Cloudy modeling suggests a diversity of ionization mechanisms for diffuse extraplanar gas

TL;DR

This study models the ionization mechanisms of diffuse extraplanar gas in galaxies, finding that leaking OB star radiation is primary, but evolved stars can significantly contribute in low star-forming galaxies.

Contribution

It introduces a comprehensive photoionization model including both OB stars and evolved stars, applied to a sample of nearby edge-on galaxies using MUSE data.

Findings

Leaking OB star radiation is the main ionization source for eDIG.

In situ evolved stars can significantly ionize extraplanar gas in low star-forming galaxies.

The model successfully fits observed line ratio profiles across the galaxy sample.

Abstract

The ionization of diffuse gas located far above the energetic midplane OB stars poses a challenge to the commonly accepted notion that radiation from OB stars is the primary ionization source for gas in galaxies. We investigated the sources of ionizing radiation, specifically leaking midplane HII regions and/or in situ hot low-mass evolved stars (HOLMES), in extraplanar diffuse ionized gas (eDIG) in a sample of eight nearby (17-52 Mpc) edge-on disk galaxies observed with the Multi Unit Spectroscopic Explorer (MUSE). We constructed a model for the photoionization of eDIG clouds and the propagation of ionizing radiation through the eDIG using subsequent runs of Cloudy photoionization code. Our model includes radiation originating both from midplane OB stars and in situ evolved stars and its dilution and processing as it propagates in the eDIG. We fit the model to the data using the vertical line ratio profiles of our sample galaxies, and find that while the ionization by in situ evolved stars is insignificant for most of the galaxies in our sample, it may be able to explain the enhanced high-ionization lines in the eDIG of the green valley galaxy ESO 544-27. Our results show that while leaking radiation from midplane HII regions is the primary ionization source for eDIG, in situ evolved stars can play a significant part in ionizing extraplanar gas in galaxies with low star forming rates.