Astronomical Science The VISTA Near-infrared YJKs Public Survey of the Magellanic Clouds System (VMC) Maria-Rosa Cioni1, 2 13 UK Astronomy Technology Centre, debate as to whether it is also of tidal Gisella Clementini3 United Kingdom origin from the interaction of the Large Leo Girardi4 14 Centre for Astrophysics, University of Magellanic Cloud (LMC) with the Small Roald Guandalini1 Central Lancshire, United Kingdom Magellanic Cloud (SMC), or produced by Marco Gullieuszik5 15 University of Cambridge, Institute of ram pressure between the two galaxies. Brent Miszalski1 Astronomy, United Kingdom Maria-Ida Moretti6 16 ESO The VMC1 survey is acquiring near-infra­ Vincenzo Ripepi7 17 UPMC, University of Paris, Institut red (NIR) data of unprecedented sensi- Stefano Rubele4 d’Astrophysique de Paris, France tivity in the Magellanic system that is of Gemma Bagheri1 18 CNRS, Institut d’Astrophysique de immense value for the astronomical com­ Kenji Bekki8 Paris, France munity. The survey represents the only Nick Cross9 19 LERMA, Observatoire de Paris, France NIR counterpart to existing optical sur­ Erwin de Blok10 20 University of Zurich, Institute of Theo­ veys and for the large number of unclas­ Richard de Grijs11 retical Physics, Switzerland sified ob­­jects observed with the Spitzer Jim Emerson12 21 University of Exeter, School of Physics, Space Telescope in the mid-infrared. The Chris Evans13 United Kingdom VMC will cover the bulk of the Magellanic Brad Gibson14 22 University of Keele, School of Physical system as opposed to the tiny regions Eduardo Gonzales-Solares15 and Geographical Sciences, United sampled by the Hubble Space Telescope, Martin Groenewegen5 Kingdom and the limited area covered by most of Mike Irwin15 23 University of Leicester, United King­ the other ground­based observations at Valentin Ivanov16 dom the same sensitivity. Jim Lewis15 24 Mount Stromlo Observatory, RSSA, Marcella Marconi7 Australia Jean-Baptiste Marquette17, 18 Chiara Mastropietro19 Ben Moore20 The VISTA public survey project VMC Ralf Napiwotzki1 targets the Large Magellanic Cloud, Tim Naylor21 the Small Magellanic Cloud, the Bridge Joana Oliveira22 and two fields in the Stream. The VMC Mike Read9 survey is a uniform and homogeneous 9 Eckhard Sutorius survey in the Y, J and Ks near-infrared Jacco van Loon22 filters. The main goals are the determi- Mark Wilkinson23 nation of the star formation history and Peter Wood24 the three-dimensional structure of the Magellanic system. The survey is there- Figure 1. VMC logo fore designed to reach stars as faint 1 University of Hertfordshire, Physics as the oldest main sequence turn-off Astronomy and Mathematics, United point and to constrain the mean magni- Current open questions about the for- Kingdom tude of pulsating variable stars such mation and evolution of the Magellanic 2 University Observatory Munich, as RR Lyrae and Cepheids. We provide Clouds will be addressed by the VMC Germany a brief overview of the survey strategy survey, such as: How have the SFHs of 3 INAF, Osservatorio Astronomico di and first science results. Further details the LMC and SMC been influenced by Bologna, Italy are given in Cioni et al. (2011). interaction? Does the geometry of the 4 INAF, Osservatorio Astronomico di Magellanic system depend on age and Padova, Italy metallicity? Why is there a significant 5 Royal Observatory of Belgium, Belgium Introduction difference in structure between the gas 6 University of Bologna, Department of and stars in the SMC? Astronomy, Italy The Magellanic Clouds offer an excellent 7 INAF, Osservatorio Astronomico di laboratory for near-field cosmology. They Capodimonte, Italy represent the closest prototype for stud­ Observations and data reduction 8 ICRAR, University of Western Australia, ies of interacting galaxies and their low Australia metallicity and high gas content provide VISTA 2 (Emerson et al. 2006) is the larg­ 9 University of Edinburgh, Institute for information about galaxies at an early est wide-field NIR imaging telescope Astronomy, United Kingdom stage of evolution. The Magellanic Clouds designed to perform survey operations. 10 University of Cape Town, South Africa have experienced an extended star for­ The performance during commissioning 11 Peking University, Kavli Institute for mation history (SFH) and their dynamical is presented by Emerson et al. (2010), Astronomy and Astrophysics, China interaction may be responsible for the while science verification programmes 12 Queen Mary, University of London, formation of the Magellanic Bridge. The are summarised in Arnaboldi et al. (2010). United Kingdom origin of the Magellanic Stream is under The Messenger 144 – June 2011 25 Astronomical Science Cioni M.