The SAMI Galaxy Survey: using tidal streams and shells to trace the dynamical evolution of massive galaxies

Slow rotator galaxies are distinct amongst galaxy populations, with simulations suggesting that a mix of minor and major mergers are responsible for their formation. A promising path to resolve outstanding questions on the type of merger responsible, is by investigating deep imaging of massive galaxies for signs of potential merger remnants. We utilize deep imaging from the Subaru-Hyper Suprime Cam Wide data to search for tidal features in massive [log₁₀(M_∗/M_☉) > 10] early-type galaxies (ETGs) in the SAMI Galaxy Survey. We perform a visual check for tidal features on images where the galaxy has been subtracted using a Multi-Gauss Expansion (MGE) model. We find that 31₋₂⁺² per cent of our sample show tidal features. When comparing galaxies with and without features, we find that the distributions in stellar mass, light-weighted mean stellar population age, and Hα equivalent width are significantly different, whereas spin (λ_Re), ellipticity, and bulge-to-total ratio have similar distributions. When splitting our sample in age, we find that galaxies below the median age (10.8 Gyr) show a correlation between the presence of shells and lower λ_Re, as expected from simulations. We also find these younger galaxies which are classified as having ‘strong’ shells have lower λ_Re. However, simulations suggest that merger features become undetectable within ∼2–4 Gyr post-merger. This implies that the relationship between tidal features and merger history disappears for galaxies with older stellar ages, i.e. those that are more likely to have merged long ago.