  Upcoming large imaging surveys will allow detailed studies of the structure
and morphology of galaxies aimed at addressing how galaxies form and evolve.
Computational approaches are needed to characterize their morphologies over
large samples. We introduce an automatic method to quantify the outer structure
of galaxies. The key to our approach is the division of a galaxy image into two
sections delineated by the isophote which encloses half the total brightness of
the galaxy. We call the central section the inner half-flux region (IHR) and
the outer section the outer half-flux region (OHR). From this division, we
derive two parameters: $A_{\rm o}$, which measures the asymmetry of the OHR,
and $D_{\rm o}$, which measures the deviation of the intensity weighted
centroid of the OHR from that of the IHR relative to the effective radius. We
derive the two parameters from $HST$/ACS $z_{850}$-band images for a sample of
764 galaxies with $z_{850}<22$ mag and $0.35<z<0.9$ selected from GEMS and
GOODS-South surveys. We show that the sample galaxies having strong asymmetric
structures, in particular tidal tails, are well-separated from those with
regular morphologies in the $A_{\rm o}$-$D_{\rm o}$ space. Meanwhile, the
widely used $CAS$ and Gini-$M_{20}$ methods turn out to be insensitive to such
morphological features. We stress that the $A_{\rm o}$-$D_{\rm o}$ method is an
efficient way to select galaxies with significant asymmetric features like
tidal tails and study galaxy mergers in the dynamical phase traced by these
delicate features.
