Abstract
We present LOFAR High-Band Array (HBA) observations of the Herschel-ATLAS North Galactic Pole survey area. The survey we have carried out, consisting of four pointings covering around 142 square degrees of sky in the frequency range 126--173 MHz, does not provide uniform noise coverage but otherwise is representative of the quality of data to be expected in the planned LOFAR wide-area surveys, and has been reduced using recently developed `facet calibration' methods at a resolution approaching the full resolution of the datasets ($\sim 10 \times 6$ arcsec) and an rms off-source noise that ranges from 100 $\mu$Jy beam$^{-1}$ in the centre of the best fields to around 2 mJy beam$^{-1}$ at the furthest extent of our imaging. We describe the imaging, cataloguing and source identification processes, and present some initial science results based on a 5-$\sigma$ source catalogue. These include (i) an initial look at the radio/far-infrared correlation at 150 MHz, showing that many Herschel sources are not yet detected by LOFAR; (ii) number counts at 150 MHz, including, for the first time, observational constraints on the numbers of star-forming galaxies; (iii) the 150-MHz luminosity functions for active and star-forming galaxies, which agree well with determinations at higher frequencies at low redshift, and show strong redshift evolution of the star-forming population; and (iv) some discussion of the implications of our observations for studies of radio galaxy life cycles.
Highlights
Low-frequency continuum radio emission from galaxies originates in the synchrotron process, with the two sources of energy for the required high-energy electrons and positrons being supernovae and their remnants or the activity of radio-loud active galactic nuclei, which drive relativistic jets of magnetized plasma into the external medium
Low-Frequency Array (LOFAR) has good uv plane coverage on both long and short baselines, and so is able to image all but the very largest sources at high resolution without any loss of flux density, limited only by surface brightness sensitivity. Deep observations at these low frequencies are rare, and the previous best large-area survey at frequencies around those of the LOFAR High Band Array (HBA) is the TIFR GMRT Sky Survey2 (TGSS), full data from which were recently released (Intema et al 2016); this has a best resolution around 20 arcsec, which is substantially lower than the ∼5 arcsec that LOFAR can achieve, and, with an rms noise of ∼5 mJy beam−1, significantly lower sensitivity than will be achieved for the LOFAR Tier 1 survey
In this paper we describe an exploratory LOFAR HBA observation, of the Herschel-ATLAS survey (H-ATLAS) North Galactic Pole (NGP) field, a rectangular contiguous area of sky in the Sloan Digital Sky Survey (SDSS) sky area covering ∼170 deg2 around RA = 13.5 h and Dec = 30◦, and well positioned in the sky for LOFAR, with a substantial overlap with the position of the Coma cluster at low z
Summary
Low-frequency continuum radio emission from galaxies originates in the synchrotron process, with the two sources of energy for the required high-energy electrons and positrons being supernovae and their remnants (in star-forming galaxies, SFGs) or the activity of radio-loud active galactic nuclei, which drive relativistic jets of magnetized plasma into the external medium. LOFAR has good uv plane coverage on both long and short baselines, and so is able to image all but the very largest sources at high resolution without any loss of flux density, limited only by surface brightness sensitivity Deep observations at these low frequencies are rare, and the previous best large-area survey at frequencies around those of the LOFAR High Band Array (HBA) is the TIFR GMRT Sky Survey (TGSS), full data from which were recently released (Intema et al 2016); this has a best resolution around 20 arcsec, which is substantially lower than the ∼5 arcsec that LOFAR can achieve, and, with an rms noise of ∼5 mJy beam−1, significantly lower sensitivity than will be achieved for the LOFAR Tier 1 survey.
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