M. -R. L. Cioni, D. Kamath, S. Rubele, J. Th. van Loon, P. R. Wood, J. P. Emerson, B. K. Gibson, M. A. T. Groenewegen, V. D. Ivanov, B. Miszalski, V. Ripepi
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 studies of proper motion and of the interstellar medium along the line of sight. We search for criteria to identify candidate QSOs using near-infrared observations from the VISTA survey of the Magellanic Clouds system (VMC). The VMC survey provides photometry in the YJKs bands and 12 epochs in the Ks band with unprecedented sensitivity and spatial resolution. The (Y-J) vs. (J-Ks) diagram has been used to distinguish QSOs from Milky Way and Magellanic Cloud stars. Then, the slope of variation in the Ks band has been used to identify a sample of high confidence candidates. These criteria were developed based on the properties of 117 known QSOs. YJKs magnitudes and Ks light-curves of known QSOs behind the Magellanic system from present VMC data are presented. About 75% of them show a slope of variation >10^-4 mag/day and the shape of the light-curve is in general irregular and without any clear periodicity. A method to identify QSOs based solely on the VMC data is proposed using YJKs colours and Ks variability. The number of QSO candidates found in the South Ecliptic Pole and the 30 Doradus tiles is 22 and 26, respectively, with negligible contamination by young stellar objects, planetary nebulae, stars and normal galaxies. The high confidence in the nature of the selected objects is supported by recent studies of possible contaminants, but remains to be confirmed spectroscopically. In the entire VMC survey area we expect to find about 1500 QSOs behind the LMC, 600 behind the SMC, 300 behind the Bridge and 50 behind the Stream areas. The Ks light-curves can help support investigations of the mechanism responsible for the variations. (Abridged)
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http://arxiv.org/abs/1207.2492
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