A Search for Extraplanar Dust in Nearby Edge-on Spirals

We present high resolution BV images of 12 edge-on spiral galaxies observedwith the WIYN 3.5-m telescope. These images were obtained to search forextraplanar (|z| > 0.4 kpc) absorbing dust structures similar to thosepreviously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include asample of seven massive L_*-like spiral galaxies within D<25 Mpc that haveinclinations i > 87 deg from the plane of the sky. We find that five of theseseven systems show extraplanar dust, visible as highly-structured absorbingclouds against the background stellar light of the galaxies. The more prominentstructures are estimated to have associated gas masses >10^5 M_sun; the impliedpotential energies are > 10^(52) ergs. All of the galaxies in our sample thatshow detectable halpha emission at large z also show extraplanar duststructures. None of those galaxies for which extraplanar halpha searches werenegative show evidence for extensive high-z dust. The existence of extraplanardust is a common property of massive spiral galaxies. We discuss severalmechanisms for shaping the observed dust features, emphasizing the possibilitythat these dusty clouds represent the dense phase of a multiphase medium athigh-z in spiral galaxies. The correlation between high-z dust and extraplanarHalpha emission may simply suggest that both trace the high-z interstellarmedium in its various forms (or phases), the existence of which may ultimatelybe driven by vigorous star formation in the underlying disk. (Abstractabridged)