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

High spatial resolution observations of the 12 CO (1–0), HCN(1–0), HCO^-1.26421mu+-2.52842mu(1–0), and 13 CO(1–0) molecular lines toward the luminous infrared merger NGC 6240 have been performed using the Nobeyama Millimeter Array and the RAINBOW Interferometer. All of the observed molecular emission lines are concentrated in the region between the double nuclei of the galaxy. However, the distributions of both HCN and HCO^-1.26421mu+-2.52842mu emissions are more compact compared with that of 12 CO, and they are not coincident with the star-forming regions. The HCN / 12 CO line intensity ratio is 0.25; this suggests that most of the molecular gas between the double nuclei is dense. A comparison of the observed high HCN/13 CO intensity ratio, 5.9, with large velocity gradient calculations suggests that the molecular gas is dense [n(H₂) = 104 -- 6 \,cm⁻³] and warm (Tₖᵢₙ > 50 \,KTₖᵢₙ > 50 \,K$). The observed structure in NGC 6240 may be explained by time evolution of the molecular gas and star formation, which was induced by an almost head-on collision or very close encounter of the two galactic nuclei accompanied with the dense gas and star-forming regions.