In galaxies like the Milky Way, baryons cycle between the disk, the halo, and the intergalactic environment in ways that have profound consequences for galaxy growth and evolution. The vertical structure, support, and kinematics of gaseous, disk-halo interfaces are thus indicative of the processes driving galaxy growth at low redshift. We use observations of nearby disk galaxies viewed from a range of inclination angles to develop a three-dimensional picture of the kinematics of extraplanar diffuse ionized gas (eDIG) layers. These layers challenge our understanding of the dynamical state of disk-halo interfaces, as their observed exponential electron scale heights, h_z ∼ 1 kpc, exceed their thermal scale heights by factors of a few. For the edge-on galaxies NGC 891 and NGC 5775, we pair optical emission-line spectroscopy with radio continuum observations from Continuum Halos in Nearby Galaxies - an EVLA Survey to constrain the turbulent, magnetic field, and cosmic-ray pressure support at the disk-halo interface. A dynamical equilibrium model is only stably satisfied at large galactocentric radii where the gravitational field is relatively weak (R ≥ 8 kpc and R ≥ 10 kpc for NGC 891 and NGC 5775, respectively), suggesting that bulk flows are present in the warm ionized phase of disk-halo interfaces. We directly detect evidence of such flows in the nearby, low-inclination galaxy M83 by developing a Markov Chain Monte Carlo method to separate planar and extraplanar emission. We find indications of a galactic fountain near star-forming regions, as well as a vertical velocity dispersion ([small sigma]_z ∼ 100 km/s) that greatly exceeds the horizontal dispersions in edge-on galaxies ([small sigma]_y = 40 − 60 km/s). This suggests that a disk-halo circulation as well as anisotropic, random motions play a role in supporting the eDIG layer at its observed scale height. This work favors a non-hydrostatic, disk-halo flow model for eDIG layers in which the gas both originates in and returns to the disk. It also raises new questions about the dependence of eDIG kinematics on star-formation rate and the multiphase nature of disk-halo flows, motivating future work to further illuminate the disk-halo connection in the present-day universe.

All three of these studies suggest that OB stars, both in H II regions and in the field, play a major role in creating and maintaining the DIG, and that other mechanisms, such as shocks, may also contribute to the ionization of the DIG.

Several galaxies show decreasing rotational velocities of hydrogen gas with increasing height above the disk. This is likely due to a combination of outflowing gas from galactic fountains and infalling gas with lower angular momentum from the CGM, IGM or satellite accretion. The degree to which each scenario contributes to a galaxy's extra-planar (EP) gas affects the velocity gradient and has implications for halo formation and evolution. Until recently, it was believed that most of this gas originates from fountain flows, but simulations have shown that as much as 15% of observed EP gas is infalling material. To study the behavior of ionized EP gas, we present optical observations of 13 edge-on galaxies from a multi-slit spectroscopic setup on the ARC 3.5m telescope at the Apache Point Observatory, NM. Our setup allows us to measure velocities of H-alpha-emitting gas as a function of height above the midplane in 11 radical distance bins in a single exposure. Our sample covers a range of star formation rates (L[subscript FIR]/D[2 over 25] of 0.03 - 8.9 X 1040) and includes active and passive galaxies. To aid in the analysis of our spectra, which suffer from projection effects due to our targets' edge-on nature, we developed modeling software for our multi-slit setup. We detect lagging EP gas in four galaxies. NGC 891, NGC 4517, and NGC 4565 have large lags that likely originate from galactic flows and accretion of lower angular momentum material. NGC 4631 has a small lag on its west side that is likely from fountain flows alone. We find evidence of a warped spiral arm on the east side of NGC 4631 and large outflows near the center of the galaxy. Three galaxies (NGC 3628, NGC 4013, NGC 5907) have non-lagging EP gas. Two galaxies (NGC 3044, NGC 3079) show EP gas with complicated kinematics, and we cannot determine if the gas is lagging. We detected no EP gas in three galaxies (UGC 4278, NGC 5229, UGC 7321), and we had one non-detection (NGC 4762). We see no trend between lagging gas and star formation, but the extent of EP gas does appear to be correlated with star formation.