Massive Early-type Galaxies in the HSC-SSP: Flux Fraction of Tidal Features and Merger Rates

Here we present a statistical study on tidal features around massive early-type galaxies (ETGs). Utilizing the imaging data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), we measure the flux fraction of tidal features (f _tidal) in 2649 ETGs with stellar mass M _* > 10¹¹ M _⊙ and redshift 0.05 < z < 0.15 using automated techniques. The Wide layer of HSC-SSP reaches a depth of ~28.5 mag arcsec^-2 in the i band. Under this surface brightness limit, we find that about 28% of these galaxies harbor prominent tidal features with f _tidal > 1%, among which the number of ETGs decreases exponentially with f _tidal, with a logarithmic slope of ~100. Within the stellar mass range we probe, we note that f _tidal increases by a factor of 2 from M _* ≈ 10¹¹ to 10¹² M _⊙. We also perform a pair count to estimate the merger rate of these massive ETGs. Combining the merger rates with f _tidal, we estimate that the typical lifetime of tidal features is ~3 Gyr, consistent with previous studies.