ESO Public Surveys VST ATLAS First Science Results Tom Shanks1 Vasily Belokurov2 Ben Chehade1 Scott M. Croom3 Joe R. Findlay1 Eduardo Gonzalez-Solares2 Mike J. Irwin2 Sergey Koposov2 Robert G. Mann4 Nigel Metcalfe1 David N. A. Murphy5 Peder R. Norberg 6 Mike A. Read4 Eckhard Sutorius4 Gabor Worseck7 s urveys complement each other and provide excellent multi-wavelength data for both Galactic and extragalactic science projects. Survey description The VLT Survey Telescope (VST) ATLAS imaging exposures are 2 × 60 s in u-band, 2 × 50 s in g-band and 2 × 45 s in riz-bands. The iz images are observed in grey time and the ugr in dark. The median seeing of 1 arcsecond is well sampled by the 0.21-arcsecond pixels of VST OmegaCAM. The current status of the VST ATLAS in the ugriz band can be queried1. Ultimately we are targeting most of the available high latitude sky in the northern and southern Galactic Caps. So far we have covered approximately 2200 square degrees in each band with a little less coverage in ugr and somewhat more in iz. Thus the survey which started in September 2011 is roughly at its half-way point. The survey is unique in terms of southern surveys in having u-band sensitivity and the ATLAS Chilean u-band extension (Principal Investigator [PI]: L. Infante) doubles the u-band exposure to four minutes. As well as VHS supplying YJHK imaging, ATLAS is well complemented by the WISE satellite near-infrared (NIR) survey covering 3.4–22 μm. The WISE survey depth is particularly well matched to ATLAS in the W1 (3.4 μm) and W2 (4.6 μm) bands and several of the science aims, outlined in the following sections, exploit this complementarity. The OmegaCAM Science Archive2 will publish ATLAS data, and federate it with GALEX, WISE and the VISTA surveys, to facilitate these multi-wavelength analyses. 1 epartment of Physics, Durham University, D United Kingdom 2 Cambridge Astronomical Surveys Unit, Cambridge, United Kingdom 3 School of Physics, University of Sydney, Australia 4 Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, United Kingdom 5 Instituto de Astrofísica, Universidad Catolica de Chile, Chile 6 ICC, Department of Physics, Durham University, United Kingdom 7 Max-Planck-Institut für Astronomie, Heidelberg, Germany The VST ATLAS is a ugriz-imaging survey targeting ~ 4500 square degrees of the southern sky. It reaches similar magnitude limits to the Sloan Digital Sky Survey in the north, i.e., r ~ 22.5, but ATLAS has better median seeing of 1 arcsecond full width half maximum. ATLAS is a companion survey to the VISTA Hemisphere Survey, which supplies YJHK imaging over much of its area. In addition, the Wide-field Infrared Survey Explorer (WISE) satellite supplies a further four mid-infrared bands. Together these Y Y Both these methods distinguish the power-law spectra of quasars from the thermal spectra of stars at their respective wavelength extremes. Chehade et al. (in prep.) have made the first spectroscopic follow-up of quasars selected using ATLAS+WISE, using the Two-degreeField Galaxy Redshift Survey (2dF) and A AOmega at the Australian Astronomical Telescope (AAT). They found that while UV selection wins at the faintest magnitudes (g < 22.5), NIR selection using (g-i):(i-W1) significantly increases the quasar number counts by around ~ 15% in the fields where it has been applied to g < 22, mostly comprising a population of dust-reddened quasars (see Figure 1a), leading to quasar sky densities of up to 95 per square degree. We now have a sample of ~ 10 000 quasars with which to analyse the luminosity functions and clustering in a more complete and fainter redshift survey than the best previous surveys, such as 2SLAQ. The ATLAS quasar surveys will provide future targets for the ESO 4MOST fibre spectrograph and will also gain X-ray coverage with the eROSITA satellite, soon to be launched. 5 < z < 6 quasar redshift surveys The combination of WISE and ATLAS shows further potential. Findlay et al (in prep.) have shown that the (r-z):(i-W1) colour–colour plane (see Figure 1b) can easily distinguish 5 < z < 6 quasars from red stars and galaxies, since previously discovered SDSS quasars in this redshift range lie in a valley between the loci of stars and galaxies. This immediately reduces background contamination for such quasar searches by about two orders of magnitude to i = 21.5 even before a g-band dropout technique is applied. Using the combination of these techniques, it should be possible to add significantly to the relative dearth of known quasars in this redshift range and so bridge the gap between the z ~ 5 and z ~ 6 quasar luminosity functions. Increasing the numbers of z < 2.5 quasar redshift surveys ATLAS has two routes to quasar surveys at lower redshifts: via ultraviolet excess (UVX) selection by virtue of its u-band sensitivity; via “KX” selection using the W1 (3.4 μm) band. A @ H l 6 §L Y Y 2SDKK@Q +NBTR l 6@UDKDMFSG Ä 6@UDKDMFSG Ä 38 The Messenger 154 – December 2013 QmY #V@QER 2#22 02. Figure 1. (a) Four examples of a new population of dust-absorbed red quasars at z < 2.5 selected from (g-i):(i-W1) colour–colour plot by a combination of WISE and ATLAS. Spectra are from AAT 2dF AAOmega. (b) WISE and ATLAS (r-z):(i-W1) colour–colour plot shows high efficiency in isolating previously discovered SDSS 5 < z < 6 quasars.