VLBI OBSERVATIONS OF THE ULTRACOMPACT RADIO NUCLEUS OF THE GALAXY M81

VLBI observations of the nuclear region of the nearby spiral galaxy M8 1 reveal the smallest size yet for its core and that of any other extr agalactic 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 a xis 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 ax is 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 G Hz, equivalent to such a change on a length scale from 700 to 4000 AU. These observations exclude a starburst or supernova origin of the cor e emission and instead argue for the core being an active galactic nuc leus, perhaps with a bent jet, and with properties lying between those of Sgr A and the cores of powerful radio galaxies and quasars.