H-alpha and Free-Free Emission from the WIM
Abstract
Recent observations have found the ratio of H-alpha to free-free radio continuum to be surprisingly high in the diffuse ionized ISM (the so-called WIM), corresponding to an electron temperature of only ~3000K. Such low temperatures were unexpected in gas that was presumed to be photoionized. We consider a 3-component model for the observed diffuse emission, consisting of a mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3) cool H I gas. This model can successfully reproduce the observed intensities of free-free continuum, H-alpha, and collisionally-excited lines such as NII 6583. To reproduce the low observed value of free-free to H-alpha, the PAH abundance in the photoionized regions must be lowered by a factor ~3, and ~20% of the diffuse H-alpha must be reflected from dust grains, as suggested by Wood & Reynolds (1999).
Keywords
Cite
@article{arxiv.1007.0005,
title = {H-alpha and Free-Free Emission from the WIM},
author = {Ruobing Dong and B. T. Draine},
journal= {arXiv preprint arXiv:1007.0005},
year = {2015}
}
Comments
25 pages, 7 figures, 4 tables, single column, details of the calculation and atomic physics added, accepted by ApJ