-R.. et al., The VISTA Near-infrared YJKs Public Survey The VMC survey parameters are listed observed in the LMC. One field covers sky levels. Tile catalogues are produced in Table 1. VMC uses three filters, Y, J the famous 30 Doradus region, one field following the application of a nebulos- and Ks for the analysis of colour–colour corresponds to the South Ecliptic Pole ity filter in order to remove diffuse varying diagrams; the wider colour spacing in (SEP) region, there are a pair of fields background on scales of 3 arcseconds Y–Ks provides a good characterisation of located in the northern outer part of the or larger (Irwin 2010). The average VMC the sub-giant branch population for LMC disc while the remaining two fields parameters from all single-tile images deriving the SFH. Monitoring of variable are located towards the Bridge. The are given in Table 3 where the magnitude sources is performed in the Ks­band, progress of the VMC survey observations limit is at 5σ. This set comprises ob­­­ where variable stars obey a clear period– up to 4 March 2011 is provided in Table 2. servations obtained up to the end of magnitude relation. The VMC survey area November 2010 but does not distinguish is shown in Figure 2. It covers the area The VISTA raw images are reduced by between crowded and uncrowded fields to a limiting B magnitude of 25 mag arc­ the VISTA Data Flow System (VDFS) for which the VMC has different observ­ sec–2 for both galaxies and encompasses pipeline at the Cambridge Astronomical ing requirements. the major features traced by the distribu­ Survey Unit (CASU3). Standard reduc - tion of stars and gas. tion steps are performed and quality con­ Astrometry is based on the positions of trol parameters are calculated for both the many 2MASS sources within each Here we focus on observations obtained monitoring and evaluating observing con­ detector. The median astrometric root- during the dry-run period (November ditions retrospectively. A tile image is pro­ mean-square is 80 milliarcseconds (mas) 2009–March 2010) when VISTA was duced by combining different pawprint and is dominated by uncertainties on tested and survey operations were still images adjusted for astrometric and pho­ 2MASS coordinates. Residual systematic being defined. Six VMC fields were tometric distortions as well as different distortions across the VISTA field of view are present at the 25 mas level. The pho­ tometric calibration relies on the observa­ Table 1. VMC survey parameters. tion of stars from the 2MASS catalogue with magnitudes in the range 12–14 in all Filter Y J Ks Filter Y J Ks bands. The calibration in the Y­band is Central λ (μm) 1.02 1.25 2.15 Exposure time per epoch (s) 800 800 750 possible where the extinction is not too Bandwidth (μm) 0.10 0.18 0.30 No. of epochs 3 3 12 high, i.e. EB–V < 1.5 (Hodgkin et al. 2009). DIT (s) 20 10 5 Total exposure time (s) 2400 2400 9000 Figure 3 shows the behaviour of VISTA No. of DITs 4 8 15 Sensitivity per epoch (Vega) 21.3 20.8 18.9 photometric uncertainties in a VMC field. No. of exposures 1 1 1 S/N per epoch 5.7 5.9 2.9 Uncertainties are reduced by about 50% Micro-stepping 1 1 1 Total sensitivity (Vega) 21.9 21.4 20.3 compared to those for individual tiles No. of jitters 5 5 5 Total S/N 10 10 10 and will reduce further for deep tiles. A Pawprints in tile 6 6 6 Saturation (Vega) 12.9 12.7 11.4 morphological classification flag is also Pixel size (arcsec) 0.339 0.339 0.339 Area (deg2) 184 184 184 included in the catalogue. System FWHM 0.51 0.51 0.51 No. of tiles 110 110 110 The data reduced by the VDFS pipeline are ingested into the VISTA Science Description VMC LMC SMC Bridge Stream Table 2. VMC survey Archive4 (VSA). At present these data are No. of tiles 110 68 27 13 2 progress up to 4 March reduced with the version 1.0 of the 2011. (Items in blue refer No. of epochs 1980 1224 486 234 36 to the complete survey.) pipeline and include all VMC data ob­­ No. of Y epochs 330 204 81 39 6 served until end of November 2011. At No. of J epochs 330 204 81 39 6 the VSA the data are archived to produce No. of Ks epochs 1320 816 324 156 24 standardised data products: individual Observed Y epochs 59 37 8 8 6 passband frame association and source Observed J epochs 58.5 38.5 6.5 7.5 6 association to provide multi-colour, multi- Observed Ks epochs 98.5 75 5 4.5 14 epoch source lists; cross-associations No. of observed epochs 216 150.5 19.5 20 26 with external catalogues; and deeper Completion in Y 17.9 % 18.1 % 9.9% 20.5% 100 % stacking. The position and magnitude of Completion in J 17.7 % 18.9 % 8% 19.2 % 100 % each source in a given table refers to the Completion in Ks 7.5 % 9.2% 1.5 % 2.9% 58.3% astrometrically and photometrically cali­ Total completion 10.9 % 12.3 % 4% 8.5% 72.2% brated measurements using the parame­ ters specified in the image headers.