The Main Sources of Gas Ionization in Several Nearby Star-Forming 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 intermedi- ate cases, when the contributions of radiation from some sources mix, the identication 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-" relations) to help nding an appropriate solution. The minimum distance from the curve that bounds the H II-type ionization region for each point in BPT- diagram can be used to characterize the excitation mechanism of the ionized gas. The shock excitation in the diuse ionized gas (DIG) can be realized by the correlation between and , while the H II regions with low level turbulent motions can be characterized by the absence of this correlation. We consider the "BPT-" relation and the correlation between and 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 obser- vations 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.