The Herschel☆-ATLAS data release 1-I. Maps, catalogues and number counts

We present the first major data release of the largest single key-project in area carried out in open time with the Herschel Space Observatory. The Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) is a survey of 600 deg(2) in five photometric bands - 100, 160, 250, 350 and 500 mu m - with the Photoconductor Array Camera and Spectrometer and Spectral and Photometric Imaging Receiver (SPIRE) cameras. In this paper and the companion Paper II, we present the survey of three fields on the celestial equator, covering a total area of 161.6 deg(2) and previously observed in the Galaxy and Mass Assembly (GAMA) spectroscopic survey. This paper describes the Herschel images and catalogues of the sources detected on the SPIRE 250 mu m images. The 1 sigma noise for source detection, including both confusion and instrumental noise, is 7.4, 9.4 and 10.2 mJy at 250, 350 and 500 mu m. Our catalogue includes 120 230 sources in total, with 113 995, 46 209 and 11 011 sources detected at > 4 sigma at 250, 350 and 500 mu m. The catalogue contains detections at >3 sigma at 100 and 160 mu m for 4650 and 5685 sources, and the typical noise at these wavelengths is 44 and 49 mJy. We include estimates of the completeness of the survey and of the effects of flux bias and also describe a novel method for determining the true source counts. The H-ATLAS source counts are very similar to the source counts from the deeper HerMES survey at 250 and 350 mu m, with a small difference at 500 mu m. Appendix A provides a quick start in using the released data sets, including instructions and cautions on how to use them.