AAS 203rd Meeting, January 2004
Session 111 Interstellar Medium II
Poster, Thursday, January 8, 2004, 9:20am-4:00pm, Grand Hall

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[111.05] The Diffuse Ionized Gas as an Indicator for the Galactic Chemical Evolution

T. Elwert, R.-J. Dettmar, R. Teullmann (Astronomisches Institut Ruhr-Universität Bochum, Germany)

We present clear evidence that the line ratios [SII]/H\alpha and [NII]/H\alpha, observed in the Diffuse Ionized Gas (DIG) at certain vertical |z| heights above the midplane (up to 2 kpc), can be used to trace the chemical evolution of a galaxy.\\ Assuming that the rate of depletion onto grains is similar for sulphur and nitrogen, we conclude that the different slopes in the [SII]/H\alpha--[NII]/H\alpha--diagram are due to the different production mechanisms of these elements. Sulphur is expelled through supernova type II explosions whereas nitrogen is produced in low mass stars and emitted into the ambient medium by planetary nebulae and stellar winds. The different timescales for these distinct mechanisms cause that the ambient medium is enriched by nitrogen more slowly than sulphur. Therefore DIG--layers which show higher [SII]/H\alpha ratios than [NII]/H\alpha are younger. Hence the recent star formation activity of a galaxy is traced by these forbidden emission lines observed in the DIG.\\ We are modeling the according line ratios with photoionization models using CLOUDY (version 94.00). Previously it was assumed that the slopes are due to different global metalicities of the galaxies, the CLOUDY models rule this clearly out. Additionally we model the |z|--dependence of the forbidden emission lines of the DIG.\\ We test our models successfully applying it to 4 edge--on galaxies (NGC 3044, IC 2153, NGC 4634, NGC 5775). It is apparent that the slopes in one galaxy can differ regarding the slit position. This is expected as the local star formation rate can vary locally over the whole galaxy disk.\\ The slopes in the [SII]/H\alpha--[NII]/H\alpha--diagram give therefore contraints regarding the chemical evolution of a galaxy.

The author(s) of this abstract have provided an email address for comments about the abstract: elwert@astro.rub.de

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