IONIZATION STRUCTURE IN THE 30 DORADUS NEBULA AS SEEN WITH HUBBLE-SPACE-TELESCOPE WIDE-FIELD PLANETARY CAMERA-2

Using the Hubble Space Telescope (HST) and Wide Field Planetary Camera 2, we have imaged the central 20 pc of the giant H II region 30 Dorad us Nebula in three different emission lines. The images allow us to st udy the nebula with a physical resolution that is within a factor of 2 of that of typical ground-based observations of Galactic H II regions . We present a gallery of interesting objects within the region studie d. These include a tube blown by the wind of a high-velocity star and a discrete H II region around an isolated B star. This small isolated H II region appears to be in the midst of the champagne flow phase of its evolution. Most of the emission within 30 Dor is confined to a thi n zone located between the hot interior of the nebula and surrounding dense molecular material. This zone appears to be directly analogous t o the photoionized photoevaporative flows that dominate emission from small, nearby H II regions. For example, a column of material protrudi ng from the cavity wall to the south of the main cluster is found to b e a direct analog to elephant trunks in M16. Surface brightness profil es across this structure are very similar to surface brightness profil es taken at ground-based resolution across the head of the largest col umn in M16. The dynamical effects of the photoevaporative flow can be seen as well. An arcuate feature located above this column and a simil ar feature surrounding a second nearby column are interpreted as shock s in which the photoevaporative flow stagnates against the high-temper ature gas that fills the majority of the nebula. The ram pressure in t he photoevaporative flow, derived from thermal pressure at the surface of the column, is found to balance with the pressure in the interior of the nebula derived from previous X-ray observations. By analogy wit h the comparison of ground-based and HST images of M16, we infer that the same sharply stratified structure seen in HST images of M16 almost certainly underlies the observed structure in 30 Doradus, which is a crucial case because it allows us to bridge the gap between nearby H I I regions and the giant H II regions seen in distant galaxies. The rea l significance of this result is that it demonstrates that the physica l understanding gained from detailed study of photoevaporative interfa ces in nearby H II regions can be applied directly to interpretation o f giant H II regions. Stated another way, interpretation of observatio ns of giant H II regions must account for the fact that this emission arises not from expansive volumes of ionized gas but instead from high ly localized and extremely sharply stratified physical structures.