The Gaia-WISE Extragalactic Astrometric Catalog
  The Gaia mission has detected a large number of active galactic nuclei (AGN)
and galaxies, but these objects must be identified among the 1000-fold more
numerous stars. Extant astrometric AGN catalogs do not have the uniform sky
coverage required to detect and characterize the all-sky low-multipole proper
motion signals produced by the barycenter motion, gravitational waves, and
cosmological effects. To remedy this, we present an all-sky sample of 567,721
AGN in Gaia Data Release 1, selected using WISE two-color criteria. The catalog
has fairly uniform sky coverage beyond the Galactic plane, with a mean density
of 12.8 AGN per square degree. The objects have magnitudes ranging from $G=8.8$
down to Gaia's magnitude limit, $G=20.7$. The catalog is approximately 50%
complete but suffers from low stellar contamination, roughly 0.2%. We predict
that the end-of-mission Gaia proper motions for this catalog will enable
detection of the secular aberration drift to high significance (23$\sigma$) and
will limit the anisotropy of the Hubble expansion to about 2%.
