High spatial resolution observations of the ^ { 12 } CO ( 1–0 ) , HCN ( 1–0 ) , HCO ^ { + } ( 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 ^ { + } 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 ( { H _ { 2 } } ) = 10 ^ { 4 - 6 } cm ^ { -3 } ] and warm ( T _ { kin } > 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 .