Spectroscopy of diffuse ionized gas in halos of selected edge-on galaxies

TL;DR

This study investigates the excitation mechanisms of diffuse ionized gas in the halos of edge-on galaxies using optical spectroscopy, revealing the need for additional ionization sources beyond photoionization.

Contribution

First detailed spectroscopic analysis of halo diffuse ionized gas in multiple edge-on galaxies, highlighting the limitations of photoionization models and suggesting shock ionization as an additional mechanism.

Findings

Line ratios in halos differ from disk regions.

Photoionization models fit disk gas but not halo gas.

Shock ionization may be needed to explain halo emission.

Abstract

In order to examine the excitation and ionization mechanism of extraplanar diffuse ionized gas (DIG) we have obtained optical longslit spectra of seven edge-on spiral galaxies. In four objects the brightest emission lines can be traced out to distances of typically 1.5 kpc above the disk. For NGC1963 and NGC3044 line ratios such as [NII]6583/Ha or [SII]6717/Ha as well as [OIII]5007/Hb could be measured for the halo DIG. This allows us to discuss the DIG in the halo of these objects in the framework of diagnostic diagrams. For these two objects, the line ratios of [OIII]5007/Hb decrease with increasing z, different from the recently reported trend in NGC891 (Rand 1998). We find that emission lines from the DIG in the disks are in good agreement with photoionization models using a dilute radiation field. However, with increasing z these models fail to predict the measured [OI]6300/Ha and HeI5876/Ha line ratios for NGC1963. Diagnostic diagrams reveal for NGC1963 the need for a second ionization mechanism of the halo DIG (besides photoionization). This additional source could be shock ionization. The same diagrams demonstrate an intermediate classification for NGC3044. Plots of [SII]/[NII] vs. emission measure reveal significant changes towards the halo and seem to trace local small scale density fluctuations of the extraplanar DIG.