Extended dust emission and atomic hydrogen: A reservoir of diffuse H-2 in NGC 1068

We report on sensitive submillimeter imaging observations of the prototype Seyfert 2/starburst galaxy NGC 1068 at 850 and 450 mu m using the Submillim eter Common-User Bolometer Array on the James Clerk Maxwell Telescope. We f ind clear evidence of dust emission associated with the extended H I compon ent that, together with the very faint (CO)-C-12 J = 1-0 emission, gives a gas-to-dust ratio of M-gas/M-dust similar to 70-150. This contrasts with th e larger ratio M-gas/M-dust similar to 330 that is estimated within a galac tocentric radius of r less than or equal to 1.36 kpc, where the gas is most ly molecular and starburst activity occurs. The large gas-to-dust ratio tha t is found for the starburst region is attributed to a systematic overestim ate of the molecular gas mass in starburst environments when the luminosity of the (CO)-C-12 J = 1-0 line and a standard Galactic conversion factor ar e used. On the other hand, submillimeter imaging proves to be a more powerf ul tool than conventional CO imaging for revealing the properties of the di ffuse H, that coexists with H I. This molecular gas phase is characterized by low densities [n(H-2)< 10(3) cm(-3)], very faint emission from subtherma lly excited CO, and contains more mass than H I, namely, M(H-2)/M(H I) simi lar to 5.