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