Stellar Populations of Shell Galaxies

Abstract

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.