The Multiphase Halo of NGC 891: WIYN H-alpha and BVI Imaging

Abstract

We present new, deep optical images (BVI+H-alpha) of the interstellar medium (ISM) far above the plane of NGC 891. These sub-arcsecond images give a direct visual view of two physically distinct ``phases'' of the thick interstellar disk of this galaxy. A dense phase of the thick disk ISM is observed in our BVI images as highly-structured dust-bearing clouds viewed against the stellar light of the galaxy. These structures are traceable to heights |z|=2 kpc from the midplane. Very few highly-structured dust features are present at |z|>2 kpc. The more prominent dust structures have gas masses in excess of 10^5 solar masses, each having visual extinctions well in excess of unity. A warm ionized phase of the high-z ISM is observed through its well-studied H-alpha emission. Our images of the well-studied diffuse ionized medium, to date the highest-resolution observations of this phase of the ISM in NGC 891, show it is relatively smoothly distributed with some filamentary structure superposed on this smooth background. There is little correspondence between the H-alpha emitting material and the absorbing dust structures. These two phases of the multiphase high-z ISM are physically distinct. The H-alpha emission is being heavily extincted in many places by the dense dust-bearing medium. Our H-alpha observations show evidence for several discrete H II regions at large distances from the midplane (to |z|=2 kpc). The presence of these H II regions in the thick disk of NGC 891 suggests that on-going star formation may be present in some of the dense, high-z clouds visible in our images. (Abstract Abridged)