The Relationship Between the Dense Neutral and Diffuse Ionized Gas in
  the Thick Disks of Two Edge-On Spiral Galaxies
  We present high-resolution, optical images (BVI + Halpha) of the multiphase
interstellar medium (ISM) in the thick disks of the edge-on spiral galaxies NGC
4013 and NGC 4302. Our images from the Hubble Space Telescope, Large Binocular
Telescope, and WIYN 3.5-m reveal an extensive population of filamentary dust
absorption seen to z ~ 2-2.5 kpc. Many of these dusty thick disk structures
have characteristics reminiscent of molecular clouds found in the Milky Way
disk. Our Halpha images show the extraplanar diffuse ionized gas (DIG) in these
galaxies is dominated by a smooth, diffuse component. The strongly-filamentary
morphologies of the dust absorption have no counterpart in the smoothly
distributed Halpha emission. We argue the thick disk DIG and dust-bearing
filaments trace physically distinct phases of the thick disk ISM, the latter
tracing a dense, warm or cold neutral medium. The dense, dusty matter in the
thick disks of spiral galaxies is largely tracing matter ejected from the thin
disk via energetic feedback from massive stars. The high densities of the gas
may be a result of converging gas flows. This dense material fuels some thick
disk star formation, as evidenced by the presence of thick disk H II regions.
