We investigate the interplay between starbursts and host galaxies by studyi
ng the structure and physical characteristics of the ionized gas surroundin
g the central starbursts in the two nearby galaxies NGC 5253 and NGC 5236.
The two systems form a pair that presumably interacted about 1 Gyr ago. The
y represent very different galactic environments, NGC 5253 being a metal-po
or dwarf and NGC 5236 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 [O III] H alpha, and [S II] and in continuum U, V, and
R.
For NGC 5253, the images are deep enough that we can detect faint H alpha a
rches and filaments out to similar to 1.9 kpc and CS nl filaments out to si
milar to 1 kpc from the main ionizing cluster. The ground-based line images
are complemented with an archival HST Wide Field Planetary Camera 2 H beta
image. Line ratio maps [O III]/H beta and [S II]/H alpha show that in the
outer regions the diffuse ionized gas is partially excited by a nonphotoion
ization process (" shocks "). The "shocked" gas is mostly concentrated sout
hwest of the galaxy's center, in coincidence with the position of H alpha b
ubbles and with extended soft X-ray emission. The H alpha emission from the
shock-excited gas is approximate to 1%-2% of the total and approximate to
10%-20% of the diffuse ionized gas emission, although the mechanical input
from the starburst would be sufficient to support a shocked H alpha luminos
ity similar to 3 times the observed one. About 80%-90% of the diffuse gas i
s consistent with being photoionized, requiring that about 10% of the ioniz
ing photons escape from the starburst site. The starburst in NGC 5253 appea
rs 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 NGC 5236 are less clear than for NGC 5253, as the images ar
e not as deep. In the central region of NGC 5236, the H alpha image traces
the U emission from the ionizing stars more closely than in NGC 5253; the e
mission-line ratio maps show very little or no evidence for presence of sho
ck excitation. Very little or no ionized gas appears expanding from the cen
ter of the galaxy outward along the disk plane, and ionization is a local p
rocess. The starburst in NGC 5236 is thus more strongly confined than that
in NGC 5253; the deeper gravitational-potential well of the more massive ga
laxy probably keeps the ionized gas near to the ionizing stars.