Physical Conditions in the Ionized Gas of 30 Doradus
  We present a mid-infrared spectroscopic data cube of the central part of 30
Doradus, observed with Spitzer's IRS and MIPS/SED mode. Aromatic dust emission
features and emission lines from molecular and atomic hydrogen are detected but
not particularly strong. The dominant spectral features are emission lines from
moderately ionized species of argon, neon, and sulphur, which are used to
determine the physical conditions in the ionized gas. The ionized gas
excitation shows strong variations on parsec scales, some of which can
plausibly be associated with individual hot stars. We fit the ionic line
strengths with photoionization and shock models, and find that photoionization
dominates in the region. The ionization parameter U traces the rim of the
central bubble, as well as highlighting isolated sources of ionization, and at
least one quiescent clump. The hardness of the ionizing radiation field T_rad
reveals several "hot spots" that are either the result of individual very hot
stars or trace the propagation of the diffuse ionizing field through the
surrounding neutral cloud. Consistent with other measurements of giant
molecular hydrogen regions, log(U) ranges between -3 and -0.75, and T_rad
between 30000 and 85000K.
