A New Probe of the Molecular Gas in Galaxies: Application to M101

Recent studies of nearby spiral galaxies suggest that photodissociationregions (PDRs) are capable of producing much of the observed HI in galaxydisks. In that case, measurements of the HI column density and thefar-ultraviolet (FUV) photon flux provide a new probe of the volume density ofthe local underlying H_2. We develop the method and apply it to the giant Scdspiral M101 (NGC 5457). We find that, after correction for the best-estimategradient of metallicity in the ISM of M101 and for the extinction of theultraviolet emission, molecular gas with a narrow range of density from 30-1000cm^-3 is found near star- forming regions at all radii in the disk of M101 outto a distance of 12' (approximately 26 kpc), close to the photometric limit ofR_25 = 13.5'. In this picture, the ISM is virtually all molecular in the inner parts ofM101. The strong decrease of the HI column density in the inner disk of thegalaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gasratio there, resulting in an increase of the H_2 formation rate on grains and acorresponding disappearance of hydrogen in its atomic form.Comment: accepted for publication in The Astrophysical Journal (1 August 2000); 29 pages including 20 figures (7 gif); AAS LaTex; contact authors for full resolution versions of gif figure