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We analyze stellar proper motions in the COSMOS field to assess the presenceof bulk motions. At bright magnitudes (G-band 18.5--20.76 AB), we use theproper motions of 1,010 stars in the Gaia DR2 catalog. At the faint end, wecomputed proper motions of 11,519 point-like objects at i-band magnitudes19--25 AB using Hubble ACS and Subaru HSC, which span two epochs about 11 yearsapart. In order to measure these proper motions with unprecedented accuracy atfaint magnitudes, we developed a foundational set of astrometric tools whichwill be required for Joint Survey Processing (JSP) of data from the nextgeneration of optical/infrared surveys. The astrometric grids of Hubble ACS andSubaru HSC mosaics were corrected at the catalog level, using propermotion-propagated and parallax-corrected Gaia DR2 sources. These astrometriccorrections were verified using compact extragalactic sources. Upon comparisonof our measured proper motions with Gaia DR2, we estimate the uncertainties inour measurements to be ~2--3 mas/yr per axis, down to 25.5 AB mag. We correctedproper motions for the mean motion of the Sun, and we find that late-typemain-sequence stars predominantly in the thin disk in the COSMOS field havespace velocities mainly towards the Galactic center. We detect candidatehigh-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 sourcesfrom the faint star sample may be candidate halo members of the Sangariusstream.

Joint Survey Processing II: Stellar Proper Motions in the COSMOS Field, from Hubble Space Telescope ACS and Subaru Telescope HSC Observations