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HST Proper Motions of Andromeda V and VI

T0 review · 2 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read The paper measures absolute proper motions for two M31 dwarf satellites and derives their orbits, finding And VI co-orbits the disk while And V counter-orbits.

desk verdict First PMs for And V/VI; And VI's zero point has an internal 2.2σ chip discrepancy that should be addressed before relying on the co-rotation claim. read the letter →

arxiv 2509.06215 v1 pith:5RLVDWIM submitted 2025-09-07 astro-ph.GA

Dana I. Casetti-Dinescu , Marcel S. Pawlowski , Terrence M. Girard , Kosuke J. Kanehisa , Max Martone , Alexander Petroski This is my paper · ORCID
classification astro-ph.GA
keywords propermotionsdwarfgalaxiesAndromedaVVIHSTastrometrysatelliteplanesorbitintegrationbackground
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper measures the absolute proper motions of two dwarf satellite galaxies of Andromeda (M31): Andromeda V and Andromeda VI/Pegasus, using Hubble Space Telescope images taken about 20 years apart. The motions are converted from relative to absolute using roughly 100 background galaxies per satellite as a fixed reference frame. From these motions, combined with line-of-sight velocities and updated distances, the paper computes orbits around M31 and finds that And VI is consistent with co-orbiting along M31's disk plane, while And V is consistent with counter-orbiting, though with larger uncertainties. And VI, the farthest M31 satellite with a measured proper motion, appears to stay beyond about 90 kpc from M31, implying weak tidal influence over its orbit.

What carries the argument

The method rests on converting a relative proper motion into an absolute one using background galaxies as zero-point reference objects. For each satellite, early-epoch WFPC2 images from 2000 and late-epoch ACS/WFC images from 2020 are processed with the hst1pass code, and WFPC2 positions are refined by a deep-learning centering model that overcomes undersampling. Chip-to-chip and epoch-to-epoch transformations use polynomial fits, and the absolute proper motion is obtained as the mean motion of satellite members minus the weighted mean motion of roughly 100 galaxies per satellite. Orbit parameters come from integrating backward in time with Monte Carlo draws over distance, line-of-sight velocity, and proper-motion uncertainties, using two M31 potentials.

What would settle it

Compare the galaxy-based absolute proper motions with those from a future Gaia data release whose faint-end errors reach about 0.05-0.1 mas/yr for the same fields; a discrepancy larger than the quoted ~10-20 microarcsecond-per-year errors would falsify the galaxy calibration, as would a test with synthetic galaxies inserted into the WFPC2 and ACS frames showing systematic epoch- or filter-dependent centroid offsets.

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Extended reading notes

Core claim

The paper reports two absolute proper motions: for And V, $(\mu_\alpha, \mu_\delta) = (26.1 \pm 21.5, -74.2 \pm 19.1)\ \mu\mathrm{as}\,\mathrm{yr}^{-1}$, and for And VI, $(\mu_\alpha, \mu_\delta) = (-1.6 \pm 12.3, -52.6 \pm 11.2)\ \mu\mathrm{as}\,\mathrm{yr}^{-1}$. These are the first proper motions for M31 satellites that lie far from the Great Plane of Andromeda but close to the M31 disk plane. Orbit integrations using two M31 mass models and two M31 proper motions show that And VI's orbit is well aligned with the M31 disk and co-rotates with it, while And V's orbit is consistent with a counter-rotating alignment, though less tightly constrained. Both satellites are bound to M31, and And VI remains at distances of at least about 90 kpc, supporting the interpretation that its cuspy central mass profile is preserved by a weak tidal field.

Load-bearing premise

Background galaxies have zero net motion and their centroids, measured with a stellar point-spread-function model, are unbiased between the 2000 WFPC2 and 2020 ACS epochs and filters.

