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Astron. Astrophys. 362, 119-132 (2000)

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1. Introduction

Over the last decade a widespread and extended diffuse component of ionized hydrogen contributing 25 - 50 % to the total H[FORMULA] flux has been found well outside of classical HII regions in several face-on and edge-on galaxies (e.g., Lehnert & Heckman 1995, Rand 1996, Greenawalt et al. 1997, or Dettmar 1998). Although there is significant evidence that the presence of the extraplanar diffuse ionized gas (DIG, sometimes also called warm ionized medium, WIM) is correlated with ongoing star formation in the disk (e.g., Dettmar 1992), important aspects of its ionization and excitation are not well understood. While of all conventional ionizing sources only UV radiation of massive stars provides sufficient power to keep these thick gas layers ionized (including the Reynolds-layer of the Milky Way (Reynolds 1993)) the resulting photoionization models have to explain the long mean free paths for the UV photons and the observed emission line ratios. However, the relatively strong emission observed in the [FORMULA] lines (Dettmar & Schulz 1992, Golla et al. 1996, Rand 1997) cause a problem for pure photoionization models and an additional heating contribution, e.g. from the kinetic energy provided by supernova, is likely (Dahlem et al. 1997). An even more puzzling observation was recently presented for the DIG in NGC 891 which is up to now the best studied galaxy in this regard (Rand 1998). In complete contradiction to photoionization models here [OIII ]/H[FORMULA] is rising with distance above the galactic plane for [FORMULA] 1 kpc. Diagnostic diagrams for emission lines from DIG in outflows of dwarf galaxies (Martin 1997) also demonstrate the need for ionization and/or excitation processes beyond pure photoionization. In this paper we report results from spectroscopic observations of a small sample of edge-on galaxies. These new spectra cover a larger wavelengths range extending earlier work to the blue range of the optical spectrum. This allows us to discuss the results in the framework of diagnostic diagrams. The paper is structured as follows: In Sect. 2- 3 some essential information concerning observations and data reduction strategies, the determination of line ratios, and the use of diagnostic diagrams is given. Sect. 4 shows representative results which are discussed briefly for each galaxy with respect to the photoionization models by Mathis (1986, hereafter Ma86) and Domgörgen & Mathis (1994, hereafter DM94). Finally, our findings are summarized in Sect. 5 with special emphasis to the trend of [FORMULA], and the proposed dependence of line ratio variations with changing halo metallicities.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100