Origin of multiple nuclei in ultraluminous infrared galaxies
  Ultraluminous infrared galaxies (ULIRGs) with multiple ($\ge 3$) nuclei are
frequently observed. It has been suggested that these nuclei are produced by
multiple major mergers of galaxies. The expected rate of such mergers is,
however, too low to reproduce the observed number of ULIRGs with multiple
nuclei. We have performed high-resolution simulations of the merging of two
gas-rich disk galaxies. We found that extremely massive and compact star
clusters form from the strongly disturbed gas disks after the first or second
encounter between the galaxies. The mass of such clusters reaches $\sim 10^8
M_{\odot}$, and their half-mass radii are $20-30 \rm{pc}$. Since these clusters
consist of young stars, they appear to be several bright cores in the galactic
central region ($\sim \rm{kpc}$). The peak luminosity of these clusters reaches
$\sim 10%$ of the total luminosity of the merging galaxy. These massive and
compact clusters are consistent with the characteristics of the observed
multiple nuclei in ULIRGs. Multiple mergers are not necessary to explain
multiple nuclei in ULIRGs.
