  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.
