Testing a Dynamical Equilibrium Model of the Extraplanar Diffuse Ionized Gas in NGC 891

The observed scale heights of extraplanar diffuse ionized gas (eDIG) layers exceed their thermal scale heights by a factor of a few in the Milky Way and other nearby edge-on disk galaxies. Here, we test a dynamical equilibrium model of the eDIG layer in NGC 891, where we ask whether the thermal, turbulent, magnetic field, and cosmic-ray pressure gradients are sufficient to support the layer. In optical emission-line spectroscopy from the SparsePak integral field unit on the WIYN 3.5 m telescope, the Hα emission in position–velocity space suggests that the eDIG is found in a ring between galactocentric radii of <?CDATA Rₘᵢₙ R 8\,kpc?> , where <?CDATA Rₘᵢₙ 2\,kpc?> . We find that the thermal ( <?CDATA σ ₜₕ=11?> km s−1) and turbulent ( <?CDATA σ turb=25?> km s−1) velocity dispersions are insufficient to satisfy the hydrostatic equilibrium equation given an exponential electron scale height of <?CDATA hz=1.0\,kpc?> . Using a literature analysis of radio continuum observations from the CHANG-ES survey, we demonstrate that the magnetic field and cosmic-ray pressure gradients are sufficient to stably support the gas at <?CDATA R 8?> kpc if the cosmic rays are sufficiently coupled to the system ( <?CDATA γ cr=1.45?> ). Thus, a stable dynamical equilibrium model is viable only if the eDIG is found in a thin ring around R = 8 kpc, and nonequilibrium models such as a galactic fountain flow are of interest for further study.