A study of purely astrometric selection of extragalactic point sources with Gaia

Abstract

Context. Selection of extragalactic point sources, e.g., quasi-stellar objects (QSOs), is often hampered by significant selection effects causing existing samples to have rather complex selection functions.
Aims: We explore whether a purely astrometric selection of extragalactic point sources, e.g., QSOs, is feasible with the ongoing Gaia mission. The idea has been discussed in the context of Gaia, but it is the first time quantified numbers have been given. This kind of selection would also be interesting as it would be unbiased in terms of colors of the targets and hence would allow selection also with colors in the stellar sequence.
Methods: We have analyzed a total of 18 representative regions of the sky using GUMS, the simulator prepared for ESAs Gaia mission, both in the range of 12 ≤ G ≤ 20 mag and 12 ≤ G ≤ 18 mag. For each region we determine the density of apparently stationary stellar sources, i.e., sources for which Gaia cannot measure a significant proper motion. The density is contrasted with the density of extragalactic point sources, e.g., QSOs, to establish in which celestial directions a pure astrometric selection is feasible.
Results: When targeting regions at galactic latitude | b | ≥ 30^o the ratio of QSOs to apparently stationary stars is above 50% and when observing toward the poles, the fraction of QSOs goes up to about ~80%. We show that the proper motions from the proposed Gaia successor mission in about 20 years would dramatically improve these results at all latitudes. Detection of QSOs solely from zero proper motion, unbiased by any assumptions on spectra, might lead to the discovery of new types of QSOs or new classes of extragalactic point sources.