Constraining the extra heating of the Diffuse Ionized Gas in the Milky Way

Abstract

The detailed observations of the diffuse ionized gas through the emission lines H$α$, [NII], and [SII] in the Perseus Arm of our Galaxy by the Wisconsin H$α$ Mapper (WHAM)--survey challenge photoionization models. They have to explain the observed rise in the line ratios [NII]/H$α$ and [SII]/H$α$. The models described here consider for the first time the detailed observational geometry toward the Perseus Arm. The models address the vertical variation of the line ratios up to height of 2 kpc above the midplane. The rising trends of the line ratios are matched. The increase in the line ratios is reflected in a rise of the temperature of the gas layer. This is due to the progressive hardening of the radiation going through the gas. However an extra heating above photoionization is needed to explain the absolute values. Two different extra heating rates are investigated which are proportional to $n^0$ and $n^1$. The models show that a combination of both are best to explain the data, where the extra heating independent of density is dominant for z $>$ 0.8 kpc.