LOFAR/H-ATLAS: A deep low-frequency survey of the Herschel-ATLAS North
  Galactic Pole field
  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.
