Aperture Synthesis Images of Dense Molecular Gas in Nearby Galaxies with the Nobeyama Millimeter Array

摘要

In order to study the distribution of dense molecular gas and its relation tothe central activities (starburst and AGN) in galaxies, we have conducted animaging survey of HCN(1-0) and HCO+(1-0) emissions from nearby spiral galaxieswith the Nobeyama Millimeter Array. In starburst galaxies, we find that thereis good spatial coincidence between dense molecular gas and star-formingregions. The ratios of HCN to CO integrated intensities on the brightnesstemperature scale, R(HCN/CO), are as high as 0.1 to 0.2 in the starburstregions, and quickly decrease outside of these regions. In contrast, we find aremarkable decrease of the HCN emission in the post-starburst nuclei, despitethe strong CO concentrations there. The R(HCN/CO) values in the central a few100 pc regions of these quiescent galaxies are very low, 0.02 to 0.04. A roughcorrelation between R(HCN/CO) and Ha/CO ratios, which is an indicator of starformation efficiency, is found at a few 100 pc scale. The fraction of densemolecular gas in the total molecular gas, measured from R(HCN/CO), may be animportant parameter that controls star formation. In some Seyfert galaxies wefind extremely high R(HCN/CO) exceeding 0.3. These very high ratios are neverobserved even in strong starburst regions, implying a physical link betweenextremely high R(HCN/CO) and Seyfert activity.