New quasars behind the Magellanic Clouds. II. Spectroscopic confirmation of 136 near-infrared selected candidates

TL;DR

This study identifies and spectroscopically confirms 136 new quasars behind the Magellanic Clouds using near-infrared colors and variability, aiding astrometric reference frame creation and proper motion measurements.

Contribution

It introduces an effective method combining infrared color and variability criteria for selecting high-probability quasar candidates behind nearby galaxies.

Findings

Confirmed 136 new QSOs with redshifts from 0.1 to 2.9.

Achieved ~90% success rate in confirming quasar candidates.

Compiled a list of 3609 objects matching selection criteria across the VMC survey.

Abstract

Quasi-stellar objects (QSOs) are a basis for an absolute reference system for astrometric studies. There is a need for creating such system behind nearby galaxies, to facilitate the measuring of the proper motions of these galaxies. However, the foreground contamination from the galaxies themselves is a problem for the QSO identification. We search for new QSOs behind both Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream. 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. We confirmed the QSO nature of 136 objects. They are distributed as follows: 12 behind the LMC, 37 behind the SMC, 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 ~90% probability. Finally, we report a list of 3609 objects across the entire VMC survey that match our color and variability selection criteria; only 1249 of them have Gaia counterparts. Our combined infrared color and variability criteria for QSO selection prove to be efficient - ~90% of the observed candidates are bona fide QSOs and allow to generate a list of new high-probability quasar candidates.