The Hawk-I UDS and GOODS Survey (HUGS): Survey design and deep K-band number counts ^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Roma – via Frascati 33, Monte Porzio Catone, 00040 Rome Italy
e-mail: adriano.fontana@oa-roma.inaf.it
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
³ Department of Physics and Astronomy, Sonoma State University, Rohnert Park, CA, USA
⁴ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei Munchen, Germany
⁵ The School of Physics and Astronomy, University of Nottingham, Nottingham, UK
⁶ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ Spitzer Science Center, MC 2206, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁸ Department of Physics and Astronomy, University of California, Irvine, CA, USA
⁹ CEA-Saclay/DSM/DAPNIA/Service d’Astrophysique, 91191 Gif-sur-Yvette, France
¹⁰ National Optical Astronomy Observatories, Tucson, AZ, USA
¹¹ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, 95064, USA
¹² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
¹³ Space Telescope Science Institute, Baltimore, MD, USA
¹⁴ Department of Astronomy, University of Massachusetts, Amherst, MA, USA
¹⁵ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁶ INAF – Osservatorio Astronomico di Trieste, Trieste, Italy
¹⁷ Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
¹⁸ Osservatorio Astronomico di Padova, 35122 Padova, Italy
¹⁹ Minnesota Institute of Astrophysics and School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA
²⁰ Department of Astronomy, Steward Observatory, University of Arizona, Tucson, AZ, USA
²¹ INAF Osservatorio Astronomico di Bologna, Bologna, Italy
²² School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
²³ Department of Physics and Astronomy, University of Missouri, Columbia, MO, USA
²⁴ INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 30 January 2014
Accepted: 16 July 2014

Abstract

We present the results of a new, ultra-deep, near-infrared imaging survey executed with the Hawk-I imager at the ESO VLT, of which we make all the data (images and catalog) public. This survey, named HUGS (Hawk-I UDS and GOODS Survey), provides deep, high-quality imaging in the K and Y bands over the portions of the UKIDSS UDS and GOODS-South fields covered by the CANDELS HST WFC3/IR survey. In this paper we describe the survey strategy, the observational campaign, the data reduction process, and the data quality. We show that, thanks to exquisite image quality and extremely long exposure times, HUGS delivers the deepest K-band images ever collected over areas of cosmological interest, and in general ideally complements the CANDELS data set in terms of image quality and depth. In the GOODS-S field, the K-band observations cover the whole CANDELS area with a complex geometry made of 6 different, partly overlapping pointings, in order to best match the deep and wide areas of CANDELS imaging. In the deepest region (which includes most of the Hubble Ultra Deep Field) exposure times exceed 80 hours of integration, yielding a 1 − σ magnitude limit per square arcsec of ≃28.0 AB mag. The seeing is exceptional and homogeneous across the various pointings, confined to the range 0.38–0.43 arcsec. In the UDS field the survey is about one magnitude shallower (to match the correspondingly shallower depth of the CANDELS images) but includes also Y-band band imaging (which, in the UDS, was not provided by the CANDELS WFC3/IR imaging). In the K-band, with an average exposure time of 13 hours, and seeing in the range 0.37–0.43 arcsec, the 1 − σ limit per square arcsec in the UDS imaging is ≃27.3 AB mag. In the Y-band, with an average exposure time ≃8 h, and seeing in the range 0.45–0.5 arcsec, the imaging yields a 1 − σ limit per square arcsec of ≃28.3 AB mag. We show that the HUGS observations are well matched to the depth of the CANDELS WFC3/IR data, since the majority of even the faintest galaxies detected in the CANDELS H-band images are also detected in HUGS. Finally we present the K-band galaxy number counts produced by combining the HUGS data from the two fields. We show that the slope of the number counts depends sensitively on the assumed distribution of galaxy sizes, with potential impact on the estimated extra-galactic background light.