced some mergers or significant gas accretion. We confirm earlier work where the Fe/H and alpha/Fe parameters are correlated with the mass and velocity dispersion of ETGs. We emphasize that the strongest relation is between alpha/Fe and age. This relation falls into two regimes, one with a steep slope for old galaxies and one with a shallow slope for younger ETGs. The vast majority of ETGs in our sample do not show the "knee" in the plot of Fe/H vs. alpha/Fe commonly observed in local group galaxies. This implies that for the vast majority of ETGs, the stars younger than 9.5 Gyr are likely to have been accreted or formed from accreted gas. The properties of the intermediate-age stars in accretion-dominated ETGs indicate that mass growth through late (minor) mergers in ETGs is dominated by galaxies with low Fe/H and low alpha/Fe. The method of reconstructing the stellar enrichment histories of ETGs introduced in this paper promises to constrain the star formation and mass assembly histories of large samples of galaxies in a unique way.