2000AJ....119..644H

We present new, deep optical images (BVI+Hα) of the interstellar medium (ISM) far above the plane of the edge-on Sb galaxy NGC 891. These subarcsecond (0".8-0".9) ground-based images give a direct visual view of two physically distinct ``phases'' of the thick interstellar disk of this galaxy. A dense, likely cold, phase of the thick disk ISM is observed in our BVI images as highly structured dust-bearing clouds viewed in absorption against the background stellar light of the galaxy. These dusty structures are traceable to heights |z|∼2 kpc from the midplane. Though our data are deep enough to identify such clouds at higher z, very few highly structured dust features are present at |z|≳2 kpc. If the more prominent dust structures have Galactic gas-to-dust ratios, then they have gas masses in excess of 10⁵ M_☉, each having visual extinctions well in excess of unity. A warm ionized phase of the high-z ISM is observed through its well-studied Hα emission. Our images of this medium, to date the highest resolution observations of the warm ionized medium in NGC 891, show that the diffuse ionized medium in this galaxy is relatively smoothly distributed with some filamentary structure superposed on this smooth background. Our data show very little correspondence between the Hα-emitting material and the absorbing dust structures. These two phases of the multiphase high-z ISM are physically distinct. The material traced by Hα emission is much more smoothly distributed than that traced by the absorbing dust clouds, and it is clear that the Hα emission is being heavily extincted in many places by the dense dust-bearing medium. The high-z dust clouds in NGC 891 are a major component of the multiphase halo medium. The mass found in the cold dense, warm neutral, and warm ionized phases are for |z|>0.4 kpc are comparable and of order a fewx10⁸ M_☉. Our discussion of the physics of the multiphase medium at high z relies heavily on the theoretical work of Wolfire et al., and we suggest the dense phase of the thick disk ISM exists to |z|≲2 kpc because the thermal pressures of the warm and hot phases in this region are sufficient for its maintenance. Our Hα observations show evidence for several discrete H II regions at large distances from the midplane (0.6≲|z|≲2.0 kpc). The presence of these H II regions in the thick disk of NGC 891 suggests that ongoing star formation may be present in some of the dense, high-z clouds visible in our images.