  We investigate the relative importance of starbursts and AGN in nuclear
activities of ultra-luminous infrared galaxies (ULIRGs) based on chemodynamical
simulations combined with spectrophotometric synthesis codes. We numerically
investigate both the gas accretion rates (m_acc) onto super massive black holes
(SMBHs) and the star formation rates (m_sf) in ULIRGs formed by gas-rich galaxy
mergers and thereby discuss what powers ULIRGs. Our principal results, which
can be tested against observations, are as follows. (1) ULIRGs powered by AGN
can be formed by major merging between luminous, gas-rich disk galaxies with
prominent bulges containing SMBHs, owing to the efficient gas fuelling m_acc >
1 M_sun/yr of the SMBH. AGN in these ULIRGs can be surrounded by compact
poststarburst stellar populations (e.g., A-type stars). (2) ULIRGs powered by
starbursts with m_sf ~ 100 M_sun/yr can be formed by merging between gas-rich
disk galaxies with small bulges having the bulge-to-disk-ratio (f_b) as small
as 0.1. (3) The relative importance of starbursts and AGN can depend on
physical properties of merger progenitor disks, such as f_b, gas mass fraction,
and total masses. For example, more massive galaxy mergers are more likely to
become AGN-dominated ULIRGs. (4) For most models, major mergers can become
ULIRGs, powered either by starbursts or by AGN, only when the two bulges
finally merge. Interacting disk galaxies can become ULIRGs with well separated
two cores (> 20kpc) at their pericenter when they are very massive and have
small bulges. (5) Irrespective of the choice of model, interacting/merging
galaxies show the highest accretion rates onto the central SMBHs, and the
resultant rapid growth of the SMBHs occur when their star formation rates are
very high.
