  In order to study the distribution of dense molecular gas and its relation to
the central activities (starburst and AGN) in galaxies, we have conducted an
imaging survey of HCN(1-0) and HCO+(1-0) emissions from nearby spiral galaxies
with the Nobeyama Millimeter Array. In starburst galaxies, we find that there
is good spatial coincidence between dense molecular gas and star-forming
regions. The ratios of HCN to CO integrated intensities on the brightness
temperature scale, R(HCN/CO), are as high as 0.1 to 0.2 in the starburst
regions, and quickly decrease outside of these regions. In contrast, we find a
remarkable decrease of the HCN emission in the post-starburst nuclei, despite
the strong CO concentrations there. The R(HCN/CO) values in the central a few
100 pc regions of these quiescent galaxies are very low, 0.02 to 0.04. A rough
correlation between R(HCN/CO) and Ha/CO ratios, which is an indicator of star
formation efficiency, is found at a few 100 pc scale. The fraction of dense
molecular gas in the total molecular gas, measured from R(HCN/CO), may be an
important parameter that controls star formation. In some Seyfert galaxies we
find extremely high R(HCN/CO) exceeding 0.3. These very high ratios are never
observed even in strong starburst regions, implying a physical link between
extremely high R(HCN/CO) and Seyfert activity.
