The LOFAR LBA Sky Survey: Deep Fields I. The Bootes Field

We present the first sub-mJy (approximate to 0.7 mJy beam(-1)) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15 x 15 arcsec) image of the Bootes field at 34-75 MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. The observations and data reduction, including direction-dependent calibration, are described here. We present a radio source catalogue containing 1948 sources detected over an area of 23.6 deg(2), with a peak flux density threshold of 5 sigma. Using existing datasets, we characterise the astrometric and flux density uncertainties, finding a positional uncertainty of similar to 1.2 arcsec and a flux density scale uncertainty of about 5 per cent. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources. We calculate the Euclidean-normalised differential source counts and investigate the low-frequency radio source spectral indices between 54 and 144 MHz. Both show a general flattening in the radio spectral indices for lower flux density sources, from similar to - 0.75 at 144-MHz flux densities between 100 and 1000 mJy to similar to - 0.5 at 144-MHz flux densities between 5 and 10 mJy. Such flattening is attributable to a growing population of star forming galaxies and compact core-dominated AGN.

Automated summary

This survey is the first completed below 100 MHz and is an order of magnitude deeper than previous widefield imaging at these low frequencies. The high-resolution image and radio source catalogue of the Bootes field were created from 56 hours of LOFAR Low Band Antenna system observations. The study reveals a flattening in radio spectral indices for lower flux density sources, attributed to an increasing population of star forming galaxies and compact core-dominated AGN.