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We have obtained large scale, 1' resolution maps of the edge-on spiral galaxy NGC 891 in the 158 micron [CII] and CO(1 - 0) lines. The [CII] distribution largely follows the optical image, but is significantly asymmetric - there is a pronounced ridge of emission to the north. The CO(1 - 0) and 2.8 cm radio continuum are similarly asymmetric suggesting that the [CII] emission arises from both the UV exposed surfaces of giant molecular clouds and from the diffuse ionized medium. For low density media, we estimate ~ 25% of the observed [CII] emission arises from ionized gas clouds. The HI 21 cm line emission bears little resemblance to the other tracers, along the major axis. Assuming "standard" atomic cloud parameters, only ~15% of the observed [CII] intensity along the plane comes from the atomic medium. The remainder (60%) of the [CII] emission is associated with dense photodissociation regions (PDRs) on the surfaces of molecular clouds. The [CII]/CO(1 - 0) line intensity ratio for these regions is similar to Galactic molecular clouds, providing an important link between the molecular clouds in NGC 891 and the Milky Way.
For the outer regions (r > 4 kpc) of NGC 891, most of the [CII] emission arises from atomic clouds with Galactic parameters (T ~ 100 K, n ~ 30 cm-3). The enhanced [CII] to FIR continuum ratio in these outer regions is a reflection of the increased efficiency for photoelectic heating in low UV field environments. Integrated over the galaxy about 25% of the observed [CII] emission arises from atomic clouds.
The [CII] emission may be marginally resolved in the z direction with our 55" beam - indicating a scale height larger than that of CO. This extraplanar [CII] may be associated with the "filaments" observed in the optical recombination lines.
