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