Context : Ultra Steep Spectrum ( USS ) radio sources are one of the efficient tracers of powerful High- z Radio Galaxies ( H z RGs ) .
In contrast to searches for powerful H z RGs from radio surveys of moderate depths , fainter USS samples derived from deeper radio surveys can be useful in finding H z RGs 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 \sim 800 \mu Jy ) and 1.4 GHz VLA observations ( 5 \sigma \sim 80 - 100 \mu Jy ) 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 .

Methods : 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 .

Results : Redshift estimates are available for 86/116 ( \sim 74 \% ) USS sources in the VLA-VVDS field and for 39/44 ( \sim 87 \% ) USS sources in the SXDF fields with median values ( z _ { median } ) \sim 1.18 and \sim 1.57 , which are higher than that for non-USS radio sources ( z _ { median~ { } non - USS } \sim 0.99 and \sim 0.96 ) , in the two subfields , respectively .
The MIR color-color diagnostic and radio luminosities are consistent with a majority 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 { \sim } 0.5 to 3.8 are likely to be hosted in obscured environments .
A significant fraction ( \sim 26 \% in the VLA-VVDS and \sim 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 H z RGs , and thus , can be considered as potential H z RG 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 H z RG candidates , our faint USS sample also contain population of weaker radio - loud AGNs potentially hosted in obscured environments .