LOFAR/H-ATLAS: a deep low-frequency survey of theHerschel-ATLAS North Galactic Pole field

We present LOFAR High-Band Array (HBA) observations of the Herschel-ATLAS North Pole survey area. The survey we have carried out, consisting of four pointings covering 142 square degrees of sky in the frequency range 126–173 MHz, does not provide noise coverage but otherwise is representative of the quality of data to be expected in planned LOFAR wide-area surveys, and has been reduced using recently developed ‘facet ’ methods at a resolution approaching the full resolution of the datasets (∼ 10×6) and an rms off-source noise that ranges from 100 µJy beam−1 the centre of the best to around 2 mJy beam−1 the furthest extent of our imaging. We describe the imaging, and source identification processes, and present some initial science results on a 5-σ source catalogue. These include (i) an initial look at the radio/far-infrared 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 of star-forming galaxies; (iii) the 150-MHz luminosity functions for active and starforming, which agree well with determinations at higher frequencies at low redshift, show strong redshift evolution of the star-forming population; and (iv) some discussion the implications of our observations for studies of radio galaxy life cycles.