The <i>Herschel</i>-SPIRE Dark Field I: the deepest <i>Herschel</i> image of the submillimetre Universe

ABSTRACT We present the image maps, data reduction, analysis, and the first source counts from the Herschel SPIRE Dark Field. The SPIRE Dark Field is an area of sky near the North Ecliptic Pole observed many times during the calibration phase of the Herschel mission in order to characterize the stability of the SPIRE (Spectral and Photometric Imaging Receiver) instrument and is subsequently one of the deepest imaged fields of the Universe at far-infrared-submillimetre wavelengths. The SPIRE dark field is concurrent with the Spitzer Infrared Array Camera Dark Field used for a similar purpose. The final Dark Field map is comprised of 141 individual SPIRE observations in Small Map and Large Map modes defined by a deep inner region approximately 12 arcmin in diameter and a slightly shallower surrounding area of diameter ${\sim} 30\,\mathrm{ arcmin}$. The depth of both regions reach well below the confusion limit of the SPIRE instrument at 250, 350, and 500$\, \mu$m. Two independent processes are used to extract sources, a standard map based method using the SUSSEXtractor algorithm and a list driven photometry approach using the XID (cross identification) algorithm with the Spitzer MIPS (Multiband Imaging Photometer for Spitzer) 24$\, \mu$m catalogue as an input prior. The resulting source counts detect the turnover in the galaxy population with both methods shown to be consistent with previous results from other Herschel surveys, with the XID process reaching approximately twice as deep compared to traditional map based algorithms. Finally, we compare our results with two contemporary galaxy evolution models, again showing a good general agreement with the modelled counts.