Ionized Gas Characteristics in the Cavities of the Gas and Dust Disc of
  the Spiral Galaxy NGC 6946
  The parameters of the ionized gas in NGC 6946 (in the [NII]6548,6583, H-alpha
and [SII]6717,6731 lines) are investigated with the SAO RAS BTA telescope along
three positions of the long slit of the SCORPIO focal reducer, passing through
a number of large and small cavities of the gaseous disc of the galaxy. Most of
these cavities correspond exactly to the cavities in warm dust. We found that
everywhere in the direction of NGC 6946 the lines of ionized gas are decomposed
into two Gaussians, one of which shows almost constant [SII]/H-alpha and
[NII]/H-alpha ratios, as well as an almost constant radial velocity within the
measurement errors (about -35 - -50 km/s). This component is in fact the
foreground radiation from the diffuse ionized gas of our Galaxy; a similar
component is also present in the emission of neutral hydrogen. The analysis of
the component of ionized gas, occurring in NGC 6946, has revealed that it shows
signs of shock excitation in the cavities of the gaseous disc of the galaxy. It
is as well typical for the extraplanar diffuse ionized gas (EDIG), observed in
a number of spiral galaxies at their high Z-coordinates. This can most likely
be explained by low density of the gas in the NGC 6946 disc (with the usual
photoionization) inside the cavities, due to what we see the spectral features
of the EDIG gas of NGC 6946, projected onto them. In the absence of separation
of ionized gas into two components by radial velocities, there is an increasing
contribution to the integral line parameters by the EDIG of our Galaxy when the
gas density in NGC 6946 decreases, which explains some strange results,
obtained in the previous studies. The morphology of warm dust, visible in the
infrared range and HI is almost the same. The shock excitation of the ionized
gas is detected also in the smallest holes, well distinguishable only in the IR
images.
