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arxiv: 2603.25848 · v2 · pith:J2ZXAOBWnew · submitted 2026-03-26 · 🌌 astro-ph.GA · astro-ph.IM· astro-ph.SR

The Hubble Missing Globular Cluster Survey. III. Astro-photometric catalogs, artificial-star tests, and improved absolute proper motions

M. Libralato , A. Bellini , D. Massari , M. Bellazzini , F. Aguado-Agelet , S. Cassisi , E. Ceccarelli , E. Dalessandro
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E. Dodd F. R. Ferraro C. Gallart B. Lanzoni M. Monelli A. Mucciarelli E. Pancino R. Pascale L. Rosignoli M. Salaris S. Saracino C. Zerbinati
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Pith reviewed 2026-05-21 10:09 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.IMastro-ph.SR
keywords globular clustersproper motionsHSTGaiaastro-photometric catalogsartificial-star testsMilky Way progenitorsgalaxy assembly
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The pith

Combining HST and Gaia data refines absolute proper motions of globular clusters to roughly three times the precision of Gaia alone.

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

This paper releases the official astro-photometric catalogs for the 34 globular clusters observed in the Hubble Missing Globular Cluster Survey. It describes the HST data reduction, shows how Gaia enables homogeneous photometric calibration across the set, and supplies artificial-star tests to quantify completeness and systematics. The central advance is the merging of HST and Gaia measurements to derive improved absolute proper motions. These motions are then applied to revise the links between the clusters and their likely parent galaxies, with first determinations for five systems. Readers care because tighter motions help map how the Milky Way assembled from earlier galaxies.

Core claim

We present the official astro-photometric catalogs for all globular clusters imaged by the Hubble Missing Globular Cluster Survey. By combining HST and Gaia data we refine the absolute proper motions of our GCs, reaching a precision ∼3 times better than that of Gaia alone. We use these new proper motions to update (and to determine for the first time for five systems) the associations between GCs and their putative galaxy progenitors.

What carries the argument

The joint use of HST high-resolution imaging with Gaia astrometry to produce absolute proper motions and uniform photometry across the sample.

Load-bearing premise

Systematic errors in HST data reduction, photometric calibration against Gaia, and absolute proper-motion derivation remain small enough that the reported factor-of-three precision gain is real.

What would settle it

Independent proper-motion measurements from a future Gaia data release or another high-precision facility that fail to confirm the claimed improvement or the revised galaxy-progenitor associations.

Figures

Figures reproduced from arXiv: 2603.25848 by A. Bellini, A. Mucciarelli, B. Lanzoni, C. Gallart, C. Zerbinati, D. Massari, E. Ceccarelli, E. Dalessandro, E. Dodd, E. Pancino, F. Aguado-Agelet, F. R. Ferraro, L. Rosignoli, M. Bellazzini, M. Libralato, M. Monelli, M. Salaris, R. Pascale, S. Cassisi, S. Saracino.

Figure 1
Figure 1. Figure 1: Comparison of our photometry with that in the Gaia synthetic-photometry catalog. In each row (=camera/filter), the first two panels from the left show the HST-Gaia magnitude difference for all stars in all our GCs as a function of magnitude and color, respectively. The red line is set to 0 as a reference, while the light-blue lines are set at the median (solid line) ± 1σ (dashed lines) magnitude difference… view at source ↗
Figure 1
Figure 1. Figure 1: We find an overall agreement between the two sets for [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Overview of the artificial-star tests for the GC ESO-452-11. Panel (a) shows the input CMD for the ACS/WFC data. Black dots are taken from the observed catalog, while gray points are artificial stars in input. Panel (b) presents the Hess diagram of recovered stars, color-coded by the completeness level as in the colorbar at the top. The analogs of these plots for the WFC3/UVIS parallel field are provided i… view at source ↗
Figure 3
Figure 3. Figure 3: Completeness level for the GC ESO-452-11 as a function of F606W magnitude (instrumental at the top and calibrated at the bottom) for the ACS/WFC (black line) and WFC3/UVIS (red line) data. The 50% completeness level is highlighted with a gray, dashed line. (2026, hereafter, Paper II) discuss two methods devised to facili￾tate the analysis of the MGCS GCs: a statistical approach purely based on photometry (… view at source ↗
Figure 4
Figure 4. Figure 4: Absolute PMs for a sample of GCs in our program. For each GC (two for each row), from left to right we present: the VPD of the Gaia DR3 PMs; the VPD of the PMs from GaiaHub; and a calibrated CMD. In the rightmost CMD, stars in red are likely cluster members according to their PM (made with an arbitrary magnitude-dependent PM selection). This Figure includes examples of a very-crowded region (Palomar 8); a … view at source ↗
Figure 5
Figure 5. Figure 5: Projections (left panel: E ver￾sus Lz ; right panel: L⊥ versus Lz) of the integral-of-motion space for the GCs an￾alyzed in our work, color-coded accord￾ing to the association with their progen￾itor (see legend in the right panel). The gray points represent all other GCs in￾cluded in the repository of Pace (2025) as a reference. Given the higher precision achieved, we combined our newly-computed PMs with p… view at source ↗
read the original abstract

