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

TL;DR

This paper introduces a high-precision infrared astrometric reference frame near the Galactic Center, combining HST and Gaia data to enable detailed dynamical studies of stars around the supermassive black hole.

Contribution

It presents the first high-precision proper motion catalog near the Galactic Center, using a novel Gaussian Process method to improve accuracy and establish a new ICRS-based reference frame.

Findings

Proper motions measured with ~0.03 mas/yr precision

Reference frame consistent with Gaia-CRF3 within 0.025 mas/yr

Agreement between HST-Gaia and radio measurements within 0.041 mas/yr

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.