Numerical Models for the Diffuse Ionized Gas in Galaxies. I. Synthetic spectra of thermally excited gas with turbulent magnetic reconnection as energy source

TL;DR

This study investigates whether turbulent magnetic reconnection can supply sufficient energy to ionize the diffuse ionized gas in galaxies, using detailed non-LTE radiative transfer simulations informed by MHD models, revealing the necessity of photoionization.

Contribution

It demonstrates that magnetic reconnection alone can energize the gas but must be combined with photoionization to match observed spectra, advancing understanding of DIG ionization mechanisms.

Findings

Magnetic reconnection can provide enough energy for ionization.

Purely thermally excited spectra do not match observations.

Photoionization remains essential for accurate spectral modeling.

Abstract

Aims: The aim of this work is to verify whether turbulent magnetic reconnection can provide the additional energy input required to explain the up to now only poorly understood ionization mechanism of the diffuse ionized gas (DIG) in galaxies and its observed emission line spectra. Methods: We use a detailed non-LTE radiative transfer code that does not make use of the usual restrictive gaseous nebula approximations to compute synthetic spectra for gas at low densities. Excitation of the gas is via an additional heating term in the energy balance as well as by photoionization. Numerical values for this heating term are derived from three-dimensional resistive magnetohydrodynamic two-fluid plasma--neutral-gas simulations to compute energy dissipation rates for the DIG under typical conditions. Results: Our simulations show that magnetic reconnection can liberate enough energy to by itself fully or partially ionize the gas. However, synthetic spectra from purely thermally excited gas are incompatible with the observed spectra; a photoionization source must additionally be present to establish the correct (observed) ionization balance in the gas.