2025ApJ...989...79H

We present the first high-precision proper-motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R ≤ 25″ (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains ∼2900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from Hubble Space Telescope (HST) observations (14 epochs, 2010-2023) and transformed into the ICRS using 40 stars in common with Gaia-DR3. We implement a new method for modeling proper motions via Gaussian processes that accounts for systematic errors, greatly improving measurement accuracy. Proper-motion and position measurements reach precisions of ∼0.03 mas yr^–1 and ∼0.11 mas, respectively, representing a factor of ∼20 improvement over previous ICRS proper-motion catalogs in the region. These measurements define a novel HST-Gaia reference frame that is consistent with Gaia-CRF3 to within 0.025 mas yr^–1 in proper motion and 0.044 mas in position, making it the first ICRS-based reference frame precise enough to probe the distribution of extended mass within the orbits of stars near SgrA*. In addition, HST-Gaia provides an independent test of the radio measurements of stellar masers that form the basis of current GC reference frames. We find that the HST-Gaia and radio measurements are consistent to within 0.041 mas yr^–1 in proper motion and 0.54 mas in position at 99.7% confidence. Gaia-DR4 is expected to reduce the HST-Gaia reference-frame uncertainties by another factor of ∼2, further improving the reference frame for dynamical studies.