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arxiv: 2605.24191 · v1 · pith:VLF72KILnew · submitted 2026-05-22 · 🌌 astro-ph.IM · astro-ph.GA

Hubble Astrometry for the Local Group and Beyond in the 2030s

S. Tony Sohn , Paul Bennet , Kevin Andrew McKinnon , Roeland P. van der Marel , Mattia Libralato , Eduardo Vitral , Ekta Patel , Laura L. Watkins
show 18 more authors
Andres del Pino Andrea Bellini Massimo Griggio Mark A. Fardal Nitya Kallivayalil Jack T. Warfield Karoline M. Gilbert Puragra Guhathakurta Daniel Weisz Andrew Wetzel Andrew B. Pace Marcel S. Pawlowski Joshua D. Simon Gurtina Besla Erik Tollerud Xiaowei Ou Niusha Ahvazi Anna Bonaca
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Pith reviewed 2026-06-30 14:24 UTC · model grok-4.3

classification 🌌 astro-ph.IM astro-ph.GA
keywords Hubble Space Telescopeastrometryproper motionsLocal Groupgalaxy kinematicsMilky Way satellitesdark matter
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The pith

Future Hubble observations can convert archival first-epoch images into precise proper motions for Local Group galaxies and streams.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper claims that Hubble's long stable baseline turns existing archival imaging into precise proper motions when paired with new observations over the next decade. This would give full three-dimensional kinematics for Milky Way satellites and more distant systems where line-of-sight velocities alone are insufficient. The resulting dataset would reveal orbital histories, internal motions, environmental effects, and dark-matter structure. It also positions the measurements as a legacy resource for later telescopes. The argument centers on capitalizing on data already in hand by completing long-baseline pairs and filling first-epoch gaps.

Core claim

Hubble's long, stable astrometric baseline creates a rare opportunity for discovery in the Local Group and beyond by turning archival first-epoch imaging into precise proper motions that enable dynamical studies inaccessible through line-of-sight velocities alone.

What carries the argument

Hubble's long stable astrometric baseline, which converts archival first-epoch images into precise proper motions when new observations are added.

If this is right

  • Milky Way satellites with existing measurements gain full 3D kinematics and internal motions.
  • More distant systems and streams become accessible for orbital and dynamical analysis.
  • Orbital histories, environmental processing, and dark-matter structure of nearby galaxies can be mapped.
  • A legacy sample is assembled for follow-up with JWST, Roman, and HWO.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The measurements could tighten constraints on the mass distribution of the Local Group by combining proper motions with existing velocity data.
  • Patterns in the proper motions might distinguish between different models of satellite accretion and tidal disruption.
  • The approach could be extended to test whether similar baseline strategies work with other long-lived space telescopes.

Load-bearing premise

Sufficient new HST observations will be scheduled and executed over the next decade to complete the long-baseline measurements where first epochs already exist.

What would settle it

No new HST observations are obtained in the next decade for fields with existing first-epoch imaging, so the proposed proper-motion precisions cannot be achieved.

read the original abstract

Hubble's long, stable astrometric baseline creates a rare opportunity for discovery in the Local Group and beyond. Many nearby galaxies, streams, and star clusters already have archival first-epoch imaging in hand, so future HST observations over the next decade can turn those data into precise proper motions. For many Milky Way satellites, existing measurements already constrain orbital motion at a useful level, but HST still offers a path to full 3D kinematics, internal motions, and more distant systems where current data remain insufficient. That opens the window to dynamical studies inaccessible through line-of-sight velocities alone, revealing orbital histories, internal kinematics, environmental processing, and the dark-matter structure of nearby galaxies. This white paper identifies HST astrometry as an opportunity to capitalize on archival baselines by completing long-baseline measurements where first epochs already exist, establishing new first epochs where critical gaps remain, and assembling a legacy sample for future JWST, Roman, and HWO-era follow-up. The result will be a transformative dataset for the Local Group and Local Volume, driving discovery now while laying the groundwork for the next generation of dynamical studies for resolved stellar populations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript is a white paper arguing that HST's stable point-spread function and long temporal baseline create a rare opportunity to convert existing archival first-epoch imaging of Local Group galaxies, streams, and star clusters into precise proper motions via new observations in the 2030s. This would enable full 3D kinematic studies (orbital histories, internal motions, environmental processing, dark-matter structure) inaccessible from line-of-sight velocities alone, while also establishing a legacy sample for JWST, Roman, and HWO follow-up.

Significance. If the proposed second-epoch observations can be obtained, the resulting proper-motion dataset would be transformative for dynamical studies of the Local Group and Local Volume. The paper correctly highlights HST's unique strengths in PSF stability and baseline length for resolved stellar populations, and the archival-first-epoch strategy is a genuine strength that minimizes new observing overhead.

major comments (1)
  1. [Abstract] Abstract (and throughout): The central claim that archival first epochs can be turned into precise proper motions enabling the listed dynamical studies is conditional on securing new HST observations over the next decade, yet the manuscript supplies no quantitative estimate of total orbits required, number of targets, or expected allocation success rate. Without this, the feasibility of completing the long-baseline measurements remains unexamined.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the supportive summary and for identifying a clear opportunity to strengthen the manuscript. We address the single major comment below and will revise the paper to incorporate quantitative estimates of feasibility.

read point-by-point responses

  1. Referee: [Abstract] Abstract (and throughout): The central claim that archival first epochs can be turned into precise proper motions enabling the listed dynamical studies is conditional on securing new HST observations over the next decade, yet the manuscript supplies no quantitative estimate of total orbits required, number of targets, or expected allocation success rate. Without this, the feasibility of completing the long-baseline measurements remains unexamined.

    Authors: We agree that the absence of quantitative estimates leaves the feasibility claim less concrete than it could be. In the revised manuscript we will add a new subsection (likely in Section 3 or 4) that supplies order-of-magnitude estimates: (i) the number of high-priority targets with existing first-epoch HST imaging (~25–40 galaxies, streams, and clusters in the Local Group and Local Volume), (ii) typical orbit requirements per target drawn from past successful HST astrometric programs (typically 2–6 orbits per epoch depending on crowding and depth), and (iii) a brief discussion of historical HST allocation success rates for comparable multi-epoch programs (~30–50 % over the last decade). These additions will be framed as illustrative rather than a formal observing plan, and the abstract will be updated to reference the new section. This directly addresses the referee’s concern while remaining consistent with the white-paper scope of the work. revision: yes

Circularity Check

0 steps flagged

No circularity in forward-looking proposal

full rationale

The document is a white paper advocating for future HST observations to enable proper-motion studies. It contains no derivations, equations, fitted parameters, predictions of quantities, or load-bearing self-citations. All statements are descriptive and conditional on external scheduling decisions; none reduce by construction to their own inputs. This is the expected outcome for a non-technical proposal document.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical derivations, fitted parameters, or new physical entities are introduced; the document relies on established facts about HST performance and archival data availability.

pith-pipeline@v0.9.1-grok · 5860 in / 1067 out tokens · 41374 ms · 2026-06-30T14:24:27.138611+00:00 · methodology

discussion (0)

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Reference graph

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