Shell-type Tidal Features Are More Frequently Detected in Slowly Rotating Early-type Galaxies than Stream- and Tail-type Features

Abstract: To enhance our understanding of the impact of galaxy mergers on the kinematics of early-type galaxies (ETGs), we examine differences in specific stellar angular momentum within the half-light radius ($\lambda_{R_e}$) among ETGs with different types of tidal features and those without such features. This is accomplished by categorizing tidal features, which serve as direct evidence of recent mergers, into shells, streams, and tails, through deep images from the DESI Legacy Survey, and by using MaNGA data for the analysis of the kinematics of 1244 ETGs at $z<0.055$. We find that ETGs with tidal features typically have reduced $\lambda_{R_e}$ values that are lower by 0.12 dex than ETGs without tidal features. ETGs with shells contribute most to the reduction in $\lambda_{R_e}$. Consequently, nearly half of ETGs with shells are classified as slow rotators, a fraction that is more than twice as high as that of ETGs with tails or streams, and over three times higher than that of ETGs without tidal features. These trends generally remain valid even when ETGs are divided into several mass bins. Our findings support the idea that radial mergers, which are more effective at reducing $\lambda_{R_e}$ than circular mergers, are more closely associated with the formation of shells rather than streams or tails. The detection of shells in slightly more massive ETGs compared to streams and tails may be attributed to the fact that massive satellite galaxies are more likely to be accreted through radial orbits, due to the nature of dynamical friction.