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

TL;DR

This study examines the origins and ionization mechanisms of extraplanar diffuse ionized gas in five low-mass edge-on galaxies, revealing complex ionization sources and kinematic behaviors that challenge simple models.

Contribution

It provides new insights into the multifaceted ionization sources and kinematic properties of eDIG in low-mass galaxies, highlighting the dominance of OB stars and the complexity of gas origins.

Findings

eDIG scale heights correlate with star formation rates

Ionized gas shows rotation velocity lags above the midplane

Multiple ionization mechanisms, including OB stars, HOLMES, and shocks, are involved

Abstract

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.