A dust component ~2 kpc above the plane in NGC 891

Abstract

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