8 research outputs found
High-z radio starbursts host obscured X-ray AGN
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.Comment: 4 pages, 2 figures, To appear in the proceedings of 'At the Edge of
the Universe' (9-13 October 2006, Sintra, Portugal
Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region
A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy,
resolved by MERLIN+VLA at 0".2-2".0 resolution. 55 have Chandra X-ray
counterparts including 18 with a hard X-ray photon index and high luminosity
characteristic of a type-II (obscured) AGN. >70% of the radio sources have been
classified as starbursts or AGN using radio morphologies, spectral indices and
comparisons with optical appearance and MIR emission. Starbursts outnumber
radio AGN 3:1. This study extends the VO methods previously used to identify
X-ray-selected obscured type-II AGN to investigate whether very luminous radio
and X-ray emission originates from different phenomena in the same galaxy. The
high-redshift starbursts have typical sizes of 5--10 kpc and star formation
rates of ~1000 Msun/yr. There is no correlation between radio and X-ray
luminosities nor spectral indices at z>~1.3. ~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, of which 11 are type-II AGN including 7 at z>1.5. This distribution
overlaps closely with the X-ray detected radio sources which were also detected
by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray
sources is correlated with X-ray hardness. Most extended radio starbursts at
z>1.3 host X-ray selected obscured AGN. Radio emission from most of these
ultra-luminous objects is dominated by star formation but it contributes 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 an order of magnitude more
extended and more copious, it is closely linked to AGN activity and it is
triggered differently, compared with star formation at lower redshifts.Comment: 24 pages, 12 figures, uses graphicx, rotating, natbib, supertabular
packages and aa.cls. Accepted for publication in A&