Synthesis Imaging of Dense Gas in Nearby Galaxies

We present BIMA observations of the HCN emission from five nearby spiral galaxies. The HCN observations comprise the first high-resolution (5--10\arcsec) survey of dense molecular gas from a sample of normal galaxies, rather than galaxies with prolific starburst or nuclear activities. The images show compact structure, demonstrating that the dense gas emission is largely confined to the central kiloparsec of the sources. In one of the galaxies, NGC 6946, the ratio of HCN to CO integrated intensities ranges from 0.05--0.2 within the extent of the HCN emission (r = 150 pc), with an average value of 0.11 \pm 0.01 over the whole region; the range and average values of the ratios in NGC 6946 are very similar to what is observed in the central r = 250 pc of the Milky Way. In NGC 6946, NGC 1068 and the Milky Way, the ratio I{HCN}/I{CO} is 5 to 10 times higher in the bulge regions than in their disks; this suggests that the physical conditions in their bulges and disks are very different. In NGC 4826 and M51, as in the Milky Way and NGC 1068, there is a linear offset of continuum emission