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

Abstract

Abstract High spatial resolution observations of the ${{12\atop} \mathrm{CO}}$ (1–0), HCN(1–0), $\mathrm{HCO}^{\mkern-1.26421mu{+}\mkern-2.52842mu}$(1–0), and ${{13\atop} \mathrm{CO}}$(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 $\mathrm{HCO}^{{\mkern-1.26421mu{+}\mkern-2.52842mu}}$ emissions are more compact compared with that of ${{12\atop} \mathrm{CO}}$, and they are not coincident with the star-forming regions. The $\mathrm{HCN} / {{12\atop} \mathrm{CO}}$ 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 $\mathrm{HCN}/{{13\atop} \mathrm{CO}}$ intensity ratio, 5.9, with large velocity gradient calculations suggests that the molecular gas is dense [$n(\mathrm{H}_2) = 10^{4 \hbox{--} 6} \,\mathrm{cm}^{-3}$] and warm ($T_{\mathrm{kin}} \gt 50 \,\mathrm{K}$$T_{\mathrm{kin}} \gt 50 \,\mathrm{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.