Photographic UBV surface photometry and gross structure of the halo of M 82

We present a surface photometry of M 82 of medium spatial resolution. Special attention was paid to good systematic photometric accuracy. The results are discussed in terms of the distribution of intensity, colour, reddening free parameteer Q and colour excess b -V. The bright elongated body of the galaxy consists of a moderately old stellar population (1 – 3 · 10⁹a) and is only slightly reddned with the exception of the central absorption lane and the southern central region. The chaotic fan-like disturbance south of the center shows the colours of a very young (10⁶ – 10⁷a) highly reddened group of stars; according to polarimetric data from the literature we see their light scattered by dust. The population index Q of the inner halo is bluer than that of the disc everywhere (with the possible exception of the extreme western part). Two sources must be responsible for the illumination of the halo; one of them is responsible for the light from the great disturbances near the minor axis. Both may be located near center but must be partly shadowed. A major contribution of the disc to the illumination is less probable because the population index is different in most places of the halo from that of the disc. In addition to the dusty halo, obscuring dust is probably located to the south of the galaxy. This can be interpreted as part of a thick disc of dust embedding the stellar disc or as beeing connected to a proposed strip of B stars at the southern boundary of the visible disc. Both these interpretations lead to opposite results concerning the direction of the inclination of the disc. The inclination itself is estimated to lie between 80° and 90°.