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

V. Singh*, A. Beelen, Y. Wadadekar, S. Sirothia, C. H. Ishwara-Chandra, A. Basu, A. Omont, K. McAlpine, R. J. Ivison, S. Oliver, D. Farrah, M. Lacy

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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 (5 sigma similar to 800 mu Jy) and 1.4 GHz VLA observations (5 sigma similar to 80-100 mu Jy) 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 (similar to 74%) USS sources in the VLA-VVDS field and for 39/44 (similar to 87%) USS sources in the SXDF fields with median values (z(median)) similar to 1.18 and similar to 1.57, respectively, which are higher than estimates for non-USS radio sources (z(median) non-USS similar to 0.99 and similar to 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 (mu m)) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z similar to 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (similar to 26% in the VLA-VVDS and similar to 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.

Original languageEnglish
Article number52
Number of pages19
JournalAstronomy & Astrophysics
Volume569
DOIs
Publication statusPublished - Sep 2014

Keywords

  • galaxies: nuclei
  • galaxies: active
  • radio continuum: galaxies
  • galaxies: high-redshift
  • galaxies: general
  • galaxies: evolution
  • ACTIVE GALACTIC NUCLEI
  • ULTRALUMINOUS INFRARED GALAXIES
  • SCALE STRUCTURE SURVEY
  • NEWTON-DEEP-FIELD
  • EIS-NVSS SURVEY
  • Z-ALPHA CORRELATION
  • XMM-LSS SURVEY
  • K-Z RELATION
  • SKY SURVEY
  • ENERGY-DISTRIBUTIONS

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