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W.C. Keel (U. Ala.)
Strong Lyman \alpha emission from star-forming galaxies, especially as seen at high redshifts, poses an interesting puzzle. Optical depth effects, particularly resonant trapping, can destroy these photons very effectively in the interstellar medium, yet some galaxies have strong net emission in this line. Previous observations have suggested a strong role for the detailed density and velocity structure of the ISM, and that Lyman \alpha photons may escape preferentially from the diffuse ISM rather than individual (giant) H II regions. HST STIS and WFPC2 observations of the starburst system Mkn 357 at z=0.0532 support the latter notion. This is a clumpy, interacting starburst system with morphological evidence for a global wind, rather than the paired spiral suggested by ground-based images. A STIS slitless spectrum traces Lyman \alpha emission over a 3-kpc region, which can be compared with the Balmer emission using a WFPC2 LRF image at H\alpha and the locations of ionizing stars via a UV STIS image. The emission equivalent width of Lyman \alpha increases by a factor 20 (2--40 Å) from the core to the outermost regions, and individual star-forming regions do not show Lyman \alpha counterparts, so that it is indeed the diffuse (possible outflowing) gas from which we see the line emission. The emission is everywhere systematically redshifted (by 150-300 km/s) with respect to the H\alpha velocity, consistent with radiative transfer in a net outflow. The net line width increases with distance from the core, as the peak redshift decreases, also indicating a change in the effects of radiative transfer. Thus, strong Lyman \alpha emission in high-redshift galaxies may trace the state of the ISM as well as metallicity or dust content; a diffuse ionized-gas component favors strong Lyman \alpha, particularly when there is a large-scale outflow. This work was supported by NASA through STScI grant GO-8565.01-A.