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

TL;DR

This study tests whether magnetic fields and cosmic rays can support the extraplanar diffuse ionized gas in NGC 891 in a stable equilibrium, finding support only at certain radii and suggesting non-equilibrium models for other regions.

Contribution

It demonstrates that magnetic and cosmic ray pressures can support the eDIG layer in NGC 891 at specific radii, providing a dynamical equilibrium model constrained by observations.

Findings

Thermal and turbulent pressures are insufficient for hydrostatic support.

Magnetic and cosmic ray pressures can support the gas at R >= 8 kpc.

eDIG is likely confined to a ring around R = 8 kpc.

Abstract

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.