Modeling the Diffuse Ionized Gas with Cloudy

Abstract

The power source of the Diffuse Ionized Gas (DIG)/Warm Ionized Medium (WIM) is still a matter of debate. Energy estimations favor photoionization by OB stars but photoionization models had so far difficulties in reproducing the rise in the [NII]/Hα and [SII]/Hα line ratios observed in the DIG with increasing distance z from the midplane. We are investigating this question by modeling the observed line ratios with the photoionization code CLOUDY (version 94.00; Ferland 2000) using three different stellar input spectra, the LTE model Atlas of Kurucz (1991) and the Non–LTE models CoStar from Schaerer et al. (1996), and WMbasic from Pauldrach et al. (2001). Observations of the Perseus Arm of the Milky Way are complemented and enlarged with a sample of 8 edge–on galaxies for which more emission lines were analyzed. In modeling the data we took care of the difference in the sight– line integration for the Perseus Arm compared to the other external galaxies. Our detailed analysis of the first 2 kpc of the DIG shows that photoionization models can indeed account for the increase in the line ratios [NII]/Hα and [SII]/Hαwith z. Depending on the object the absolute values can be reproduced or are being underestimated. The oxygen lines [OIII] and [OI] give serious constrains for the models. In order to increase the validity of the models the importance of other heating sources have to be examined. Furthermore we present clear evidence that the [SII]/[NII] ratio cannot be explained by merely metalicity effects as was so far assumed.