Extraplanar Dust and Star Formation in Nearby Edge-On Galaxies

We present high-resolution (∼< 0. 7) ground-based images of the edge-on spiral galaxies NGC 891 (D ∼ 9.5 Mpc) and NGC 4013 (D ∼ 17 Mpc) acquired with the WIYN 3.5-m telescope. These BVI+Hα images reveal complex webs of dusty interstellar material far above the midplanes of both galaxies (0.5 ∼< |z| ∼< 2 kpc) as discussed previously by Howk & Savage (1997). The dusty high-z clouds, visible in absorption against the background stellar light of the galaxies, have widths 50–100 pc and lengths 100–400 pc. An analysis of their absorbing properties suggests they have AV ∼> 0.5 − 2.0. If Milky Way gas-to-dust relationships are appropriate, then these structures have gaseous column densities NH ∼> 10 21 cm, with very large masses (∼> 10 5 − 10 M⊙) and gravitational potential energies (∼> 10 51 − 10 ergs relative to z = 0). The estimated column densities suggest molecular gas may be present, and with the estimated masses allows for the possibility of star formation in these dusty clouds. Recent star formation is the likely cause of the discrete H II regions, in some cases associated with relatively blue continuum sources, observed at heights 0.6 ∼< |z| ∼< 0.8 kpc above the disks of these galaxies. The presence of early-type stars at high-z in these galaxies may be related to the extraplanar dust structures seen in our images.