Extraplanar dust in the edge-on spiral NGC 891

We present high-resolution (0.6 ''-0.65 '') optical broad-band images of the edge-on Sb galaxy NGC 891 obtained with the WIYN 3.5-m telescope. These BVR images reveal a complex network of hundreds of dust absorbing structures far from the mid-plane of the galaxy. The dust structures have a wide range of morphologies and are clearly visible to \z\ less than or equal to 1.5 kpc from the mid-plane. In this paper we discuss the general characteristics of the population of absorbing structures, as well as physical properties of 12 individual features. These 12 structures are characterized by N-H greater than or similar to 10(21) cm(-2), with masses estimated to be more than 2 X 10(5)-5 x 10(6) M., assuming Galactic gas-to-dust relationships. The gravitational potential energies of the individual dust structures, given their observed heights and derived masses, lie in the range of 20-200x10(51) ergs, which represents the energy input of at least tens to hundreds of supernovae. Rough number counts of extraplanar dust features find greater than or similar to 120 structures at \z\>400 pc with apparent B-band extinction greater than or similar to 0.25 mag. If these have similar properties to those structures studied in detail, the mass of high-z gas associated with extraplanar dust in NGC 891 likely exceeds 2 x 10(8) M., which is similar to 2% of the total neutral ISM mass of the galaxy, The absorbing properties of extraplanar dust structures in NGC 891 are best fit with R-V=A(V)/E(B - V)= 3.6+/-0.4. A comparison of the new WIYN images with an archival Hubble Space Telescope image of the central region of NGC 891 suggests that the quantities we measure for the extraplanar dust features are not seriously affected by atmospheric blurring in the WIYN images. Some of the dust features seen in NGC 891 suggest supernova-driven galactic fountain or chimney phenomena are responsible for their production and are clearly associated with ionized gas structures thought to be tracing the violent disk-halo interface of this galaxy, However, other structures are not so readily associated with these energetic processes. We discuss several mechanisms which may produce high-z dust structures such as those seen in the WIYN and HST images presented here. It is, however, less than clear which of these mechanisms are primarily responsible for the extensive extraplanar dust structures seen in our images. The data presented are part of a larger program to search for and characterize off-plane dust structures in edge-on systems. (C) 1997 The American Astronomical Society. [S0004-6256(97)02812-4].