We have shown that a SMBH with mass 
can be made in a
ULIG during the course of merger between/among gas rich galaxies
regardless of the presence of a seed SMBH in progenitor galaxies; i.e.,
(a) if one progenitor galaxy had a seed SMBH with mass of 
,
this seed SMBH can grow up to 
because of
efficient Bondi-type gas accretion during the course of merger given
a gas density in the circumnuclear region of 
cm
, and
(b) even if there was no progenitor galaxy with a seed SMBH, the star clusters of
compact remnants made in the circumnuclear starbursts can merge into
the merger center within a dynamical time scale of 
years
to form a SMBH with 
.
Note, however, that the contribution of compact remnants supplied from
hidden star clusters is necessary to explain the formation of SMBHs.
In conclusion, the ultraluminous infrared galaxies observed in the local universe
can make a SMBH in their center during the course of merger either by
the gas accretion onto a seed SMBH or by the dynamical relaxation of
star clusters of compact remnants made in the violent circumnuclear starbursts.
The presence of a SMBH with 
is a crucially important necessary condition
for the occurrence of quasar activity.
In the local universe, the masses of SMBHs in giant spiral galaxies
(e.g., our Milky Way Galaxy, M31, NGC 1068, and NGC 4258) are as low as
(Kormendy et al. 1998 and references therein).
Therefore, it is suggested that isolated, typical spiral galaxies
cannot harbor quasar nuclei. However,
mergers between/among gas-rich galaxies can cause efficient gas fueling
toward the nuclear regions of the merging systems and then trigger
intense starbursts either as a result of the piling of a lot of gas
(Mihos & Hernquist 1994) or by the dynamical effect of SMBH
binaries (Taniguchi & Wada 1996; Taniguchi & Shioya 1998).
Furthermore, as demonstrated in the present work, these mergers
provide a possible way to form SMBHs with 
.
In this respect,
it is quite likely that the ULIGs will finally evolve to optically luminous
quasars as suggested by Sanders et al. (1988a, 1988b).
Finally it is worthwhile noting that some elliptical galaxies
could be formed by galaxy mergers (e.g., Toomre 1977; Barnes 1989;
Ebisuzaki et al. 1991).
We also note that some elliptical galaxies
(e.g., M87, NGC 3115, NGC 3377, NGC 4261, and so on) have
SMBHs with 
(Kormendy et al. 1998
and references therein). Actually, investigating the physical
conditions of ULIGs (i.e., mass density and velocity dispersion),
Kormendy and Sanders (1992) found evidence that ULIGs are elliptical galaxies
forming by merger-induced dissipative collapse (see also Wright et al. 1990;
Baker & Clements 1997).
Therefore, we suggest that almost all SMBHs with 
in the local universe were made by galaxy mergers.
We would like to thank an anonymous referee for his/her useful comments. This work was supported in part by the Ministry of Education, Science, Sports and Culture in Japan under Grant Nos. 07055044, 10044052, and 10304013.
表 1: The number of massive stars per unit mass (
)