Editorial extensions

If this is right

  • The sample of M31 satellites with measured proper motions grows to six, including two objects outside the Great Plane of Andromeda but near the M31 disk.
  • And VI's orbit, which keeps it beyond about 90 kpc from M31, corroborates the idea that its steep central mass profile results from a weak tidal history.
  • Both satellites are well bound to M31, so they do not directly constrain M31's total mass, but they can be used in satellite-based mass constraints.
  • The last pericenters of both satellites occurred on the far side of M31, offering a possible clue to the observed lopsidedness of the M31 satellite system.
  • A future Gaia data release with improved faint-end precision could check the galaxy-based zero-point against the EDR3 stars identified in these fields.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If And VI genuinely co-orbits the disk plane at about 280 kpc, the disk-aligned population of M31 satellites extends far beyond the Great Plane; a testable prediction is that other distant dwarf spheroidals near the disk plane will show the same co-rotation.
  • The deep-learning centering of undersampled WFPC2 images could be applied to other archival HST two-epoch pairs, potentially yielding proper motions for many faint Local Group systems.
  • A confirmed counter-orbiting And V would imply that disk-plane orbits around M31 are not all coherent in sense, complicating the single-rotating-plane picture.
  • The zero-point precision is limited by galaxy centroid scatter; using a larger or morphologically curated galaxy sample could push the absolute proper motions below the current 10-20 microarcsecond-per-year level.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper measures absolute proper motions of the M31 dwarf satellites Andromeda V and Andromeda VI using HST WFPC2 (epoch ~2000) and ACS/WFC (epoch ~2020) images spanning a 20-year baseline. The WFPC2 data are processed with a deep-learning centering model and updated astrometric calibrations; the absolute zero-point is set by roughly 100 background galaxies per field. The resulting proper motions are combined with published distances, line-of-sight velocities, and two M31 proper-motion determinations to integrate orbits under two M31 mass models. The authors report that And VI is consistent, within uncertainties, with a co-planar, co-rotating orbit around M31's disk, while And V is consistent (with large errors) with a counter-orbiting alignment. They also find both satellites are well bound to M31 and that And VI remains beyond ~90 kpc, implying weak tidal effects.

Significance. If the measurements are correct, this is a valuable observational contribution: it increases the sample of M31 satellites with measured proper motions to six, adds the most distant satellite (And VI, ~280 kpc) to that sample, and provides the first orbit determinations for two satellites far from the Great Plane of Andromeda. The paper is careful with random-error propagation (Monte Carlo orbit realizations), uses an external reference frame (background galaxies) for the absolute zero-point, and makes the data sets available via MAST DOIs. The orbit analysis explores multiple M31 mass models and proper-motion choices, and the conclusions are appropriately hedged for And V. The central results, if robust, bear on the kinematics of M31's satellite system, the lopsidedness of the satellite distribution, and the tidal history of And VI.

major comments (2)
  1. [§4, Table 2] The absolute zero-point rests on the assumption that background-galaxy centroids measured with a stellar PSF model (hst1pass for ACS, the deep-learning model for WFPC2) are unbiased between the 2000 WFPC2 and 2020 ACS epochs and across filters. The manuscript presents no test of this assumption and assigns no systematic error to it. The data in Table 2 already contain a check: for And VI, the two ACS-chip zero-points are (µ_cor_alpha, µ_cor_delta) = (−27±12, 72±11) and (34±25, 69±21) µas/yr, which disagree in RA by 61±28 µas/yr (≈2.2σ), while the adopted absolute RA proper motion is only −1.6±12.3 µas/yr. Because And VI's disk-alignment conclusion depends on the direction of its ~50 µas/yr proper-motion vector, a shift of this size—well within the chip-2 solution's uncertainty—can rotate the orbital pole and weaken the co-planar, co-rotating claim. I request either a quantitative systematic-error term for the galaxy-centroid zero-point or a validation (e.g., splitting galaxies by morphology or color, or using the available Gaia EDR3 stars as a cross-check), and a discussion of the chip discrepancy.
  2. [§2.1, §4] The F450W WFPC2 data are processed with the F555W deep-learning centering model and PSF library (Section 2.1), while the ACS data use filter-specific models. For unresolved stars this may be acceptable, but for background galaxies—which are the sole absolute reference—a filter- or morphology-dependent centroid offset between epochs would appear directly as a spurious proper motion. The paper notes only that random positional errors for F450W are a few percent larger; it does not address the possibility of a systematic bias. This is a load-bearing point for the absolute proper motions and should be investigated or explicitly argued to be negligible.
minor comments (5)
  1. [Figure 7 caption] The caption reads 'Same an in Fig. 6' and should be corrected to 'Same as in Fig. 6'.
  2. [§6] The word 'immediatly' in the summary is a typo and should be 'immediately'.
  3. [§4] The sentence 'The final zero-point correction is a weighted average of all galaxies with total proper-motion values less than 2 mas yr−1' should specify whether the cut is on the quadrature sum of the two proper-motion components or on each component separately.
  4. [§5.2] The statement that the choice of M31 mass and proper motion 'does not strongly change the expected orbital characteristics' is followed by pericenter-time ranges of 1.5–2.0 Gyr for And VI; please quantify what 'not strongly' means in the text.
  5. [Keywords] The keyword list appears incomplete; consider adding 'dwarf galaxies' and 'galaxy kinematics' to improve discoverability.

