  We investigate the relative significance of radiation pressure and gas
pressure in the extended narrow line regions (ENLRs) of four Seyfert galaxies
from the integral field Siding Spring Southern Seyfert Spectroscopic Snapshot
Survey (S7). We demonstrate that there exist two distinct types of
starburst-AGN mixing curves on standard emission line diagnostic diagrams which
reflect the balance between gas pressure and radiation pressure in the ENLR. In
two of the galaxies the ENLR is radiation pressure dominated throughout and the
ionization parameter remains constant (log U ~ 0). In the other two galaxies
radiation pressure is initially important, but gas pressure becomes dominant as
the ionization parameter in the ENLR decreases from log U ~ 0 to -3.4 <= log U
<= -3.2. Where radiation pressure is dominant, the AGN regulates the density of
the interstellar medium on kpc scales and may therefore have a direct impact on
star formation activity and/or the incidence of outflows in the host galaxy to
scales far beyond the zone of influence of the black hole. We find that both
radiation pressure dominated and gas pressure dominated ENLRs are dynamically
active with evidence for outflows, indicating that radiation pressure may be an
important source of AGN feedback even when it is not dominant over the entire
ENLR.
