Multiwavelength characterization of faint Ultra Steep Spectrum radio
  sources : A search for high-redshift radio galaxies
  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. Using our 325 MHz GMRT observations (5-sigma ~ 800 microJy) and 1.4
GHz VLA observations (5-sigma ~ 80 - 100 microJy) available in two subfields
(viz., 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. The optical, 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. 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, which are higher than that for
non-USS radio sources (z_median non-USS ~ 0.99 and ~ 0.96), in the two
subfields, respectively. The flux ratio of radio to mid-IR (S_1.4 GHz/S_3.6
micron) 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 thus, can be considered as
potential HzRG candidates.
