Ionization Structure in the 30 Doradus Nebula as seen with HST/WFPC-2

Abstract

Using the Hubble Space Telescope and WFPC2 we have imaged the central 20pc of the giant H II region 30 Doradus nebula in three different emission lines. The images allow us to study the nebula with a physical resolution that is within a factor of two of that typical of ground based observations of Galactic H II regions. Most of the emission within 30 Dor is confined to a thin zone located between the hot interior of the nebula and surrounding dense molecular material. This zone appears to be directly analogous to the photoionized photoevaporative flows that dominate emission from small, nearby H II regions. The dynamical effects of the photoevaporative flow can be seen. The ram pressure in the photoevaporative flow, derived from thermal pressure at the surface of the ionization front, is found to balance with the pressure in the interior of the nebula derived from previous x-ray observations. By analogy with the comparison of ground 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 Dor. 30 Doradus is a crucial case because it allows us to bridge the gap between nearby H II regions and the giant H II regions seen in distant galaxies. The real significance of this result is that it demonstrates that the physical understanding gained from detailed study of photoevaporative interfaces in nearby H II regions can be applied directly to interpretation of giant H II regions.