  We use Virtual Observatory methods to investigate the association between
radio and X-ray emission at high redshifts. Fifty-five of the 92 HDF(N) sources
resolved by combining
  MERLIN+VLA data were detected by Chandra, of which 18 are hard enough and
bright enough to be obscured AGN. The high-z population of microJy radio
sources is dominated by starbursts an order of magnitude more active and more
extended than any found at z<1 and at least a quarter of these simultaneously
host highly X-ray-luminous obscured AGN.
