Afonso, J. and Mauduit, J. -C. and Surace, J. (2011) Ultra Steep Spectrum Radio Sources in the Lockman Hole: SERVS Identifications and Redshift Distribution at the Faintest Radio Fluxes. Astrophysical Journal, 743 (2). Art. No. 122. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120201-075154677
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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 μJy, is assembled. Deep infrared data at 3.6 and 4.5 μm from the Spitzer Extragalactic Representative Volume Survey (SERVS) are 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 μm (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 indicates 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.
|Additional Information:||© 2011 The American Astronomical Society. Received 2010 May 26; accepted 2011 September 4; published 2011 November 29. This work is based in part on observations made with Spitzer, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. J.A.,H.M., M.G., L.B., and C.F. gratefully acknowledge support from the Science and Technology Foundation (FCT, Portugal) through the research grant PTDC/FIS/100170/2008 and the Fellowships SFRH/BD/31338/2006 (H.M.) and SFRH/BPD/62966/ 2009 (L.B.). J.S.D. acknowledges the support of the Royal Society via a Wolfson Research Merit Award and the support of the European Research Council via an ERC Advanced Grant. C.M. and J.P. acknowledge support from the Marie Curie Excellence Team Grant MEXT-CT-2006-042754 “UniMass” of the Training and Mobility of Researchers programme financed by the European Community.|
|Subject Keywords:||galaxies: active; galaxies: evolution; galaxies: high-redshift; radio continuum: galaxies; surveys|
|Official Citation:||Ultra Steep Spectrum Radio Sources in the Lockman Hole: SERVS Identifications and Redshift Distribution at the Faintest Radio Fluxes J. Afonso et al. 2011 ApJ 743 122|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||01 Feb 2012 16:40|
|Last Modified:||26 Dec 2012 14:46|
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