The Structure and Origins of the Thick Disk Interstellar Medium

Through imaging and spectroscopy we analyze the structure and the origins of the thick disk interstellar medium (ISM) in edge-on, spiral galaxies. Using broadband and narrowband imaging we analyze the dense, dusty material, seen in absorption, and the ionized gas, seen by Hα emission, within the thick disk of NGC 4013 and NGC 4302. Comparing the morphologies of the dust and the ionized gas, we show the thick disk to be an environment with a multi-phase ISM, having a phase structure consistent with that found in the disk. By studying the thick disk ISM with narrowband Hα imagery we identify candidate H II regions in the thick disk. With spectroscopic data on a candidate H II region located in the thick disk of NGC 4013, we confirm an H II region resulting from in situ star formation in the thick disk. The discovery of this thick disk star formation implies that the thick disk ISM has, like the disk, the necessary physical structure for star formation. These studies analyze the thick disk ISM in different ways but both reveal the thick disk to be a dynamic environment, capable of supporting a multi-phase ISM, with the physical condition necessary for star formation.