The First Deep Advanced Camera for Surveys Lyα Images of Local Starburst Galaxies

We report the first results from a deep Lyα imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have luminosities similar to those of the Magellanic Clouds but differ in their chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar, whereas SBS 0335-052 is known to have one of the lowest abundances among blue galaxies (~1/30). The ACS imaging reveals a complex Lyα morphology, with sometimes strong offsets between the emission of Lyα and the location of stellar light, ionized gas traced by Hα, and the neutral gas. Overall, more Lyα photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lyα emission over all observed knots, whatever their dust content or/and color indices, contradicts model expectations of a lower escape fraction from dust-rich gas because of destruction of Lyα photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lyα escape regulator. If the properties of the two observed galaxies are representative of starburst galaxies in general, Lyα will be difficult to interpret as a star formation indicator, in particular if based on Lyα imaging at low spatial resolution.