GaiaHub: A method for combining data from the Gaia and Hubble space telescopes to derive improved proper motions for faint stars

TL;DR

GaiaHub combines Gaia and HST data to improve proper motion measurements for faint stars, enabling advanced dynamical studies of distant stellar systems with only minimal additional observations.

Contribution

It introduces GaiaHub, a novel method that enhances proper motion accuracy for faint stars by integrating Gaia and archival HST data, requiring only a single epoch of HST observations.

Findings

Improved proper motion accuracy for faint stars (G > 18)

Measured velocity dispersions in MW globular clusters and dwarf galaxies

Detected correlations between cluster kinematics and ellipticity

Abstract

We present GaiaHub, a publicly available tool that combines $Gaia$ measurements with $Hubble$ $Space$ $Telescope$ ($HST$) archival images to derive proper motions (PMs). It increases the scientific impact of both observatories beyond their individual capabilities. $Gaia$ provides PMs across the whole sky, but the limited mirror size and time baseline restrict the best PM performance to relatively bright stars. $HST$ can measure accurate PMs for much fainter stars over a small field, but this requires two epochs of observation which are not always available. GaiaHub yields considerably improved PM accuracy compared to $Gaia$-only measurements, especially for faint sources $(G \gtrsim 18)$, requiring only a single epoch of $HST$ data observed more than $\sim 7$ years ago (before 2012). This provides considerable scientific value especially for dynamical studies of stellar systems or structures in and beyond the Milky Way (MW) halo, for which the member stars are generally faint. To illustrate the capabilities and demonstrate the accuracy of GaiaHub, we apply it to samples of MW globular clusters (GCs) and classical dwarf spheroidal (dSph) satellite galaxies. This allows us, e.g., to measure the velocity dispersions in the plane of the sky for objects out to and beyond $\sim 100$ kpc. We find, on average, mild radial velocity anisotropy in GCs, consistent with existing results for more nearby samples. We observe a correlation between the internal kinematics of the clusters and their ellipticity, with more isotropic clusters being, on average, more round. Our results also support previous findings that Draco and Sculptor dSph galaxies appear to be radially anisotropic systems.