An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253
  There are few observational constraints on how the escape of ionizing photons
from starburst galaxies depends on galactic parameters. Here, we report on the
first major detection of an ionization cone in NGC 5253, a nearby starburst
galaxy. This high-excitation feature is identified by mapping the emission-line
ratios in the galaxy using [S III] lambda 9069, [S II] lambda 6716, and H_alpha
narrow-band images from the Maryland-Magellan Tunable Filter at Las Campanas
Observatory. The ionization cone appears optically thin, which is suggestive of
the escape of ionizing photons. The cone morphology is narrow with an estimated
solid angle covering just 3% of 4pi steradians, and the young, massive clusters
of the nuclear starburst can easily generate the radiation required to ionize
the cone. Although less likely, we cannot rule out the possibility of an
obscured AGN source. An echelle spectrum along the minor axis shows complex
kinematics that are consistent with outflow activity. The narrow morphology of
the ionization cone supports the scenario that an orientation bias contributes
to the difficulty in detecting Lyman continuum emission from starbursts and
Lyman break galaxies.
