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: Star forming galaxies are thought to dominate the sub-mJy radio population, but recent work has shown that low luminosity AGN can still make a significant contribution to the faint radio source population. Spectral indices are an important tool for understanding the emission mechanism of the faint radio sources. We have observed the extended Chandra Deep Field South at 5.5 GHz using a mosaic of 42 pointings with the Australia Telescope Compact Array (ATCA). Our image reaches an almost uniform sensitivity of ~12 microJy rms over 0.25 deg^2 with a restoring beam of 4.9 x 2.0 arcsec, making it one of the deepest 6cm surveys to date. We present the 5.5 GHz catalogue and source counts from this field. We take advantage of the large amounts of ancillary data in this field to study the 1.4 to 5.5 GHz spectral indices of the sub-mJy population. For the full 5.5 GHz selected sample we find a flat median spectral index, alpha_med = -0.40, which is consistent with previous results. However, the spectral index appears to steepen at the faintest flux density levels (S_{5.5 GHz} < 0.1 mJy), where alpha_med = -0.68. We performed stacking analysis of the faint 1.4 GHz selected sample (40 < S_{1.4 GHz} < 200 microJy) and also find a steep average spectral index, alpha = -0.8, consistent with synchrotron emission. We find a weak trend of steepening spectral index with redshift. Several young AGN candidates are identified using spectral indices, suggesting Gigahertz Peaked Spectrum (GPS) sources are as common in the mJy population as they are at Jy levels.
    Monthly Notices of the Royal Astronomical Society 08/2012; 426(3). · 5.52 Impact Factor
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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.
    The Astrophysical Journal Supplement Series 12/2011; 198(1):1. · 16.24 Impact Factor
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    ABSTRACT: In this article we present deep, high-resolution radio interferometric observations at 153 MHz to complement the extensively studied NOAO Boötes field. We provide a description of the observations, data reduction and source catalog construction. From our single pointing GMRT observation of ~12 h we obtain a high-resolution (26" × 22") image of ~11.3 square degrees, fully covering the Boötes field region and beyond. The image has a central noise level of ~1.0 mJy beam-1, which rises to 2.0-2.5 mJy beam-1 at the field edge, placing it amongst the deepest ~150 MHz surveys to date. The catalog of 598 extracted sources is estimated to be ~92 percent complete for > 10 mJy sources, while the estimated contamination with false detections is < 1 percent. The low rms position uncertainty of 1.24" facilitates accurate matching against catalogs at optical, infrared and other wavelengths. Differential source counts are determined down to &lsim;10 mJy. There is no evidence for flattening of the counts towards lower flux densities as observed in deep radio surveys at higher frequencies, suggesting that our catalog is dominated by the classical radio-loud AGN population that explains the counts at higher flux densities. Combination with available deep 1.4 GHz observations yields an accurate determination of spectral indices for 417 sources down to the lowest 153 MHz flux densities, of which 16 have ultra-steep spectra with spectral indices below -1.3. We confirm that flattening of the median spectral index towards low flux densities also occurs at this frequency. The detection fraction of the radio sources in NIR KS-band is found to drop with radio spectral index, which is in agreement with the known correlation between spectral index and redshift for brighter radio sources. Full Table 3 is only available at the CDS via anonymous ftp to ( or via
    Astronomy and Astrophysics 01/2011; 535. · 5.08 Impact Factor

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