A compilation of known QSOs for the Gaia mission

Quasars are essential for astrometry in the sense that they are spatially stationary because of their large distance from the Sun. The European Space Agency (ESA) astrometric satellite Gaia is scanning the whole sky with unprecedented accuracy up to the level of a few mu as. However, Gaia's two-fields-of-view-observation strategy may introduce a parallax bias in the Gaia catalog. Since it presents no significant parallax, a quasar is the perfect natural object to detect such bias. More importantly, quasars can be used to construct a celestial reference frame in the optical wavelengths for the Gaia mission. In this paper, we compile a list of known quasars that have been published in the literatures. The final compilation (designated as Known Quasars Catalog for Gaia mission, KQCG) contains 1 842 076 objects, among which 797 632 objects are found in Gaia DR1 after cross-identifications. Redshift, color information and position uncertainties of the objects are also provided if available. This catalog will be very useful for the Gaia mission