The VMC Survey

VI. Quasars behind the Magellanic system^⋆

¹ University Observatory Munich, Scheinerstrasse 1, 81679 München, Germany
e-mail: mcioni@usm.uni-muenchen.de
² University of Hertfordshire, Physics Astronomy and Mathematics, Hatfield AL10 9AB, UK
³ RSAA, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
⁴ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Keele University, Lennard-Jones Laboratories, ST5 5BG, UK
⁶ School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁷ Jeremiah Horrocks Institute, University of Central Lancshire, Preston PR1 2HE, UK
⁸ Royal Observatory of Belgium, Ringlaan 3, 1180 Ukkel, Belgium
⁹ European Southern Observatory, Av. Alonso de Córdoba 3107, Casilla 19, Santiago, Chile
¹⁰ South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
¹¹ Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, South Africa
¹² INAF, Osservatorio Astornomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy

Received: 28 May 2012
Accepted: 10 October 2012

Abstract

Context. The number and spatial distribution of confirmed quasi-stellar objects (QSOs) behind the Magellanic system is limited. This undermines their use as astrometric reference objects for different types of studies.

Aims. We have searched for criteria to identify candidate QSOs using observations from the VISTA survey of the Magellanic Clouds system (VMC) that provides photometry in the YJK_s bands and 12 epochs in the K_s band.

Methods. The (Y − J) versus (J − K_s) diagram has been used to distinguish QSO candidates from Milky Way stars and stars of the Magellanic Clouds. Then, the slope of variation in the K_s band has been used to identify a sample of high confidence candidates. These criteria were developed based on the properties of 117 known QSOs presently observed by the VMC survey.

Results. VMC YJK_s magnitudes and K_s light-curves of known QSOs behind the Magellanic system are presented. About 75% of them show a slope of variation in K_s > 10^-4 mag/day and the shape of the light-curve is in general irregular and without any clear periodicity. The number of QSO candidates found in tiles including the south ecliptic pole and the 30 Doradus regions is 22 and 26, respectively, with a  ~20% contamination by young stellar objects, planetary nebulae, stars and normal galaxies.

Conclusions. By extrapolating the number of QSO candidates to the entire VMC survey area we expect to find about 1200 QSOs behind the LMC, 400 behind the SMC, 200 behind the Bridge and 30 behind the Stream areas, but not all will be suitable for astrometry. Further, the K_s band light-curves can help support investigations of the mechanism responsible for the variations.