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

¹ Space Physics and Astronomy research unit, University of Oulu, 90014 Oulu, Finland
e-mail: riku.rautio93@gmail.com
² Centre of Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK
³ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Department of Physics, Engineering Physics and Astrophysics, Queen’s University, Kingston, ON K7L 3N6, Canada
⁶ Finnish Centre of Astronomy with ESO (FINCA), Vesilinnantie 5, 20014 University of Turku, Finland
⁷ Specim, Spectral Imaging Ltd., Elektroniikkatie 13, 90590 Oulu, Finland

Received: 14 October 2021
Accepted: 13 December 2021

Abstract

Aims. We investigate the origin of the extraplanar diffuse ionized gas (eDIG) and its predominant ionization mechanisms in five nearby (17–46 Mpc) low-mass (10⁹–10¹⁰ M_⊙) edge-on disk galaxies: ESO 157-49, ESO 469-15, ESO 544-27, IC 217, and IC 1553.

Methods. We acquired Multi Unit Spectroscopic Explorer (MUSE) integral field spectroscopy and deep narrowband Hα imaging of our sample galaxies. To investigate the connection between in-plane star formation and eDIG, we measure the star formation rates (SFRs) and perform a photometric analysis of our narrowband Hα imaging. Using our MUSE data, we investigate the origin of eDIG via kinematics, specifically the rotation velocity lags. We also construct standard diagnostic diagrams and emission-line maps (EW(Hα), [N II]/Hα, [S II]//Hα, [O III]/Hβ) and search for regions consistent with ionization by hot low-mass evolved stars (HOLMES) and shocks.

Results. We measure eDIG scale heights of h_zeDIG = 0.59−1.39 kpc and find a positive correlation between them and specific SFRs. In all galaxies, we also find a strong correlation between extraplanar and midplane radial Hα profiles. These correlations along with diagnostic diagrams suggest that OB stars are the primary driver of eDIG ionization. However, we find regions consistent with mixed OB–HOLMES and OB–shock ionization in all galaxies and conclude that both HOLMES and shocks may locally contribute to the ionization of eDIG to a significant degree. From Hα kinematics, we find 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.

Conclusions. Despite our galaxies’ similar structures and masses, our results support a surprisingly composite image of ionization mechanisms and a multifarious origin for the eDIG. Given this diversity, a complete understanding of eDIG will require larger samples and composite models that take many different ionization and formation mechanisms into account.