SCUBA-selected AGN - from the sub-mm to X-rays. Liz Puchnarewicz, Mullard Space Science Laboratory, University College, London Walter Gear, University of Cardiff Steve Eales, University of Cardiff.
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SCUBA-selected AGN - from the sub-mm to X-rays
Liz Puchnarewicz, Mullard Space Science Laboratory, University College, London
Walter Gear, University of Cardiff
Steve Eales, University of Cardiff
Merged (MOS1+MOS2+pn) image of the CFRS3h field in the 0.3 to 10keV band. Plotted as circles are the positions of the SCUBA sources. There are 83 X-ray detections – none are coincident with SCUBA AGN.
The deep Chandra surveys (Brandt et al 2001) have provided evidence thatthe missing population of heavily absorbed AGN does exist. Thesesurveys have resolved virtually all of the X-ray background,finding that at the faintest fluxes there are a large numberof sources with the highly-obscured spectra needed to producethe hard X-ray background. But are these very faint, hard X-ray-emitting AGN also SCUBA sources? Are the X-ray and sub-mm/IR backgrounds generated by the same population of objects? We present the initial results from our programme of deep X-ray surveys of well-studied SCUBA fields, with multiwavelength data from the radio to X-rays, to address this question.
SCUBA image of the CFRS 3-hour field (03h02m38 6, 00°07 40: J2000.0). Becauseof our observing procedure, a real object appears asa positive source (white) with two negative sources (black) of halfthe amplitude 30 arcsec on either side.
Would Chandra have been able to find SCUBA-like sources anyway?
Three objects in the merged EPIC image are coincident with ISO sources. The ISO sources are all "hot" i.e. strong in the mid-IR, rather than in the far-IR, and hence evidence for an AGN rather than star-formation. Sub-mm emission is dominated by the cool dust associated with star-formation and any AGN contribution is relatively weak. This would explain why some X-ray sources can be seen by ISO but not with SCUBA.
Deep Chandra observations have failed to detect 90% of the SCUBA sources, implying, at first sight,that the populations producing the submillimetre and X-raybackgrounds are not the same (Barger et al 2001, Almaini et al. 2001). However, a simple calculation shows that this does not fullyanswer the question of whether the X-ray and submillimetrebackgrounds are produced by similar objects. Existing observations arealmost certainly not deep enough.
BeppoSAX andXMM-Newton observations of the ultraluminous infrared galaxy NGC 6240 have shown that it hasa hard Compton-thick X-ray spectrum and that its spectral energydistribution is remarkably similar to the extragalactic background asa whole [below, left]. We have calculated how the 5-10 keV flux of anobject like NGC6240 depends on redshift [below right]. The emissionof the Compton-thick component is vanishingly smallat lower energies and the observable flux falls off very quickly with redshift. Even with a 50ks XMM-Newtonobservation, a galaxy like NGC 6240 would be detectedout to a redshift of only ~1, but the median redshiftof SCUBA sources is ~ 2-3 (Lilly et al 1999).
Hard sources in the CFRS3h field
A long Chandra observationwould not detect an object like this even at modest redshifts thus Chandra surveys remain inconclusive.
There are few hard X-ray (2-10keV) bright sources in the CFRS3h field [above left] and fewer with high hardness ratios (2-10/0.3-2 keV; above right) – and none of these are SCUBA sources. If a population of Compton-thick AGN is responsible for producing the sub-mm and the X-ray backgrounds, then we might expect to see SCUBA sources preferentially coincident with hard X-ray objects. Much deeper XMM-Newton observations are essential however, if we are to reach the faint X-ray fluxes we expect [see right].
Almaini, O. et al. 2001, MNRAS, submitted (astro-ph 0108400).
Barger, A.J. et al., 2001, ApJL submitted (astro-ph 0107252)
Brandt, W.N. et al. 2001, AJ, in press (astro-ph 0108404)
Lilly et al. 1999, ApJ, 518, 641.