The First Deep ACS Lyman alpha Images of Local Starburst Galaxies

We report the first results from a deep Ly α imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) of 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 Ly α morphology, with sometimes strong offsets between the emission of Ly α and the location of stellar light, ionized gas traced by H α , and the neutral gas. Overall, more Ly α photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Ly α 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 due to destruction of Ly α photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Ly α escape regulator. If the properties of the two observed galaxies are representative for starburst galaxies in general, Ly α will be difficult to interpret as a star-formation indicator, in particular if based on Ly α imaging at low spatial resolution.