We investigate the interplay between starbursts and host galaxies by
studying the structure and physical characteristics of the ionized gas
surrounding the central starbursts in the two nearby galaxies NGC5253
and NGC5236. The two systems form a pair which presumably interacted
about 1 Gyr ago. They represent very different galactic
environments, NGC5253 being a metal-poor dwarf, and NGC5236 being a
metal-rich, massive, grand-design spiral. We present images of the
starburst regions in these two galaxies in the light of the line
emission [OIII], H
, and [SII], and in continuum U, V, R.
For NGC5253, the images are deep enough that we can detect faint H
arches and filaments out to ~1.9 kpc and [SII] filaments out
to ~1 kpc from the main ionizing cluster. The ground-based
line images are complemented with an archival HST-WFPC2 H
image. Line ratio maps [OIII]/H and [SII]/H
show that in the outer regions the diffuse ionized gas is partially
excited by a non-photoionization process (`shocks'). The `shocked' gas
is mostly concentrated south-west of the galaxy's center, in
coincidence with the position of H bubbles and with
extended soft X-ray emission. The H emission from the
shock-excited gas is 
1-2% of the total and 10-20%
of the diffuse ionized gas emission, although the mechanical input
from the starburst would be sufficient to support a shocked H
luminosity ~3 times the observed one. About 80-90% of the diffuse gas
is consistent with being photoionized, requiring that about 10% of the
ionizing photons escape from the starburst site. The starburst in
NGC5253 appears to be fed by gas infalling along the galaxy's optical
minor axis, while hot gas expanding from the starburst has a
preferential direction along the major axis.
The results for NGC5236
are less clear than for NGC5253, as the images are not as deep. In the
central region of NGC5236, the H image traces the
U emission from the ionizing stars more closely than in
NGC5253; the emission line ratio maps show very little or no evidence
for presence of shock excitation. Very little or no ionized gas
appears expanding from the center of the galaxy outward along the disk
plane, and ionization is a local process. The starburst in NGC5236 is
thus more strongly confined than that in NGC5253; the deeper
gravitational potential well of the more massive galaxy probably keeps
the ionized gas near to the ionizing
stars.
