Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Markarian 273, and IRAS 17208-0014, Based on HCN-to-HCO+Ratios

We present interferometric observations of three ultraluminous infraredgalaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mmwavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) andHCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission wasclearly detected at the nuclear positions of these ULIRGs, and HCO+ emissionwas detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCNto HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGswere derived and compared with those of lower luminosity galaxies known to bedominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk273, where there is evidence for obscured AGNs from previous observations atother wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the rangefound for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of apowerful obscured AGN has been unclear, the ratio (1.7) is in between theobserved values for starburst- and AGN-dominated galaxies. The high HCN/HCO+brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequenceof an HCN abundance enhancement, which is expected from chemical effects of thecentral X-ray emitting AGN on the surrounding dense molecular gas. Our proposedmillimeter interferometric method based on HCN/HCO+ ratios may be an effectivetool for unveiling elusive buried AGNs at the cores of ULIRGs, especiallybecause of the negligible dust extinction at these wavelengths.