The Main Sources of Gas Ionization in Several Nearby Star-Forming Galaxies

TL;DR

This study uses emission-line ratios and gas velocity dispersion to identify ionization sources in the interstellar medium of nearby star-forming galaxies, improving classification accuracy in mixed ionization cases.

Contribution

It introduces the use of the 'BPT-sigma' relation and sigma-rho correlation to distinguish ionization mechanisms in galactic ISM, combining observational data from different instruments.

Findings

Shock excitation correlates with sigma and rho in diffuse ionized gas.

H II regions with low turbulence lack sigma-rho correlation.

The method effectively characterizes ionization sources in multiple galaxies.

Abstract

The emission-line intensity ratios are used to distinguish the main sources of gas ionization to study the state of galactic interstellar medium (ISM). In intermediate cases, when the contributions of radiation from some sources mix, the identification becomes uncertain. As an extra parameter, the gas velocity dispersion in the line-of-sight can be added to classical diagnostic diagrams (i.e., "BPT-sigma" relations) to help finding an appropriate solution. The minimum distance from the curve that bounds the H II-type ionization region for each point in BPT-sigma diagram can be used to characterize the excitation mechanism of the ionized gas. The shock excitation in the diffuse ionized gas (DIG) can be realized by the correlation between sigma and rho, while the H II regions with low level turbulent motions can be characterized by the absence of this correlation. We consider the "BPT-sigma" relation and the correlation between sigma and rho to determine the ionized gas excitation in several nearby star-forming galaxies. Distributions of the velocity dispersion are obtained from the scanning Fabry-Perot interferometer observations at the SAO RAS 6-m telescope, whereas the emission-line ratios are calculated from the archival long-slit spectroscopic data. The results of this study are reported for Mrk 370, NGC 4068, UGC 8313, and UGC 8508.