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Ultra Steep Spectrum (USS) radio sources are one of the efficient tracers ofpowerful High-z Radio Galaxies (HzRGs). In contrast to searches for powerfulHzRGs from radio surveys of moderate depths, fainter USS samples derived fromdeeper radio surveys can be useful in finding HzRGs at even higher redshiftsand in unveiling a population of obscured weaker radio-loud AGN at moderateredshifts. Using our 325 MHz GMRT observations (5-sigma ~ 800 microJy) and 1.4GHz 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 sourcesand characterize their nature. The optical, IR counterparts of our USS samplesources are searched using existing deep surveys, at respective wavelengths. Weattempt to unveil the nature of our faint USS sources using diagnostictechniques based on mid-IR colors, flux ratios of radio to mid-IR, and radioluminosities. Redshift estimates are available for 86/116 (~ 74%) USS sourcesin the VLA-VVDS field and for 39/44 (~ 87%) USS sources in the SXDF fields withmedian values (z_median) ~ 1.18 and ~ 1.57, which are higher than that fornon-USS radio sources (z_median non-USS ~ 0.99 and ~ 0.96), in the twosubfields, respectively. The flux ratio of radio to mid-IR (S_1.4 GHz/S_3.6micron) versus redshift diagnostic plot suggests that more than half of our USSsample sources distributed over z ~ 0.5 to 3.8 are likely to be hosted inobscured environments. A significant fraction (~ 26% in the VLA-VVDS and ~ 13%in the SXDF) of our USS sources without redshift estimates mostly remainunidentified in the existing optical, IR surveys, and exhibit high radio tomid-IR flux ratio limits similar to HzRGs, and thus, can be considered aspotential HzRG candidates.

Multiwavelength characterization of faint Ultra Steep Spectrum radio sources : A search for high-redshift radio galaxies