Proposed Hubble Space Telescope Legacy Program Would Map Star Cluster Orbits in Andromeda and Triangulum Galaxies
A new white paper submitted to the Space Telescope Science Institute proposes a 'Hubble Local Group Astrometric Legacy' program that would use 2030s Hubble observations combined with archival data stretching back to the mid-1990s to measure the motions of star clusters in the Andromeda (M31) and Triangulum (M33) galaxies. The proposal leverages existing sky surveys — PHAT and PHATTER — which have already resolved tens to hundreds of millions of stars and identified thousands of star clusters in these nearby spirals. If approved, the program would produce the first systematic orbital maps of star clusters outside the Milky Way, shedding light on galaxy formation, cluster disruption, and the gravitational interaction between M31 and M33.
Researchers have submitted a white paper to STScI arguing that the Hubble Space Telescope's most valuable asset in the 2030s is not its optics alone, but the decades-long time baseline its archive provides. The proposed program would acquire new ACS/WFC and WFC3/UVIS imaging of selected M31 and M33 star cluster fields in the 2030s, then carefully cross-match those images against archival observations — some dating to the mid-1990s — to measure precise transverse (across-the-sky) velocities of star clusters. The PHAT and PHATTER survey programs already supply high-quality first-epoch data resolving individual stars across both galaxies and cataloguing thousands of clusters and compact background reference objects suitable for astrometric anchoring. The authors note that earlier targeted HST programs can extend the time baseline further in select fields, but only after rigorous calibration work addressing differences in cameras, chips, filters, dithering strategies, crowding, and reference frames. Scientific goals include constraining disk heating, cluster disruption rates, accretion histories, stream associations, the dynamical interaction between M31 and M33, and the mass distributions of both spirals. The program would also serve as a calibration testbed for the future Habitable Worlds Observatory, preserving geometric-distortion solutions, PSF and charge-transfer-efficiency models, and long-term reference frame infrastructure.
What's missing
The white paper does not specify the number of star cluster fields targeted, the total observing time requested, or the estimated cost of the proposed campaign. It also does not address the likelihood of HST remaining operational into the 2030s, which is a key practical uncertainty for the proposal's feasibility.
What different sources said
- arXiv astro-phCenter
From Images to Orbits: A Hubble 2030s Astrometric Legacy for the M31-M33 Star Cluster Systems
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