New quasars behind the Magellanic Clouds

Context. Quasi-stellar objects (QSOs) are a basis for an absolute reference system for astrometric studies. A system like this at the far side of nearby galaxies is required to facilitate measuring of the proper motions of these galaxies. However, the foreground contamination from the galaxies themselves is a problem for the QSO identification. Aims. We search for new QSOs behind the two Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream. Methods. We identify QSO candidates with a combination of near–infrared colors and variability criteria from the public ESO Visual and Infrared Survey Telescope for Astronomy (VISTA) Magellanic Clouds (VMC) survey. We confirm their nature from broad emission lines with low-resolution optical spectroscopy. Results. We confirmed the QSO nature of 136 objects. They are distributed as follows: 12 behind the Large Magellanic Cloud, 37 behind the Small Magellanic Cloud, 63 behind the Bridge, and 24 behind the Stream. The QSOs span a redshift range from z ~0.1 to z ~2.9. A comparison of our quasar selection with the Quaia quasar catalog, based on Gaia low-resolution spectra, yields a selection and confirmation success rate of 6–19%, depending on whether the quality of the photometry, the magnitude ranges, and the colors are considered. Our candidate list is rather incomplete, but the objects in it are likely to be confirmed as quasars with a probability of ~90%. Finally, we report a list of 3609 objects in the entire VMC survey that match our color and variability selection criteria; only 1249 of them have Gaia counterparts. Conclusions. Our combined infrared color and variability criteria for the QSO selection prove to be efficient: ~90% of the observed candidates are bona fide QSOs and allow us to generate a list of new high-probability quasar candidates.