Comparing the CO and HI Distributions in Nearby Spiral Galaxies at High Resolution

With wide-field CO maps of spiral galaxies at ∼6′′resolution provided by the BIMA Survey of Nearby Galaxies (BIMA SONG),it is now feasible to compare the CO and HI distributions in disks onscales of ∼1 kpc or less. The results of such a comparisonsuggest that, when azimuthally averaged, the ratio of CO to HIemission, and by implication the molecular-to-atomic gas ratio, isdetermined by the midplane interstellar pressure, with the ISMmetallicity as a secondary factor. A variety of observed phenomena,including the inhibition of molecular cloud and star formation in theouter disks of galaxies, can be understood in terms of thisdependence. We present recent data on the edge-on galaxy NGC 891 thatallow a comparison of the vertical distribution of CO and HI, and willtherefore provide an independent test of these ideas.