ar X iv : a st ro - p h / 00 03 39 4 v 1 2 4 M ar 2 00 0 A New Probe of the Molecular Gas in Galaxies : Application to M 101

Recent studies of nearby spiral galaxies suggest that photodissociation regions (PDRs) are capable of producing much of the observed HI in galaxy disks. In that case, measurements of the observed HI column density and the far–ultraviolet (FUV) photon flux responsible for the photodissociation process provide a new probe of the volume density of the local underlying molecular hydrogen. We develop the method and apply it to the giant Scd spiral M101 (NGC 5457). The HI column density and amount of FUV emission have been measured for a sample of 35 candidate PDRs located throughout the disk of M101 using the Very Large Array and the Ultraviolet Imaging Telescope. We find that, after correction for the best-estimate gradient of metallicity in the ISM of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30–1000 cm is found near star-forming regions at all radii in the disk of M101 out to a distance of 12 ≈ 26 kpc, close to the photometric limit of R25 ≈ 13.5 . In this picture, the ISM is virtually all molecular in the inner parts of M101. The strong decrease of the HI column density in the inner disk of the galaxy at RG < 10 kpc is a consequence of a strong increase in the dust-to-gas ratio there, resulting in an increase of the H2 formation rate on grains and a corresponding disappearance of hydrogen in its atomic form. Subject headings: galaxies: individual (M101) — galaxies: ISM — ISM: clouds — ISM: molecules — HII regions — radio lines: galaxies — ultraviolet: galaxies