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Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Hα emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. Aims. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in Hα emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. Methods. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. Results. From the 24 µm emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., ∼30%. The additional asymmetry in Hα is modeled as additional symmetric dust attenuation which extends up to ∼40 (1.9 kpc) above the plane of the galaxy with a mid-plane value of τ = 0.8 and a scale height of 0.5 kpc

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-Pérot data cube in Hα 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 τ Hα = 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 km s −1 kpc −1 . We find that the scale length of the Hα 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.

A technique is introduced for deriving the gaseous rotation curves of edge-on spiral galaxies. The entire major axis position-velocity (XV) diagram is modelled with a set of rings in a leastsquares sense, allowing for the effects of beam-smearing and line-of-sight projection. The feasibility of the technique is demonstrated by applying it to good quality H I XV diagrams of eight edge-on spirals. For seven additional spirals the XV diagrams are of insufficient quality, and the H I rotational velocities derived earlier using the envelope-tracing method are retained. The H I results are augmented with the optical emission line (H II) kinematics to arrive at estimates of the full rotation curves. A detailed comparison of the H I and H II kinematics shows that the discs in our sample are sufficiently transparent at the heights above the plane where we have taken our optical spectra to derive the stellar kinematics. In several of these spirals the H II is mainly confined to the spiral arms and does not extend out to the edge of the H I layer, which may have caused the H II velocity profiles to be significantly narrower than those of H I.

This is the first paper of a series in which we will attempt to put constraints on the flattening of dark halos in disk galaxies. We observe for this purpose the HI in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. In this paper, we define a sample of 8 HI rich late-type galaxies suitable for this purpose and present the HI observations.