Hubble Space Telescope Imaging of Extraplanar Dust Structures in the Edge-On Spiral NGC 4217^*

We present high-resolution (∼ 01) B, V, and I images of the edge-on Sb galaxy NGC 4217 obtained with the Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). We make use of these images to study extraplanar (z > 0.4 kpc) dust structures visible in absorption against the background stellar light of the galaxy, similar to those observed previously in NGC 891 and other spiral galaxies by Howk & Savage. The HST images of NGC 4217 reveal extensive high-z dust to both sides of its midplane. The dust structures have a range of morphologies, and some of the more tenuous features are traceable to heights ∼2 kpc from the midplane. We investigate the physical properties of 12 individual features. We measure the apparent optical extinctions of the features and use these to infer the hydrogen column densities and masses of the structures. The extraplanar dust structures in NGC 4217 typically have N_H ≳ 4 × 10²⁰ cm^-2, with associated gas masses estimated to be ≳2 × 10⁵ M_⊙, assuming Milky Way gas-to-dust relationships apply to this spiral galaxy. The energy requirements for the expulsion of such massive structures into the lower halo are large; the gravitational potential energies of similar individual structures in NGC 891 (>10⁵² ergs) represent the energy input of at least tens of supernovae. The morphologies of some of the extraplanar dust structures in NGC 4217 suggest supernova-driven galactic fountain or chimney phenomena may be responsible for their production. Other features at high-z are not readily linked to energetic processes in the disk.