The HST-Gaia Near-Infrared Astrometric Reference Frame near the Milky Way Galactic Center

Matthew W. Hosek Jr.; Tuan Do; Gregory D. Martinez; Rebecca Lewis-Merrill; Andrea M. Ghez; Jessica R. Lu; Shoko Sakai; Jay Anderson

arXiv · 2506.19933

The HST-Gaia Near-Infrared Astrometric Reference Frame near the Milky Way Galactic Center

Matthew W. Hosek Jr.·Tuan Do·Gregory D. Martinez·Rebecca Lewis-Merrill·Andrea M. Ghez·Jessica R. Lu·Shoko Sakai·Jay Anderson

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Abstract

We present the first high-precision proper motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R $\leq$ 25" (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains $\sim$2,900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from 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 $\sim$0.03 mas/yr and $\sim$0.11 mas, respectively, representing a factor of $\sim$20x 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 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 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 $\sim$2x, further improving the reference frame for dynamical studies.

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Matthew W. Hosek Jr., Tuan Do, Gregory D. Martinez, Rebecca Lewis-Merrill, Andrea M. Ghez, Jessica R. Lu, Shoko Sakai, Jay Anderson. 2025-06-24. The HST-Gaia Near-Infrared Astrometric Reference Frame near the Milky Way Galactic Center. https://arxiv.org/abs/2506.19933

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