Context. The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas.
Aims. Here we report observations of the kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to determine the rotation curve of the halo gas.
Methods. We have obtained a Fabry-Perot data cube in H alpha to measure the kinematics of the halo gas with angular resolution much higher than obtained from HI 21 cm observations. The data cube was obtained with the TAURUS II spectrograph at the WHT on La Palma. The velocity information of the diffuse ionized gas extracted from the data cube is compared to model distributions to constrain the distribution of the gas and in particular the halo rotation curve.
Results. The best fit model has a central attenuation tau H alpha = 6, a dust scale length of 8.1 kpc, an ionized gas scale length of 5.0 kpc. Above the plane the rotation curve lags with a vertical gradient of -18.8 kms(-1) kpc(-1). We find that the scale length of the H alpha must be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation curve above the plane rises less steeply than in the plane. This is all in agreement with the velocities measured in the HI.