| Abstract: | Published low-resolution measurements of colour and polarisation over the face of M82 are discussed to separate the contribution of starlight and scattered light. We show that in all places of the middle and outer halo the scattered light comes predominantly from a central source of very high ultraviolett excess, the contribution of the disc is negligble there. The projected distributions of Hα-light and scattered continuum are of considerable similarity. Major extinction occurs in the southern half of the main body and of the inner halo; the northern half of the bright body, and the northern halo, are free of extinction, excluding some regions near the minor axis. The light of the central source is reddened only before it is scattered in the halo. The variation of the true degree of polarisation (after correction for starlight) is interpreted in terms of the variation of the mean scattering angle. From this, conclusions can be drawn concerning the location of the dust and the geometry of the illumination. The high brightness of the scattered light near the minor axis is caused axis is caused by a bright illuminating beam there, strengthened in some places by comparatively low scattering angles (45°) and a higher (projected) density of the scattering material. The stellar populations seen in M82 are different in the northern and in the southern halfs of the galaxy. The main body and the region of the northern “halo” consist of an old population of normal metal content (pop. I); the colours of the southern parts – which are partly considerably influenced by extinction – can be due to either metal poor F-stars (pop. II) or to young B-stars. To solve the latter ambiguity and at the same time the question in what direction the plane of the galaxy is tilted, good spectra of the faint southern parts of M82 outside the minor axis are needed. |
