Dr. Tony Wong, Australia Telescope National Facility

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

Abstract:
                               
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
two-dimensional CO and HI distributions in disks on scales of ~1 kpc or less.
The results of such a comparison suggest that, when azimuthally averaged, the
ratio of CO to HI emission, and by implication the molecular-to-atomic gas
ratio, is determined by the midplane interstellar pressure, with the ISM
metallicity as a secondary factor.  A variety of observed phenomena, including
central HI depressions in disks, the utility of CO as a tracer of spiral
structure, and the inhibition of molecular cloud and star formation in the
outer disks of galaxies, can be understood in terms of this dependence.  We
present recent data on the edge-on galaxy NGC 891 that allow a comparison of
the vertical distribution of CO and HI, and will therefore provide an
independent test of these ideas.