The multifarious ionization sources and disturbed kinematics of extraplanar gas in five low-mass galaxies

We investigate the origin of the extraplanar diffuse ionized gas (eDIG) and its predominant ionization mechanisms in five nearby (17-46 Mpc) low-mass ($10^9\text{-}10^{10}$ $M_{\odot}$) edge-on disk galaxies: ESO 157-49, ESO 469-15, ESO 544-27, IC 217, and IC 1553. We acquired Multi Unit Spectroscopic Explorer (MUSE) integral field spectroscopy and deep narrowband H$\alpha$ imaging of our sample galaxies. To investigate the connection between in-plane star formation and eDIG, we perform a photometric analysis of our narrowband H$\alpha$ imaging. We measure eDIG scale heights of $h_{z\text{eDIG}} = 0.59 \text{-} 1.39$ kpc and find a positive correlation between them and specific star formation rates. In all galaxies, we also find a strong correlation between extraplanar and midplane radial H$\alpha$ profiles. Using our MUSE data, we investigate the origin of eDIG via kinematics. We find ionized gas rotation velocity lags above the midplane with values between 10 and 27 km s$^{-1}$ kpc$^{-1}$. While we do find hints of an accretion origin for the ionized gas in ESO 157-49, IC 217, and IC 1553, overall the ionized gas kinematics of our galaxies do not match a steady galaxy model or any simplistic model of accretion or internal origin for the gas. We also construct standard diagnostic diagrams and emission-line maps (EW(H$\alpha$), [NII]/H$\alpha$, [SII]//H$\alpha$, [OIII]/H$\beta$) and find regions consistent with mixed OB star and hot low-mass evolved stars (HOLMES) ionization, and mixed OB-shock ionization. Our results suggest that OB stars are the primary driver of eDIG ionization, while both HOLMES and shocks may locally contribute to the ionization of eDIG to a significant degree. Despite our galaxies' similar structures and masses, we find a surprisingly composite image of ionization mechanisms and a multifarious origin for the eDIG.
Comment: 21 pages, 14 figures, accepted to Astronomy & Astrophysics