Deep low-frequency radio observations of the NOAO Boötes field

I. Data reduction and catalog construction^⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
² National Radio Astronomy Observatory, Charlottesville, VA, USA
e-mail: hintema@nrao.edu
³ National Centre for Radio Astrophysics, Univerity of Pune, India
⁴ Max-Planck-Institut für Radioastronomie, Bonn, Germany

Received: 12 February 2010
Accepted: 23 September 2011

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  ≲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 K_S-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.