Spatially-Resolved Temperature and Density Structures of Nearby HII regions

Abstract

Photoionization models frequently assume constant temperature or density within HII regions. We investigate this assumption by measuring the detailed temperature and density structures of four HII regions in the Large Magellanic Cloud and the Small Magellanic Cloud, using integral-field spectroscopic data from the Wide-Field Spectrograph on the ANU 2.3m telescope. We analyse the distribution of emission-lines of low-ionization species, intermediate-ionization species and high-ionization species. We present the complex electron temperature and density structures within HII regions. All four nebulae present a negative gradient in the electron density profile. Both positive and negative temperature gradients are observed in the nebulae. We create a series of nebula models with a constant ISM pressure and varying temperature and density distributions. Comparison of the line ratios between our HII regions and models suggests that none of the simple nebula models can reproduce the observed temperature and density structures. Comparison between the models and the data suggests that the ISM pressure of nebulae in LMC and SMC is between log(P/k)=6-7.5. Complex internal structures of the nebulae highlight the importance of future Monte-Carlo photoionization codes for accurate nebula modeling, which include a comprehensive consideration of arbitrary geometries of HII regions.