VLBI observations of the ultracompact radio nucleus of the galaxy M81

VLBI observations of the nuclear region of the nearby spiral galaxy M81 reveal the smallest size yet for its core and that of any other extragalactic nucleus: 0.18 mas x 0.07 mas (700 AU x 300 AU) at 22.2 GHz. Images show no brightness structure outside the core region above the sensitivity limit of similar to 2% of the peak brightness. The core is slightly asymmetric with its brightness falling off along its major axis toward the northeast. There has been no significant change in the size and orientation since the last VLBI observation of this source in 1981, giving a nominal expansion velocity of -60 +/- 60 km s(-1). The size varies with observing frequency, with the length of the major axis being proportional to nu(-0.8). The apparent position angle is also frequency dependent, and changes by 35 degrees between 22.2 and 2.3 GHz, equivalent to such a change on a length scale from 700 to 4000 AU. These observations exclude a starburst or supernova origin of the core emission and instead argue for the core being an active galactic nucleus, perhaps with a bent jet, and with properties lying between those of Sgr A* and the cores of powerful radio galaxies and quasars.