Major mergers between galaxies of comparable mass are expected to occur frequently in hierarchical models of galaxy formation and evolution. Galaxy merging may be a crucial process that regulates galaxy mass assembly, galaxy morphology reshaping, growth of supermassive black holes, and enhancement of star formation. Measuring the galaxy merger rate over cosmic time is thus a key to understanding the origin of galaxies and a central task in today's extragalactic astronomy. Upcoming large and deep imaging surveys will allow detailed studies of the structure and morphology of galaxies aimed at addressing the importance of the merging process relative to that of other physical processes (e.g., feedback and gas accretion) to driving galaxy evolution. 

    Dr Xian-zhong Zheng and his students at the Purple mountain observatory have developed a new automatic method to measure the morphology of galaxies, especially to select galaxies with strong asymmetric structures like tidal tails. The key to their 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: Ao, which measures the asymmetry of the OHR, and Do, which measures the deviation of the intensity weighted centroid of the OHR from that of the IHR relative to the effective radius. The galaxies with stronger disturbance in morphology are expected to have higher Ao and Do. Moreover, the two parameters are designed to be less affected by the central high surface brightness section of galaxies, and thus are sensitive to low surface brightness features in the OHR. A sample of 764 galaxies with log(M/M_) > 3 × 1010 and 0.35 < z < 0.9 selected from the GEMS and GOODS-S surveys is used to verify our method (see figure 1). Our investigation shows that all sample galaxies fall on a sequence in the Ao–Do space. The position along the sequence is generally correlated with the degree of morphological disturbance. Galaxies with more disturbed morphologies have higher Ao and Do in a statistical sense. The merging galaxies with tidal tails are well separated from regular galaxies (spheroids and disks) along the sequence. Meanwhile, the widely used CAS and Gini–M20 methods turn out to be insensitive to such morphological features. We stress that the Ao–Do 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. 

   The paper is now published by the The Astrophysical Journal ( Volume 787, Issue 2, article id. 130, 9 pp. 2014, http://iopscience.iop.org/0004-637X/787/2/130). This work is supported by National Basic Research Program of China (973 Program 2013CB834900) and the Strategic Priority Research Program “The Emergence of Cosmological Structures” of the Chinese Academy of Sciences (grant No. XDB09000000). 

 

By with WEN Zhangzhen 

Fig.  1 

     Ao–Do diagram of our sample of 764 galaxies with  and 0.35 < z < 0.9 in the ECDFS. The two parameters are derived from HST z850-band images corresponding to the rest-frame optical for the redshift range examined here. The dashed line is the criterion for selecting tidal tails described as log Ao > −1.6 logDo−1.1. The solid line represents the sequence of regular galaxies best-fitted by log Ao = 0.6 log Do.