Unidentified quasars among stationary objects from Gaia DR2
  We here apply a novel technique selecting quasar candidates purely as sources
with zero proper motions in the Gaia data release 2 (DR2). We demonstrate that
this approach is highly efficient toward high Galactic latitudes with < 25%
contamination from stellar sources. Such a selection technique offers a very
pure sample completeness, since all cosmological point sources are selected
regardless of their intrinsic spectral properties within the limiting magnitude
of Gaia. We carry out a pilot-study by defining a sample compiled by including
all Gaia-DR2 sources within one degree of the North Galactic Pole (NGP)
selected to have proper motions consistent with zero within 2-sigma
uncertainty. By cross-matching the sample to the optical Sloan Digital Sky
Survey (SDSS) and the mid-infrared AllWISE photometric catalogues we
investigate the colours of each of our sources. Together with already
spectroscopically confirmed quasars we are therefore able to determine the
efficiency of our selection. The majority of the zero proper motion sources
have optical to mid-infrared colours consistent with known quasars. The
remaining population may be contaminating stellar sources, but some may also be
quasars with colours similar to stars. Spectroscopic follow-up of the zero
proper motion sources is needed to unveil such a hitherto hidden quasar
population. This approach has the potential to allow substantial progress on
many important questions concerning quasars such as determining the fraction of
dust-obscured quasars, the fraction of broad absorption line (BAL) quasars, and
the metallicity distribution of damped Lyman-$\alpha$ absorbers. The technique
could also potentially reveal new types of quasars or even new classes of
cosmological point sources.
