With the goal of investigating the nature and the environment of the faint radio sources ( at { mJy } level ) , here are presented results of X-ray identifications of Faint Imaging Radio Survey at Twenty centimetres ( FIRST ) in the 9 square degrees Boötes field of the NOAO Deep Wide Field Survey ( NDWFS ) , using data from the Chandra XBoötes survey .
A total of 92 ( 10 \% ) FIRST radio sources are identified above the X-ray flux limit f _ { X } ( 0.5 - 7 ) ~ { } { keV } = 8 \times 10 ^ { -15 } ~ { } { erg~ { } s ^ { -1 } ~ { } cm ^ { -2 } } , and 79 optical counterparts are common to both the radio and X-ray sources .
Spectroscopic identifications were available for 22 sources ( 27 \% ) .
Multi-wavelength optical/infrared photometric data ( Bw \sim 25.5 mag , R \sim 25.8 mag , I \sim 25.5 mag and K \sim 19.4 mag ) were available for this field and were used to derive photometric redshift for the remaining 57 sources without spectroscopic information .
Most of the radio-X-ray matches are optically extended objects in the R band with a photometric redshift distribution peaking at z \sim 0.7 .
Based on the hardness ratio and X-ray luminosity , 37 sources ( 89 % ) were classified as AGN-1 , 19 as AGN-2 , 12 as QSO-1 , 2 as QSO-2 and 9 sources as normal galaxies .
While the majority of these sources have a hard X-ray luminosity L _ { X } ( 2 - 7 ) keV > 10 ^ { 42 } erg s ^ { -1 } , about one third of the sources have L _ { X } ( 2 - 7 ) keV > 10 ^ { 44 } erg s ^ { -1 } and therefore classified as QSO-1 .
The majority ( 68 % ) of the radio-X-ray matched population are found to have \log f _ { X } / f _ { opt } within 0.0 \pm 1 region indicative of AGNs , 23 \% with high X-ray-to-optical flux ratio ( \log f _ { X } / f _ { opt } > 1 ) , suggesting high redshift and/or dust obscured AGN , and 11 \% of the radio-X-ray matches that are X-ray faint optically bright sources with \log f _ { X } / f _ { opt } < -1 , and most of these sources are optically extended .
These objects are low- z , normal galaxies or low luminosity AGNs ( LINERS ) .