Physical conditions in the nucleus of M 81.

Abstract

IUE and visual observations of the emission lines in the nucleus of M8 1 are presented. The permitted lines show broad and narrow features qualitatively similar to those in Seyfert galaxies. The observed luminosity for the broad component is L(Hβ) = 7.4 × 10³⁷ ergs s^-1, which is about two orders of magnitude smaller than in the faintest Seyfert galaxy reported to date. The narrow lines are produced in an extended region and are perhaps ionized by shock wave heating; the N/O abundance ratio is higher than solar by a factor of 3. The FW0I of the broad components of Mg II and Hα lines are 10,200 km s^-1 and 5300 km s^-1, respectively. If the broadening is due to circular motions, it is found that there is a central object, possibly a black hole, with 1 × 10⁶ < M_co/M_sun < 1.8 × 10⁷ and that the broad lines originate at a radius 1.5 × 10¹⁵ < r < 4 × 10¹⁶ cm. The broad component of Hα is redshifted with respect to the narrow component; the possibility of a gravitational origin for the redshift is discussed.