Escape of Lyman alpha Emission in the Starburst Galaxy Mkn 357 - a Wind's Far Side

摘要

HST imaging and slitless spectroscopy are used to examine where the strongLyman alpha emission escapes from the interstellar medium in the starburstgalaxy Mkn 357. An H-alpha image shows that the ionized gas is mostly in aglobal wind, rather than associated with the individual star-forming regionsseen in the optical and UV continuum. The Lyman alpha emission comespredominantly from the northwest side of the wind structure spatially, andshows a significant redshift relative to the optical lines. Both of theseproperties are signatures of seeing the line photons backscattered from the farside of a prolate or bipolar starburst wind, fitting both with escapecalculations and evidence for winds in high-redshift galaxies with net Lymanalpha emission. Scattering is most important within this wind itself, ratherthan involving a surrounding neutral medium, as shown by the decreasingrelative redshift of the line peak from 250 to 30 km/s between the center andedge of the detected emission. The Lyman alpha emission exhibits strongasymmetry in comparison with both the starlight and H-alpha structures. Theseresults add to the evidence that kinematics, rather than gas metallicity ordust content, are the dominant effect in determining which galaxies have strongLyman alpha emission, and that powerful (and perhaps episodic) starbursts arecommon among Lyman-break galaxies as well as those discovered from Lyman alphaline emission.