A Search for Extraplanar Dust in Nearby Edge-on Spirals

Abstract

We present high-resolution (06 to ∼ 10) BV images of 12 edge-on spiral galaxies observed with the WIYN 3.5 m telescope. These images were obtained to search for extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those previously found in NGC 891 (Howk & Savage). Many of these galaxies have been previously searched for diffuse ionized gas at high z. Our imaged galaxies include a sample of seven massive L*-like spiral galaxies within D ≲ 25 Mpc that have inclinations i ≳ 87° from the plane of the sky. We find that five of these seven systems show extraplanar dust, visible as highly structured absorbing clouds against the background stellar light of the galaxies. These dust structures lie at heights |z| ≳ 0.4 kpc, which should be above most of the thin disk molecular material in these galaxies. The more prominent structures are estimated to have associated gas masses ≳105 M⊙; the implied potential energies are ≳1052 ergs. All of the galaxies in our sample that show detectable Hα emission at large z also show extraplanar dust structures. None of those galaxies for which extraplanar Hα searches were negative show evidence for extensive high-z dust. The existence of extraplanar dust is a common property of massive spiral galaxies. We discuss several mechanisms for shaping the observed dust features. We emphasize in this discussion the possibility that these dusty clouds represent the dense phase of a multiphase medium at high z in spiral galaxies. In a few cases interactions with close galaxy companions could be responsible for the high-z dust, either through dynamical stripping or triggered star formation. We can rule out warps as the source of the observed high-z dust. Flaring gas layers seem an unlikely source of the observed material but cannot be ruled out at this time, except for those features that clearly connect to energetic processes in the disk. The correlation between high-z dust and extraplanar Hα emission may simply suggest that both trace the high-z interstellar medium in its various forms (or phases), the existence of which may ultimately be driven by vigorous star formation in the underlying disk. The absorption produced by high-z dust and associated gas in spiral galaxies must be accounted for when studying extraplanar emission from spiral galaxies over much of the electromagnetic spectrum.