Circularity Check

0 steps flagged · score 1.0 of 10

No circular derivation: absolute proper motions are anchored to an external background-galaxy reference frame, and the orbit conclusions use independent literature inputs; self-citations supply only centering and calibration tools.

full rationale

The measurement chain is self-contained against an external reference frame. Relative proper motions are derived from stellar positions across the 2000 WFPC2 and 2020 ACS epochs (Section 3), and the conversion to absolute proper motion is obtained by subtracting a zero point set by on the order of 100 background galaxies per satellite (Section 4: 'we rely solely on background galaxies to determine the correction to absolute proper motions'). The galaxies provide an inertial anchor independent of the satellite's own motion; no parameter is fitted to the final orbital alignment. The orbit analysis (Section 5) combines these proper motions with RR Lyrae distances (Savino et al. 2022), line-of-sight velocities (Collins et al. 2013), M31 potentials (Patel et al. 2017), and two M31 proper-motion estimates (Sohn et al. 2020; Pawlowski & Sohn 2021), with Monte Carlo propagation of uncertainties. The paper explicitly tests both M31 proper motions and both mass models, and the co-rotation/alignment conclusions are stated to hold across realizations, so the choice of M31 proper motion is not load-bearing. Self-citations (Casetti-Dinescu et al. 2021, 2024a, 2024b) provide WFPC2 distortion corrections and the deep-learning centering model; these are calibration tools validated on stellar images, not quantities fitted to the target proper motions, and they do not encode the orbit results. The noted 2.2-sigma disagreement between the two ACS-chip zero-point solutions for And VI is an internal systematic-precision issue, not a circularity.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The central measurement depends on the background-galaxy reference frame and on cross-filter calibration transfers, both flagged in the text. The orbit analysis depends on external mass models, distances, and M31 proper motions, all acknowledged. No ad hoc entities are introduced; the four dynamical-friction parameters are the only hand-chosen numbers, and the authors state they have a small effect.

free parameters (4)
  • Dynamical friction mass for And V (M_DF, AndV) = 2e9 M_sun
    Chosen by hand to model Chandrasekhar dynamical friction for And V; the paper calls it an upper bound and states modifying it has little effect on the orbits (Sec 5.2).
  • Dynamical friction scale radius for And V (R_DF, AndV) = 0.7 kpc
    Companion scale radius for the And V Plummer sphere in Sec 5.2; a modeling choice, not fitted to data.
  • Dynamical friction mass for And VI (M_DF, AndVI) = 4e9 M_sun
    Chosen by hand for And VI's dynamical friction treatment in Sec 5.2; stated to have small impact on resulting orbits.
  • Dynamical friction scale radius for And VI (R_DF, AndVI) = 1.0 kpc
    Companion scale radius for the And VI Plummer sphere in Sec 5.2; a modeling choice.
assumptions (4)
  • domain assumption Background galaxies have zero net proper motion and provide an inertial reference frame
    Used in Sec 4 to convert relative proper motions to absolute ones; if the galaxies' measured centroids carry a systematic offset, every absolute proper motion and orbit shifts.
  • domain assumption The F555W deep-learning centering model and distortion corrections are valid for F450W WFPC2 data
    Sec 2.1 states no F450W model exists, so F555W calibrations are used with errors stated to be a few percent larger; this is an unverified transfer between filters.
  • domain assumption The Patel et al. (2017) M31 mass models (virial masses 1.5e12 and 2e12 M_sun) represent the gravitational potential
    Used for orbit integration in Sec 5.2; the orbit parameters change somewhat between the two models, but the qualitative conclusions of boundness and disk alignment are robust to this choice.
  • domain assumption Published distances (Savino et al. 2022) and M31 proper motions (Sohn et al. 2020; Salomon et al. 2021) are adopted as inputs
    These external measurements directly enter the space-velocity calculation and orbit integrations in Secs 5.1 and 5.2; the paper varies the M31 proper motion between two published values but does not derive them.

