We present high-resolution ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 0 % \hbox { $ . ^ { \prime \prime } $ } 7 ) ground-based images of the edge-on spiral galaxies NGC 891 ( D \sim 9.5 Mpc ) and NGC 4013 ( D \sim 17 Mpc ) acquired with the WIYN 3.5-m telescope .
These BVI+H \alpha images reveal complex webs of dusty interstellar material far above the midplanes of both galaxies ( 0.5 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % |z| \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 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 A _ { V } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } % } } 0.5 - 2.0 .
If Milky Way gas-to-dust relationships are appropriate , then these structures have gaseous column densities N _ { H } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { % $ > $ } } } 10 ^ { 21 } cm ^ { -2 } , with very large masses ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 10 ^ % { 5 } -10 ^ { 6 } M _ { \odot } ) and gravitational potential energies ( \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 10 ^ % { 51 } -10 ^ { 52 } 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 \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % |z| \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } % 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 .