Frequency of Tidal Features Correlates with Age and Internal Structure of Early-type Galaxies

Abstract: Previous studies suggest that compact young early-type galaxies (ETGs) were formed by recent mergers. However, it has not yet been revealed whether tidal features that are direct evidence of recent mergers are detected frequently around compact young ETGs. Here, we investigate how the fraction of ETGs having tidal features ($f_{T}$) depends on age and internal structure (compactness, color gradient, and dust lanes) of ETGs, using 650 ETGs with $M_r\le-19.5$ in $0.015\le z\le0.055$ that are in deep coadded images of the Stripe 82 region of the Sloan Digital Sky Survey. We find that tidal features are more frequent in younger ETGs and more compact ETGs, so that compact young ETGs with ages $\lesssim6$ Gyr have high $f_{T}$ of $\sim0.7$ compared to their less compact or old counterparts with ages $\gtrsim9$ Gyr that have $f_{T}\lesssim0.1$. Among compact young ETGs, those with blue cores have $\sim3$ times higher $f_{T}$ than those with red cores. In addition, ETGs with dust lanes have $\sim4$ times higher $f_{T}$ than those without dust lanes. Our results provide direct evidence that compact young ETGs especially with blue cores and ETGs with dust lanes are involved in recent mergers. Based on our results and several additional assumptions, we roughly estimate the typical visible time of tidal features after a merger, which is $\sim3$ Gyr in the depth of the Stripe 82 coadded images.