Spatially Resolved Temperature and Density Structures of Nearby H ii Regions

Abstract

Photoionization models frequently assume constant temperature or density within H ii regions. We investigate this assumption by measuring the detailed temperature and density structures of four H ii regions in the Large Magellanic Cloud and the Small Magellanic Cloud, using integral-field spectroscopic data from the Wide-Field Spectrograph on the ANU 2.3 m telescope. We analyze the distribution of emission lines of low-ionization, intermediate-ionization, and high-ionization species. We present the complex electron temperature and density structures within H ii 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 constant interstellar medium (ISM) pressure and varying temperature and density distributions. A comparison of the line ratios between our H ii 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 the LMC and SMC is between log( P / k ) = 6 and 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 H ii regions.

Keywords

• H II regions