HubPUG: Proper Motions for Local Group Dwarfs observed with HST utilizing Gaia as a Reference Frame

TL;DR

HubPUG is a new software method that combines HST observations with Gaia data to accurately measure the proper motions of dwarf galaxies in the Local Group, even for faint targets.

Contribution

The paper introduces HubPUG, a novel technique that leverages Gaia star data as a reference frame for HST proper motion measurements, improving accuracy for faint objects.

Findings

Recovered proper motions for Sculptor and Draco with comparable uncertainties to Gaia-only data.

Demonstrated the method's potential for measuring proper motions of M31 satellites, including Andromeda VII.

Showed Gaia stars can serve as effective reference points for HST proper motion studies.

Abstract

We present the method behind HubPUG, a software tool built for recovering systemic proper motions (PMs) of Hubble Space Telescope (HST) fields with two epochs of observations by utilizing stars observed by Gaia as a foreground frame of reference. HST PM experiments have typically relied on the use of distant background galaxies or quasi-stellar objects (QSOs) as stationary sources against which to measure PMs. Without consistent profiles, background galaxies are more difficult to centroid, but benefit on-aggregate from their large numbers. QSOs, though they can be fit with stellar point-spread functions, are sparse, with most fields containing none. Historically, the use of stars as references against which to measure PMs would have been difficult because they have individual PMs of their own. However, Gaia has now provided positions and PMs for over 1.4 billion stars, which are much more likely to be well-imaged in the fields around targets versus background sources, have predictable stellar profiles, and require less observing time per-image for good signal-to-noise. This technique allows us to utilize the power of Gaia to measure the PM of targets too faint for Gaia to observe itself. We have recovered PMs for the Milky Way satellites Sculptor and Draco with comparable uncertainties over HST-only and Gaia-only measurements, limited primarily by the current capabilities of the Gaia data. We also show the promise of this method for satellites of M31 with a new PM measurement for Andromeda VII.