  We present interferometric observations of three ultraluminous infrared
galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm
wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and
HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was
clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission
was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN
to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs
were derived and compared with those of lower luminosity galaxies known to be
dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk
273, where there is evidence for obscured AGNs from previous observations at
other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range
found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a
powerful obscured AGN has been unclear, the ratio (1.7) is in between the
observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+
brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence
of an HCN abundance enhancement, which is expected from chemical effects of the
central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed
millimeter interferometric method based on HCN/HCO+ ratios may be an effective
tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially
because of the negligible dust extinction at these wavelengths.
