Context : A 10-arcmin region around the Hubble Deep Field ( North ) contains 92 radio sources brighter than 40 \mu Jy which are well-resolved by MERLIN+VLA at 0 \aas@@fstack { \prime \prime } 2-2″ resolution ( average size \sim 1 ″ ) .
55 of these have Chandra X-ray counterparts in the 2-Ms CDF ( N ) field including at least 17 with a hard X-ray photon index and high luminosity characteristic of a type-II ( obscured ) AGN .
More than 70 % of the radio sources have been classified as starbursts or AGN using radio morphologies , spectral indices and comparisons with optical appearance and rest-frame MIR emission .
On this basis , starbursts outnumber radio AGN 3:1 .

Aims : We investigate the possibility that very luminous radio and X-ray emission originates from different phenomena in the same high-redshift galaxies .

Methods : This study extends the Virtual Observatory ( VO ) methods previously used to identify X-ray-selected obscured type-II AGN , to examine the relationship between radio and X-ray emission .
We describe a VO cut-out server for MERLIN+VLA 1.4-GHz radio images in the HDF ( N ) region .

Results : The high-redshift starbursts have typical sizes of 5–10 kpc and star formation rates of \sim 1000 M _ { \odot } yr ^ { -1 } , an order of magnitude more extended and intense than in the local universe .
There is no obvious correlation between radio and X-ray luminosities nor spectral indices at z \ga 1.3 .
About 70 % of both the radio-selected AGN and the starburst samples were detected by Chandra .
The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts .
Eleven of these are type-II AGN , of which 7 are at z \geq 1.5 .
This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA .
In contrast , all but one of the AGN-dominated radio sources are at z < 1.5 , including the 4 which are also X-ray selected type-II AGN .
The stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness .

Conclusions : Almost all extended radio starbursts at z > 1.3 host X-ray selected obscured AGN .
The radio emission from most of these ultra-luminous objects is dominated by star formation although the highest redshift ( z = 4.424 ) source has a substantial AGN contribution .
Star-formation appears to contribute less than 1/3 of their X-ray luminosity .
Our results support the inferences from SCUBA and IR data , that at z \ga 1.5 , star formation is observably more extended and more copious , it is closely linked to AGN activity and it is triggered differently , compared with star formation at lower redshifts .