Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region^*

¹ Jodrell Bank Observatory, University of Manchester, SK11 9DL, Macclesfield, UK e-mail: amsr@jb.man.ac.uk
² Centre de Données astronomiques de Strasbourg (UMR 7550), 67000 Strasbourg, France
³ Mullard Space Science Laboratory, UCL, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁴ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
⁶ European Southern Observatory, 85748 Garching bei München, Germany
⁷ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁸ Department of Physics, Talledega College, Talledega, Alabama 35160, USA
⁹ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 3 April 2007
Accepted: 21 June 2007

Abstract

Context.A 10-arcmin region around the Hubble Deep Field (North) contains 92 radio sources brighter than 40 μJy which are well-resolved by MERLIN+VLA at 02-2´´resolution (average size ~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 ~1000  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 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 ≥ 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 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.