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Published online by Cambridge University Press: 04 August 2017
New observations of the radio nucleus of the nearby bright spiral galaxy M81 (NGC 3031) show that the structure of the nucleus is considerably more complex than previously thought. The radio nucleus has a slightly inverted, variable spectrum (de Bruyn et al. 1976; Crane, Guffrida, and Carlson 1976). The variability and VLBI observations (Kellermann et al. 1976; Jones, Sramek, and Terzian 1981) both indicate a linear dimension of ~1500 AU. The recent VLBI observations of Bartel et al. (1982) determined that nearly 100% of the emission originates in an elongated region with linear dimensions of 1000–4000 AU. Peimbert and Torres-Peimbert (1981) have classified the optical nucleus of M81 as Seyfert type 1.5 (the weakest known) with the narrow-emission-line region extending over ~5″ (Münch 1959).