Physical Models for the 21 cm Emission and Absorption Spectra from the Galactic Interstellar Medium

Abstract

Understanding the Interstellar Medium (ISM) is crucial to understand the structure, formation and dynamics of a galaxy. Interstellar HI provides a great probe of the ISM and high quality data are constantly being produced by the best radio telescopes. Analyzing this data can provide great insights for understanding our galaxy. However, so far, we have been limited in interpreting the high quality data. We here work with some high quality absorption and emission spectra and using a special solution of the radiative transfer equation, we try to fit the spectra jointly with our model. A C-based version of Minuit (a minimization algorithm maintained by scientists at the CERN lab) was used to minimize the chi -square and obtain good fits to the data. With the radiative transfer equation taken into account, the spatial ordering built into the model and a multiparameter fi tting routine with all parameters free, our model is a new approach to dealing with this problem and is likely to be useful for analyzing the data in the future. Using our model, we can use the spectra to describe sight-lines in detail and estimate the distribution of the interstellar HI into three types of components - Cold Neutral Medium (40- 200 K), Warm Neutral Medium (>5000 K) and thermally unstable medium (500-5000 K) - in the direction of the sight-lines. We have described 3 such sight-lines - PKS 0407-658, 3C286 and PKS 1814-637. PKS 0407-658 could be fit with 11 components and is estimated to have 69.9 +/- 6.4% of the NHI in the WNM, 16.7 +/- 1.0% in the CNM and 13.5 +/- 2.1% in the unstable medium. 3C286 could be fit with 6 components and is estimated to have 74 +/- 36% of the NHI in the WNM, 12.9 +/- 2.1% in the CNM and 13 +/- 16% in the unstable medium. PKS 1814-637 could be fi t with 8 components and is estimated to have 26.0 +/- 2.1% of the NHI in the WNM, 21.0 +/- 1.6% in the CNM and 52.9 +/- 2.1% in the unstable medium.