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Pith. "Pith review of HST Proper Motions of Andromeda V and VI." pith.science (2026). https://pith.science/paper/5RLVDWIM

@misc{pith2026250906215,
  author       = {Pith},
  title        = {Pith review of: HST Proper Motions of Andromeda V and VI},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5RLVDWIM}},
  note         = {Machine review of arXiv:2509.06215}
}
abstract

We measure the absolute proper motions of Andromeda V (And V) and Andromeda VI/Pegasus (And VI) dwarf galaxies, satellites of M31 located near its galactic plane. And VI is located the farthest from M31 among the six satellites with currently measured proper motions. A combination of ACS/WFC and WFPC2 exposures are utilized, spanning a 20-year time baseline. The WFPC2 exposures are processed using a recently developed deep-learning centering procedure as well as the most up-to-date astrometric calibration of the camera. We use on the order of 100 background galaxies per satellite to determine the correction to absolute proper motion. For And V we obtain an absolute proper motion of $(\mu_{\alpha} , \mu_{\delta})_{And\,V} = (26.1\pm21.5, -74.2\pm19.1)~\mu$as yr$^{-1}$. For And VI we obtain an absolute proper motion of $(\mu_{\alpha} , \mu_{\delta})_{And\,VI} = (-1.6\pm12.3, -52.6\pm11.2)~\mu$as yr$^{-1}$. Orbit integrations and analyses are made for these two Andromeda satellites using two estimates of both the mass and proper motion of M31. It is found that And V has an orbit consistent within errors with alignment with M31's disk and counter orbiting it, although this alignment is not well constrained. And VI's orbit is better determined and is very much consistent with co-orbiting with M31's disk. While currently at a distance of ~280 kpc from M31, And VI will remain beyond a distance of ~ 90 kpc from M31, thus experiencing low tidal influence compared to the other M31 satellites with known orbits. Both satellites are determined to be well-bound to M31.

Figures

Figures reproduced from arXiv: 2509.06215 by the authors.

Figure 1
Figure 1. Overlap between the early WFPC2 exposures (red symbols) and the late ACS/WFC exposures (blue symbols). Coordinates are a gnomonic projection of RA and DEC with the tangent point (ξ, η) = (0, 0) at satellite’s center. The one half-light ellipses are also shown according to the parameters in (McConnachie 2012). pipeline calibration. Detections, positions and magni￾tudes are obtained using hst1pass. The positions ob￾ta… view at source ↗
Figure 2
Figure 2. Proper-motion uncertainty as a function of instrumental magnitude in F814W for both ACS catalogs/chips and satellites. A moving median (1-mag bin) is represented with a solid line for each sample. The horizontal branch of each system is at magF 814W ∼ 21.5, where a higher stellar density is apparent. proper-motion solution, and propagated into its proper￾motion uncertainty estimate. Thus, the expected larger random … view at source ↗
Figure 3
Figure 3. Proper-motion catalogs: spatial distribution (top), and relative proper motions for stars with proper-motion uncer￾tainties ≤ 0.5 mas yr−1 (bottom). In the top panels we mark the direction of the satellite proper motion relative to that of M31 (red arrow) together with its error ellipse and the direction toward M31. The half-light ellipse, according to the parameters in (McConnachie 2012), is represented with a dash… view at source ↗
Figures from the paper (3 more)
Figure 4
Figure 4. Figure 4: Spatial distribution and motions of satellite galaxies around M31. The position and orientation of the M31 disk is indicated with a black ellipse, the white area in￾dicates the extent of the PAndAS footprint. Red upward tri￾angles mark dwarf galaxies with receding line…
Figure 6
Figure 6. Figure 6: Backward orbit integrations of And V around M31 over the last 5 Gyr for a high-mass (Mvir = 2×1012 M⊙) and low-mass (Mvir = 1.5 × 1012 M⊙) M31 model, in solid and dashed lines respectively. The blue orbits assume the HST+Sats M31 proper motion, while the black orbits a…
Figure 9
Figure 9. Figure 9: The M31-centric distance and total velocity of And V and VI using the HST+Sats M31 proper motion (blue square) and its weighted average with EDR3 (black dia￾mond). Escape velocity curves of the two M31 mass mod￾els adopted in this paper are shown as solid and dashed li…

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