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We have carried out systematic HCN (1-0) observations (complimentary with CO observation) in a sample of $\sim 40 $ IR/CO bright/luminous galaxies using the NRAO 12-m, IRAM 30-m and FCRAO 14-m telescopes. HCN traces much higher density molecular gas ($\sim 10^5 cm^{-3}$) than that of CO. $L_{HCN}$ is a measure of the mass of dense molecular gas and and $L_{HCN}/L_{CO}$ is an indicator of the fraction of dense molecular gas. For 10 nearby galaxies we present HCN maps along the major axis. A high detection rate of nearly $\sim 75 \%$ is obtained. This is one of the largest HCN surveys undertaken so far.
We show that all galaxies observed so far follow the correlations between $L_{IR}$ and $L_{HCN}$ and between $L_{IR}/L_{CO}$ and $L_{HCN}/L_{CO}$ proposed by Solomon et al (1992). This suggests that the star formation efficiency depends upon the fraction of dense molecular gas and the ratio of $L_{IR}/L_{HCN}$ is similar in all galaxies, whether normal, starburst or ultraluminous. Dense molecular gas is not confined to the inner $\sim 1 kpc$ nuclear regions, and can be detected to a radius as large as $\sim 5 kpc$, perhaps to a diameter of $\sim D_{25}/4$, although the highest concentration of dense molecular gas is in the central region. A significant fraction of dense molecular gas is distributed in the inner disks of galaxies outside the nuclear or inner ring starburst region. The starburst cores have the highest fraction of dense molecular gas with surface brightness ratio $I_{HCN}/I_{CO}$ comparable to that of ultraluminous IR galaxies, whereas this ratio falls off quickly in the galaxy disks at larger radii (M82 is probably an exception). HCN emission is thus a much better indicator of star forming molecular gas than is CO emission.
