An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253

TL;DR

This paper reports the first detection of an ionization cone in the starburst galaxy NGC 5253, indicating potential pathways for ionizing photon escape and implications for galaxy observations.

Contribution

It provides the first observational evidence of an ionization cone in NGC 5253, linking galaxy morphology to ionizing photon escape mechanisms.

Findings

Detection of a narrow ionization cone covering 3% of 4pi steradians.

The cone is optically thin, suggesting ionizing photon escape.

Kinematic evidence of outflow activity along the cone.

Abstract

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.