158 MICRON [C-II] MAPPING OF NGC 6946 - PROBING THE ATOMIC MEDIUM

We present a well-sampled map of the strong 158 mum [C II] cooling line in the Scd galaxy NGC 6946 at 55'' resolution taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). Significant far-infared line emission extends over an area of 8' x 6' (23 x 17 kpc) with a total luminosity of 3.0 x 10(8) L. about 1% of the total far-infrared luminosity of the galaxy. The [C II] emission comes from a mixture of components of interstellar gas. The brightest emission is associated with the nucleus, a second component traces the spiral arms and the largest star-forming/H II regions contained within them, and a third extended component of low brightness can be detected at least 12 kpc from the nucleus. The nuclear and spiral arm components are most likely associated with dense (n(H) greater-than-or-equal-to 10(3) cm-3) photodissociation regions at molecular cloud surfaces that are exposed to ultraviolet radiation produced by young massive stars. We interpret the extended component as originating in the diffuse atomic (H I) medium of NGC 6946, representing the first detection of far-infrared line emission from atomic interstellar gas. The extended [C II] emission probably originates mainly in cold (T approximately 100 K), neutral hydrogen clouds having a pressure between 6.3 x 10(3) and 1.0 x 10(4) cm-3 K, similar to H I clouds in our Galaxy. We infer the volume filling factor of the cold H I clouds in NGC 6946 to be about 1%. Low-density H II regions can also contribute up to 50% of the extended [C II] emission. Hot (T approximately 8000 K) diffuse H I gas can only significantly contribute to the observed [C II] emission if it is clumped with a filling factor of less than 0.1 or has an ionization fraction exceeding 10%. The [C II] cooling rate in the atomic medium of NGC 6946 is approximately 2 x 10(-25) ergs s-1 per hydrogen atom and is consistent with photoelectric heating by diffuse ultraviolet radiation.