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

TL;DR

This study measures stellar proper motions in the COSMOS field using Hubble and Subaru data, developing new astrometric tools for future surveys, and identifies high-velocity star candidates indicating Galactic structure features.

Contribution

The paper introduces advanced astrometric techniques for faint star proper motion measurements and applies them to the COSMOS field, combining Gaia, Hubble, and Subaru data.

Findings

Proper motions measured with ~2-3 mas/yr accuracy down to 25.5 AB mag.

Detection of candidate high-velocity stars potentially linked to the Sangarius stream.

Most thin disk stars show velocities toward the Galactic center.

Abstract

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.