Title: Galaxies with Shells in the Illustris Simulation: Metallicity Signatures,
Abstract: Stellar shells are low surface brightness arcs of overdense stellar regions,
extending to large galactocentric distances. In a companion study, we
identified 39 shell galaxies in a sample of 220 massive ellipticals
($\mathrm{M}_{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}_\odot$) from the
Illustris cosmological simulation. We used stellar history catalogs to trace
the history of each individual star particle inside the shell substructures,
and we found that shells in high-mass galaxies form through mergers with
massive satellites (stellar mass ratios $\mu_{\mathrm{stars}}\gtrsim1:10$).
Using the same sample of shell galaxies, the current study extends the stellar
history catalogs in order to investigate the metallicity of stellar shells
around massive galaxies. Our results indicate that outer shells are often times
more metal-rich than the surrounding stellar material in a galaxy's halo. For a
galaxy with two different satellites forming $z=0$ shells, we find a
significant difference in the metallicity of the shells produced by each
progenitor. We also find that shell galaxies have higher mass-weighted
logarithmic metallicities ([Z/H]) at $2$-$4\,\mathrm{R}_{\mathrm{eff}}$
compared to galaxies without shells. Our results indicate that observations
comparing the metallicities of stars in tidal features, such as shells, to the
average metallicities in the stellar halo can provide information about the
assembly histories of galaxies.