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

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.

Deep GMRT 610 MHz observations of the ELAIS N1 field: catalogue and source counts

“…Studies have shown USS are good candidates for high redshift radio galaxies (Riseley et al 2016) which are among the most luminous and massive galaxies (e.g. De Breuck & Reuland 2005;Bornancini et al 2007;Singh et al 2014) and are believed to be progenitors of the massive elliptical galaxies in the local Universe. Their extremely steep spectrum is generally attributed to radiation losses of relativistic electrons in the radio lobes, meaning they are most luminous at lower radio frequencies (Mahony et al 2016).…”

Infrared-faint radio sources are at high redshifts

“…Moreover, the extremely high radio-to-IR flux density ratios of IFRS overlap with those of this HzRG sample. Recently, Singh et al (2014) searched for HzRGs in faint ultra steep spectrum (USS) radio sources and found several sources overlapping with the selection criteria of IFRS.…”

The nature of faint radio galaxies at high redshifts

“…A successful method of identification of HzRGs from allsky radio surveys is to first create samples of radio sources that display an ultra-steep spectral index (α, where S ∝ ν α ) and then follow these up at optical and infrared wavelengths (Tielens et al 1979;Röttgering et al 1994;Chambers et al 1996;Blundell et al 1999;De Breuck et al 2000a;Ishwara-Chandra et al 2010;Afonso et al 2011;Singh et al 2014). This method is effective owing to the apparent correlation that exists between the redshift of a radio source and the steepness of its low-frequency radio spectrum (see Ker et al 2012, for example).…”