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Sub-millimetre source identifications and the microjansky source population at 8.4 GHz in the William Herschel Deep Field.

Heywood, I. and Bielby, R.M. and Hill, M.D. and Metcalfe, N. and Rawlings, S. and Shanks, T. and Smirnov, O.M. (2013) 'Sub-millimetre source identifications and the microjansky source population at 8.4 GHz in the William Herschel Deep Field.', Monthly notices of the Royal Astronomical Society., 428 (2). pp. 935-951.


Sub-millimetre observations of the William Herschel Deep Field (WHDF) using the Large Apex Bolometer Camera (LABOCA) revealed possible sub-mm counterparts for two X-ray-absorbed quasars. The primary aim here is to exploit Expanded Very Large Array (EVLA) radio continuum imaging at 8.4 GHz to establish the absorbed quasars as radio/sub-mm sources. The main challenge in reducing the WHDF EVLA data was the presence of a strong 4C source at the field edge. A new calibration algorithm was applied to the data to model and subtract this source. The resulting thermal noise limited radio map covers a sky area which includes the 16 × 16 arcmin2 Extended WHDF. It contains 41 radio sources above the 4σ detection threshold, 17 of which have primary beam corrected flux densities. The radio observations show that the two absorbed active galactic nuclei (AGN) with LABOCA detections are also coincident with radio sources, confirming the tendency for X-ray-absorbed AGN to be sub-mm bright. These two sources also show strong ultraviolet excess (UVX) which suggest that the nuclear sightline is gas absorbed but not dust absorbed. Of the three remaining LABOCA sources within the ≈5 arcmin half-power diameter of the EVLA primary beam, one is identified with a faint nuclear X-ray/radio source in a nearby galaxy, one with a faint radio source and the other is unidentified in any other band. More generally, differential radio source counts calculated from the beam-corrected data are in good agreement with previous observations, showing at S < 50 μJy a significant excess over a pure AGN model. In the full area, of 10 sources fainter than this limit, six have optical counterparts of which three are UVX (i.e. likely quasars) including the two absorbed quasar LABOCA sources. The other faint radio counterparts are not UVX but are only slightly less blue and likely to be star-forming/merging galaxies, predominantly at lower luminosities and redshifts. The four faint, optically unidentified radio sources may be either dust-obscured quasars or galaxies. These high-redshift obscured AGN and lower redshift star-forming populations are thus the main candidates to explain the observed excess in the faint source counts and hence also the excess radio background found previously by the Absolute Radiometer for Cosmology, Astrophysics and Diffuse.

Item Type:Article
Keywords:Techniques, Interferometric, Galaxies, High-redshift, Quasars, Radio continuum.
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Publisher statement:This article has been published in the Monthly Notices of the Royal Astronomical Society © 2013 RAS. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved.
Date accepted:No date available
Date deposited:14 January 2014
Date of first online publication:January 2013
Date first made open access:No date available

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