Joint Survey Processing II: Stellar Proper Motions in the COSMOS Field,
  from Hubble Space Telescope ACS and Subaru Telescope HSC Observations
  We analyze stellar proper motions in the COSMOS field to assess the presence
of bulk motions. At bright magnitudes (G-band 18.5--20.76 AB), we use the
proper motions of 1,010 stars in the Gaia DR2 catalog. At the faint end, we
computed proper motions of 11,519 point-like objects at i-band magnitudes
19--25 AB using Hubble ACS and Subaru HSC, which span two epochs about 11 years
apart. In order to measure these proper motions with unprecedented accuracy at
faint magnitudes, we developed a foundational set of astrometric tools which
will be required for Joint Survey Processing (JSP) of data from the next
generation of optical/infrared surveys. The astrometric grids of Hubble ACS and
Subaru HSC mosaics were corrected at the catalog level, using proper
motion-propagated and parallax-corrected Gaia DR2 sources. These astrometric
corrections were verified using compact extragalactic sources. Upon comparison
of our measured proper motions with Gaia DR2, we estimate the uncertainties in
our measurements to be ~2--3 mas/yr per axis, down to 25.5 AB mag. We corrected
proper motions for the mean motion of the Sun, and we find that late-type
main-sequence stars predominantly in the thin disk in the COSMOS field have
space velocities mainly towards the Galactic center. We detect candidate
high-velocity (> 220 km/s) stars, 6 of them at ~0.4-6 kpc from the Gaia sample,
and 5 of them at ~20 kpc from the faint star HSC and ACS sample. The sources
from the faint star sample may be candidate halo members of the Sangarius
stream.
