SIMBAD references

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_o, which measures the asymmetry of the OHR, and D_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 z₈₅₀-band images for a sample of 764 galaxies with z₈₅₀< 22 mag and 0.35 < z < 0.9 selected from the GEMS and GOODS-South surveys. We show that the sample galaxies having strong asymmetric structures, particularly tidal tails, are well-separated from those with regular morphologies in the A_o-D_ospace. Meanwhile, the widely used CAS and Gini-M₂₀ methods turn out to be insensitive to such morphological features. We stress that the A_o-D_omethod 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.