A new probe of the molecular gas in galaxies: Application to M101

Recent studies of nearby spiral galaxies suggest that photodissociation reg ions (PDRs) are capable of producing much of the observed H I in galaxy dis ks. In that case, measurements of the observed H I column density and the f ar-ultraviolet (FUV) photon flux responsible for the photodissociation proc ess provide a new probe of the volume density of the local underlying molec ular hydrogen. We develop the method and apply it to the giant Scd spiral g alaxy M101 (NGC 5457). The H I column density and amount of FUV emission ha ve been measured for a sample of 35 candidate PDRs located throughout the d isk of M101 using the Very Large Array and the Ultraviolet Imaging Telescop e. We find that, after correction for the best-estimate gradient of metalli city in the interstellar medium (ISM) of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30 -1000 cm(-3) is found near star-forming regions at all radii in the disk of M101 out to a distance of 12' approximate to 26 cm kpc, close to the photo metric limit of R-25 approximate to 13.'5. In this picture, the ISM is virtually all molecular in the inner parts of M 101. The strong decrease of the H I column density in the inner disk of the galaxy at R-G < 10 kpc is a consequence of a strong increase in the dust-t o-gas ratio there, resulting in an increase of the H-2 formation rate on gr ains and a corresponding disappearance of hydrogen in its atomic form.