Dense and Warm Molecular Gas between Double Nuclei of the Luminous Infrared Galaxy NGC 6240

High spatial resolution observations of the 12CO(1-0), HCN(1-0), HCO+(1-0),and 13CO(1-0) molecular lines toward the luminous infrared merger NGC 6240 havebeen performed using the Nobeyama Millimeter Array and the RAINBOWInterferometer. All of the observed molecular emission lines are concentratedin the region between the double nuclei of the galaxy. However, thedistributions of both HCN and HCO+ emissions are more compact compared withthat of 12CO, and they are not coincident with the star-forming regions. TheHCN/12CO line intensity ratio is 0.25; this suggests that most of the moleculargas between the double nuclei is dense. A comparison of the observed highHCN/13CO intensity ratio, 5.9, with large velocity gradient calculationssuggests that the molecular gas is dense [n(H_2)=10^{4-6} cm^-3] and warm(T_kin>50 K). The observed structure in NGC 6240 may be explained by timeevolution of the molecular gas and star formation, which was induced by analmost head-on collision or very close encounter of the two galactic nucleiaccompanied with the dense gas and star-forming regions.Comment: 25 pages, 8 figures, To be appeared in PASJ 57, No.4 (August 25, 2005) issu