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

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.

24 pages, 18 figures. Accepted for publication in ApJ

References in corpus (19)

• Array Programming with NumPy
• Are the Magellanic Clouds on their First Passage about the Milky Way?
• The kinematic status and mass content of the Sculptor dwarf spheroidal galaxy
• Stellar Velocities in the Carina, Fornax, Sculptor and Sextans dSph Galaxies: Data from the Magellan/MMFS Survey
• The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass
• Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results
• Hubble Space Telescope proper motion (HSTPROMO) catalogs of Galactic globular clusters. II. Kinematic profiles and maps
• The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements
• Proper Motions of Dwarf Spheroidal Galaxies from Hubble Space Telescope Imaging. IV: Measurement for Sculptor
• Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal
• Revised and new proper motions for confirmed and candidate Milky Way dwarf galaxies
• The core-cusp problem: a matter of perspective
• Breaking Beta: A comparison of mass modelling methods for spherical systems
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XVIII. Proper-motion kinematics of multiple stellar populations in the core regions of NGC 6352
• The power of teaming up HST and Gaia: the first proper motion measurement of the distant cluster NGC 2419
• Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]
• Revealing the Structure and Internal Rotation of the Sagittarius Dwarf Spheroidal Galaxy with Gaia and Machine Learning
• Hunting for intermediate-mass black holes in globular clusters: an astrometric study of NGC 6441
• Mapping Gaia parallax systematic errors over the sky with faint Milky Way stars

Cited by in corpus (2)

• Tidally induced velocity gradients in the Milky Way dwarf spheroidal satellites
• HubPUG: Proper Motions for Local Group Dwarfs observed with HST utilizing Gaia as a Reference Frame