The VMC Survey - VI. Quasars behind the Magellanic system. (arXiv:1207.2492v1 [astro-ph.CO])

The VMC Survey - VI. Quasars behind the Magellanic system. (arXiv:1207.2492v1 [astro-ph.CO]):
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)