Extended Dust Emission and Atomic Hydrogen: A Reservoir of Diffuse H2 in NGC 1068

We report on sensitive submillimeter imaging observations of the prototype Seyfert 2/starburst galaxy NGC 1068 at 850 and 450 μm using the Submillimeter Common-User Bolometer Array on the James Clerk Maxwell Telescope. We find clear evidence of dust emission associated with the extended H I component that, together with the very faint 12CO J=1-0 emission, gives a gas-to-dust ratio of Mgas/Mdust~70-150. This contrasts with the larger ratio Mgas/Mdust~330 that is estimated within a galactocentric radius of r≤1.36 kpc, where the gas is mostly molecular and starburst activity occurs. The large gas-to-dust ratio that is found for the starburst region is attributed to a systematic overestimate of the molecular gas mass in starburst environments when the luminosity of the 12CO J=1-0 line and a standard Galactic conversion factor are used. On the other hand, submillimeter imaging proves to be a more powerful tool than conventional CO imaging for revealing the properties of the diffuse H2 that coexists with H I. This molecular gas phase is characterized by low densities [n(H2)<103 cm−3], very faint emission from subthermally excited CO, and contains more mass than H I, namely, M(H2)/M(H I) ~ 5.