2005ApJ...632..277E

The detailed observations of the diffuse ionized gas through the emission lines Hα, [N II], and [S II] in the Perseus arm of our Galaxy by the Wisconsin Hα Mapper (WHAM) survey challenge photoionization models. They have to explain the observed rise in the line ratios [N II]/Hα and [S II]/Hα. The models described here are the first to consider the detailed observational geometry toward the Perseus arm. The models address the vertical variation of the line ratios up to a 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 in the temperature of the gas layer. This is due to the progressive hardening of the radiation going through the gas. However, extra heating in addition to photoionization is needed to explain the absolute values. Two different extra-heating rates are investigated, which are proportional to n⁰ and n¹. The models show that a combination of both is best to explain the data for which the extra heating independent of density is dominant for z>0.8 kpc.