Environmental Dependence of Galaxy Merger Rate in {\Lambda}CDM Universe
  We make use of four galaxy catalogs based on four different semi-analytical models (SAMs) implemented in the Millennium simulation to study the environmental effects and the model dependence of galaxy merger rate. We begin the analyses by finding that galaxy merger rate in the SAMs has mild redshift evolution, consistent with results of previous works. To study the environmental dependence of galaxy merger rate, we adopt two estimators, the local overdensity (1+{\delta}n) defined as the surface density from the nth-nearest-neighbor (n = 6 is chosen in this study) and the host halo mass Mh. We find that galaxy merger rate Fmg shows strong dependence on the local overdensity (1+{\delta}n) and the dependence is similar at all redshifts. For the overdensity estimator, the merger rate Fmg is found about twenty times larger in the densest regions than in under-dense ones in two of the four models while it is roughly four times higher in the other two. In other words, the discrepancies of the merger rate difference between two extremes can differ by a factor of ~ five depending on the SAMs adopted. On the other hand for the halo mass estimator, Fmg does not monotonically increase with the host halo mass Mh, but peaks in the $M_h$ range between 10^12 and 10^13 h-1 M{\Theta}, which corresponds to group environments. High merger rate in high local density regions corresponds primarily to the high merge rate in group environments...... 
Hung-Yu Jian, Lihwai Lin, and Tzihong Chiueh
astro-ph.CO