The Hubble Missing Globular Cluster Survey (MGCS) has taken one of the last opportunities to complete the census of Galactic globular clusters (GCs) started by past Hubble Space Telescope (HST) programs, securing high-resolution data for 34 GCs never observed before by HST. The previous papers in the series have highlighted the astrometric and photometric potential of the project by analyzing a subsample of targets. We present, and release to the community, the official astro-photometric catalogs of the MGCS for all GCs imaged by this project. We describe the data reduction using state-of-the-art techniques designed for HST. We discuss the photometric calibration and show, for the first time, the synergy with the Gaia catalog to ensure homogeneous photometry across our data set. We compute artificial-star tests that can be used to assess systematics and the completeness level of our data. We combined HST and Gaia data to refine the absolute proper motions of our GCs, reaching a precision $\sim$3 times better than that of Gaia alone. We used these new proper motions to update (and to determine for the first time for five systems) the associations between GCs and their putative galaxy progenitors. This work continues decades-long efforts of large Treasury programs in sharing precise and accurate atlases to the community for studying GCs across a wide range of scientific endeavors.

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 / 3 minor

Summary. The paper describes the release of astro-photometric catalogs for 34 globular clusters from the Hubble Missing Globular Cluster Survey (MGCS). It outlines the HST data reduction using state-of-the-art methods, the photometric calibration against the Gaia catalog to achieve homogeneity, the performance of artificial-star tests for evaluating systematics and completeness, and the combination of HST and Gaia observations to refine absolute proper motions, achieving a precision approximately three times better than Gaia alone. These improved proper motions are then used to update associations with putative galaxy progenitors, including first-time determinations for five clusters.

Significance. Should the precision improvement in proper motions hold after accounting for systematics, this manuscript offers substantial value through its public data products, which will support a wide range of investigations into globular cluster dynamics, stellar populations, and the hierarchical assembly of the Milky Way. The artificial-star tests and catalog release are particularly noteworthy strengths for reproducibility and community use.

major comments (1)
  1. The assertion in the final paragraph of the abstract that the HST+Gaia combination reaches ∼3 times better precision than Gaia alone is central to the paper's astrometric contribution. The astrometric analysis section must provide a quantitative error budget demonstrating that systematic contributions from HST distortion corrections, reference-frame alignment, photometric zero-point matching to Gaia, and residual proper-motion offsets are sub-dominant to the reported random errors and do not introduce sample-wide correlations.
minor comments (3)
  1. The data reduction description is high-level; specifying exact pipeline versions, software, and any custom steps for HST processing would improve reproducibility.
  2. In the photometric calibration discussion, include quantitative details or a figure on the Gaia matching procedure and zero-point adjustments applied across the sample.
  3. A table comparing the new absolute proper motions to prior Gaia-only values for all 34 clusters would help readers assess the claimed improvement directly.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and for recognizing the value of the public data products and the potential of the refined proper motions. We address the single major comment below and have revised the manuscript to incorporate the requested analysis.

read point-by-point responses

  1. Referee: The assertion in the final paragraph of the abstract that the HST+Gaia combination reaches ∼3 times better precision than Gaia alone is central to the paper's astrometric contribution. The astrometric analysis section must provide a quantitative error budget demonstrating that systematic contributions from HST distortion corrections, reference-frame alignment, photometric zero-point matching to Gaia, and residual proper-motion offsets are sub-dominant to the reported random errors and do not introduce sample-wide correlations.

    Authors: We agree that a quantitative error budget is necessary to support the claimed precision improvement. In the revised manuscript we have expanded the astrometric analysis section with a dedicated subsection that presents a full error budget. This budget quantifies the contributions from HST geometric-distortion corrections, reference-frame alignment to Gaia, photometric zero-point matching, and residual proper-motion offsets between the two datasets. We demonstrate that each systematic term remains sub-dominant to the random errors for the final proper-motion uncertainties. We further show, through residual maps and cross-checks against independent proper-motion catalogs, that no significant sample-wide correlations are introduced. These additions directly substantiate the abstract statement while preserving the original scientific conclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper presents astro-photometric catalogs, artificial-star tests, and refined absolute proper motions obtained by combining public HST and Gaia archival data using standard reduction techniques. The central results are direct measurements and data products; the claimed factor-of-three precision gain is presented as an empirical outcome of the combination rather than a quantity forced by construction or by a fitted parameter defined in terms of itself. No load-bearing step reduces to a self-definitional loop, a renamed known result, or a self-citation whose validity depends on the present work. Prior papers in the series are cited for context but do not supply the uniqueness or ansatz that would make the current claims circular. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work relies on standard HST photometry pipelines and Gaia astrometric reference frames; no new free parameters, ad-hoc axioms, or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption HST data reduction techniques designed for crowded fields produce unbiased photometry and astrometry when standard quality cuts are applied.
    Invoked in the description of catalog production and artificial-star tests.
  • domain assumption Gaia proper motions provide an absolute reference frame that can be directly combined with HST relative motions without significant residual systematics.
    Central to the claim of ~3x improved absolute proper motions.

pith-pipeline@v0.9.0 · 5891 in / 1391 out tokens · 54523 ms · 2026-05-21T10:09:33.294515+00:00 · methodology

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