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 extraplanar diffuse ionized gas 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-meter telescope, the H-alpha
emission in position-velocity space suggests that the eDIG is found in a ring
between galactocentric radii of R_min <= R <= 8 kpc, where R_min >= 2 kpc. We
find that the thermal (sigma_th = 11 km/s) and turbulent (sigma_turb = 25 km/s)
velocity dispersions are insufficient to satisfy the hydrostatic equilibrium
equation given an exponential electron scale height of h_z = 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 R >= 8 kpc if the cosmic rays are
sufficiently coupled to the system (gamma_cr = 1.45). Thus, a stable dynamical
equilibrium model is viable only if the extraplanar diffuse ionized gas is
found in a thin ring around R = 8 kpc, and non-equilibrium models such as a
galactic fountain flow are of interest for further study.
