Stellar Populations of Shell Galaxies
  We present a study of the inner (out to $\sim$1 R$_{\mathrm{eff}}$) stellar
populations of 9 shell galaxies. We derive stellar population parameters from
long slit spectra by both analyzing the Lick indices of the galaxies and by
fitting Single Stellar Population model spectra to the full galaxy spectra. The
results from the two methods agree reasonably well. Many of the shell galaxies
in our sample appear to have lower central $\mathrm{Mg}_{2}$ index values than
non-shell galaxies of the same central velocity dispersion, which is likely due
to a past interaction event. Our shell galaxy sample shows a relation between
central metallicity and velocity dispersion that is consistent with previous
samples of non-shell galaxies. Analyzing the metallicity gradients in our
sample, we find an average metallicity gradient of -0.16$\pm$0.10 dex per
decade in radius. We compare this with formation models to constrain the
merging history of shell galaxies. We argue that our galaxies likely have
undergone major mergers in their past but it is unclear whether the shells
formed from these events or from separate minor mergers. Additionally, we find
evidence for young stellar populations ranging in age from 500 Myr to 4--5 Gyr
in four of the galaxies, allowing us to speculate on the age of the shells. For
NGC 5670, we use a simple dynamical model to find the time required to produce
the observed distribution of shells to be consistent with the age of the young
subpopulation, suggesting that the shells likely formed from the same event
that led to the young subpopulation.
