1996A&A...313..439G

We present long-slit optical spectroscopy and a Hα map of the highly inclined interacting spiral galaxy NGC4631. This is the second object for which kinematics of the extraplanar diffuse ionized gas (eDIG) and variations of emission line ratios with position are investigated in an edge-on galaxy. We compare the measured line ratios with photoionization models. Most of the eDIG detected at approx. 1kpc above the disk is corotating within about 10km/s, with the exception of a small extraplanar feature south-east of the nucleus. We find significant deviations of 70km/s from normal differential rotation of the disk in the south-eastern disturbed part of NGC4631. All the peculiar velocities are consistent with the gravitational interaction NGC4631 is undergoing. [SII] and [NII] to Hα line intensity ratios are significantly higher in the halo than in the disk. Line ratios in the halo vary between 0.2 and 0.7 for both transitions. They can in principle be explained by photoionization in a diluted stellar radiation field. However, the very high ratios require excitation parameters much lower than for the eDIG in our Galaxy. As an inconsistency between the photoionization models and our observations, the observed [SII]/[NII] ratio variation from the disk to the halo is not as strong as the models predict.