Journal Article

X-ray identifications of FIRST radio sources in the XBoötes field

K. El Bouchefry

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 4, pages 2011-2037
Published in print July 2009 | ISSN: 0035-8711
Published online July 2009 | e-ISSN: 1365-2966 | DOI:
X-ray identifications of FIRST radio sources in the XBoötes field

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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 deg2 Boötes field of the National Optical Astronomy Observatory (NOAO) Deep Wide Field Survey (NDWFS), using data from the Chandra XBoötes survey. A total of 92 (10 per cent) FIRST radio sources are identified above the X-ray flux limit fX(0.5–7) keV = 8 × 10−15 erg s−1 cm−2, and 79 optical counterparts are common to both the radio and X-ray sources. Spectroscopic identifications [obtained from the Sloan Digital Sky Survey (SDSS) survey] were available for 22 sources (27 per cent). The majority of these sources (59 per cent) are classified as broad line active galactic nuclei (BLAGNs), and 18 per cent as low-ionization nuclear emission-line regions (LINERs), 14 per cent as star-forming galaxies and one source classified as BL Lac object. Multiwavelength optical/infrared photometric data (Bw∼ 25.5 mag, R∼ 25.8 mag, I∼ 25.5 mag and K∼ 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 red colour, their radio emission is associated with AGN activity hosted in massive early type host galaxies with a photometric redshift distribution peaking at z∼ 0.7. Based on the hardness ratio and X-ray luminosity, 37 sources (89 per cent) were classified as AGN-1, 19 as AGN-2, 12 as quasi-stellar object 1 (QSO-1), two as QSO-2 and nine sources as normal galaxies. While the majority of these sources have a hard X-ray luminosity LX(2–7) keV > 1042 erg s−1, about one third of the sources have LX(2–7) keV > 1044 erg s−1 and therefore classified as QSO-1, 92 per cent of these objects are spectroscopically identified as QSOs. I found good agreement between the X-ray classification scheme and the classical AGN classification based on optical spectroscopic diagnostics. The majority (68 per cent) of the radio–X-ray matched population are found to have log fX/fopt within 0.0 ± 1 region indicative of AGNs (Stocke et al.), 23 per cent with high X-ray to optical flux ratio (log fX/fopt > 1), suggesting high redshift and/or dust obscured AGN, and 11 per cent of the radio–X-ray matches that are X-ray faint optically bright sources with log fX/fopt < −1, and most of these sources are optically extended. These objects are low z, normal galaxies or low-luminosity AGNs (LINERs). Five sources of the radio–X-ray matches are associated with the so called extremely red object (ERO) having a colour of RK > 5. No clear correlation has been found between the RK colour and the hardness ratio. The very hard radio–X-ray matches (ignoring QSOs) all have low RK colour which could be a consequence of the correlation found between the hardness ratio and redshift, and the fact that low RK galaxies are mostly at low redshift.

Keywords: galaxies: active; galaxies: high-redshift; X-rays: diffuse background; X-rays: galaxies

Journal Article.  11952 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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