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 { 200 crit } } > 6 \times 10 ^ { 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 } } \gtrsim 1 : 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 .