Multiwavelength characterization of faint ultra steep spectrum radio sources: A search for high-redshift radio galaxies ^⋆

¹ Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
e-mail: veeresh.singh@ias.u-psud.fr
² National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganeshkhind, 411007 Pune, India
³ UPMC Univ. Paris 06 and CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
⁴ Department of Physics, University of the Western Cape, Private Bag X17, Bellville 7537, South Africa
⁵ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
⁶ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK
⁸ Department of Physics, Virginia Tech, Blacksburg VA 24061, USA
⁹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA 22903, USA

Received: 14 February 2014
Accepted: 7 May 2014

Abstract

Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts.

Aims. Using our 325 MHz GMRT observations (5σ ~ 800 μJy) and 1.4 GHz VLA observations (5σ ~ 80−100 μJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature.

Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities.

Results. Redshift estimates are available for 86/116 (~74%) USS sources in the VLA-VVDS field and for 39/44 (~87%) USS sources in the SXDF fields with median values (z_median) ~1.18 and ~1.57, respectively, which are higher than estimates for non-USS radio sources (z_{median non−USS} ~ 0.99 and ~0.96), in the two subfields. The MIR color–color diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z> 0.5) being AGN. The flux ratio of radio to mid-IR (S_{1.4 GHz}/S_{3.6 μm}) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ~ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (~26% in the VLA-VVDS and ~13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates.

Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments.