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

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5. Summary and conclusions

Line ratios for the halos of NGC 1963, NGC 3044 and NGC 4634 can be explained as a combination of photoionization by O stars and shock ionization. Since the measured FWHM increases with height above the galactic plane the results obtained from Fig. 17 support our findings that shocks can contribute to the ionization of the extraplanar DIG. The photoionization models from Mathis and Domgörgen & Mathis are not always capable to reproduce all line ratios correctly (high [NII[FORMULA]6583/[FORMULA], [OI[FORMULA]6300/[FORMULA], and [HeI[FORMULA]5876/[FORMULA]). These models were developed to describe the measured line ratios for the Milky Way (Reynolds 1985a,b). However the local environment near the sun is not necessarily representative for the more active regions in our target galaxies. Thus new models are required which should rest upon recently published extragalactic data and consider additional ionization/heating mechanisms such as shocks.

It is important to mention that the ratios for all galaxies in Fig. 16 are not constant over the whole range of z and can vary significantly on small scales ([FORMULA] 250 pc). These results deviate from observations made e.g., by Golla et al. (1996), Rand (1998), Otte & Dettmar (1999), or Haffner et al. (1999). Photoionization models can explain the general shape of [FORMULA] but cannot account for individual changes (e.g., NGC 4402). In addition non-linear changes, as visible in Fig. 16, correlate with density and are explainable by small scale density fluctuations.

If variations in [FORMULA] reflect variations in electron temperature, leading to [FORMULA], these fluctuations could be due to changing metal abundances or varying densities within the DIG. If different slopes in Fig. 15 or Fig. 16 are indicators of different metallicities (Haffner et al. 1999), NGC 4402 would have the same metallicity as the Milky Way or NGC 891. In accordance with Lehnert & Heckman (1996) a correlation between line ratios (ionization) and line widths (gas kinematics) could be established. Our results of a decrease of [OIII ]/H[FORMULA] with increasing [FORMULA] and a significant scatter in [SII ]/[NII ] are different compared to recent measurements in NGC 891. All these aspects indicate that the ionization of the extraplanar DIG varies from galaxy to galaxy and within a galaxy.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100