Ultra Steep Spectrum radio sources in the Lockman Hole: SERVS identifications and redshift distribution at the faintest radio fluxes

The Astrophysical Journal (Impact Factor: 6.73). 08/2011; 743. DOI: 10.1088/0004-637X/743/2/122
Source: arXiv

ABSTRACT Ultra Steep Spectrum (USS) radio sources have been successfully used to
select powerful radio sources at high redshifts (z>~2). Typically restricted to
large-sky surveys and relatively bright radio flux densities, it has gradually
become possible to extend the USS search to sub-mJy levels, thanks to the
recent appearance of sensitive low-frequency radio facilities. Here a first
detailed analysis of the nature of the faintest USS sources is presented. By
using Giant Metrewave Radio Telescope and Very Large Array radio observations
of the Lockman Hole at 610 MHz and 1.4 GHz, a sample of 58 USS sources, with
610 MHz integrated fluxes above 100 microJy, is assembled. Deep infrared data
at 3.6 and 4.5 micron from the Spitzer Extragalactic Representative Volume
Survey (SERVS) is used to reliably identify counterparts for 48 (83%) of these
sources, showing an average total magnitude of [3.6](AB)=19.8 mag.
Spectroscopic redshifts for 14 USS sources, together with photometric redshift
estimates, improved by the use of the deep SERVS data, for a further 19
objects, show redshifts ranging from z=0.1 to z=2.8, peaking at z~0.6 and
tailing off at high redshifts. The remaining 25 USS sources, with no redshift
estimate, include the faintest [3.6] magnitudes, with 10 sources undetected at
3.6 and 4.5 micron (typically [3.6]>22-23 mag, from local measurements), which
suggests the likely existence of higher redshifts among the sub-mJy USS
population. The comparison with the Square Kilometre Array Design Studies
Simulated Skies models indicate that Fanaroff-Riley type I radio sources and
radio-quiet Active Galactic Nuclei may constitute the bulk of the faintest USS
population, and raises the possibility that the high efficiency of the USS
technique for the selection of high redshift sources remains even at the
sub-mJy level.

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    ABSTRACT: We present broadband photometry and photometric redshifts for 187,611 sources located in ~0.5 deg2 in the Lockman Hole area. The catalog includes 388 X-ray-detected sources identified with the very deep XMM-Newton observations available for an area of 0.2 deg2. The source detection was performed on the R c-, z'-, and B-band images and the available photometry is spanning from the far-ultraviolet to the mid-infrared, reaching in the best-case scenario 21 bands. Astrometry corrections and photometric cross-calibrations over the entire data set allowed the computation of accurate photometric redshifts. Special treatment is undertaken for the X-ray sources, the majority of which are active galactic nuclei (AGNs). For normal galaxies, comparing the photometric redshifts to the 253 available spectroscopic redshifts, we achieve an accuracy of σΔz/(1 + z) = 0.036, with 12.6% outliers. For the X-ray-detected sources, compared to 115 spectroscopic redshifts, the accuracy is σΔz/(1 + z) = 0.069, with 18.3% outliers, where the outliers are defined as sources with |z phot – z spec| > 0.15 × (1 + z spec). These results are a significant improvement over the previously available photometric redshifts for normal galaxies in the Lockman Hole, while it is the first time that photometric redshifts are computed and made public for AGNs for this field.
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