The starburst in the central kiloparsec of Markarian 231

We present VLBA observations at 0.33 and 0.61 GHz, and VLA observations between 5 and 22 GHz, of subkiloparsec-scale radio emission from Mrk 231. In addition to jet components clearly associated with the active galzctic nucleus, we also find a smooth extended component of size 10-1000 pc most probably related to the purported massive star-forming disk in Mrk 231. The diffuse radio emission from the disk is found to have a steep spectrum at high frequencies, characteristic of optically thin synchrotron emission. The required relativistic particle density in the disk can be produced by a star formation rate of 220 M-circle dot, yr(-1) in the central kiloparsec. At low frequencies the disk is absorbed, most likely by ionized gas with an emission measure of 8 x 10(5) pc cm(-6). We have also identified four candidate radio supernovae that, if confirmed, represent direct evidence for ongoing star formation in the central kiloparsec.