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Interferometers allow us to determine the large scale distribution and kinematics of gas associated with high mass star formation in external galaxies, since they provide sufficient resolution to map internal structure. Typical synthesized beamsizes are 15--30$"$ for VLA and Westerbork maps and 3--7$"$ for CO maps from the present millimeter arrays. Radio continuum emission as a tracer of star formation will be discussed briefly, but the primary emphasis of the talk will be on the neutral gas.
Aperture synthesis maps of 21 cm emission confirm that spiral galaxies have uniform, extended HI disks. The centers of many galaxies, like the Milky Way, are deficient in HI although often spiral nuclei are rich in molecular gas. HI is enhanced along spiral arms, and it is better correlated with H$\alpha$ emission than with CO. The present belief is that the enhanced HI seen in spiral arms is largely a dissociation product, caused by the interaction of HII regions with molecular clouds, rather than a star formation precursor.
CO interferometry has been remarkably successful at revealing molecular structure in galaxies, including strong evidence for the density wave theory. Maps of M51 show that the CO spiral arms are systematically offset to the inner side of H$\alpha$ spiral arms, as would be expected if density wave compression or orbit crowding of molecular clouds triggers the star formation in the spiral arms. The kinematics of the CO also support this picture. Similar results have been obtained for the mini-spiral in the nucleus of IC342, indicating that density waves may determine the molecular structure in galactic nuclei as well as in the disk.
Current directions in the field include study of the relation of HI, CO, and photodissociation regions in spiral arms, the implications of the high molecular mass fractions seen in some nuclei, the study of gas temperatures and densities using higher rotational lines of CO, determination of the distribution of dense molecular gas through high dipole moment tracers such as HCN, and the effects of tidal interactions on gas structure.
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