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Kinematic Map of the Galactic Bulge Using HST Proper Motions

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Zhenhao Wang, Shude Mao, Hongjing Yang, Andrzej Udalski, Weicheng Zang, T. Dex Bhadra

Astrophysics of Galaxies · Submitted 30 Oct 2026 · 69 references

Pith's verdict: A genuinely useful new HST proper-motion catalogue whose headline bar-asymmetry claims still need the disc-contamination control the authors have in hand but did not apply to the main fits.

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arXiv ID2610.41395 (version 1)
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CitationAndrzej Udalski, Hongjing Yang, Shude Mao, T. Dex Bhadra, Weicheng Zang, Zhenhao Wang. "Kinematic Map of the Galactic Bulge Using HST Proper Motions." arXiv:2610.41395 (2026). https://pith.science/paper/JGAMMHY7
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The paper

The paper claims

HST proper motions of about 400,000 stars in 27 southern bulge fields show velocity dispersions rising toward the Galactic centre with a steeper gradient on the $l<0^\circ$ side, a kinematic signature of the bar's non-axisymmetric shape.

The reading

The paper tries to settle what the transverse motions of bulge stars look like across the whole southern bulge, not just in a few windows. From archival HST WFC3/UVIS images with 3–13 year baselines, anchored to Gaia DR3 for absolute calibration, it measures proper motions for about 400,000 stars in 27 fields at roughly 0.1 mas/yr precision for the brighter stars. The central finding is that proper-motion dispersions rise toward the Galactic centre on both sides, but the gradient is significantly steeper on the $l<0^\circ$ side, with slope ratios between the two sides of $0.70\pm0.09$ in $\sigma_l$ and $0.75\pm0.07$ in $\sigma_b$, which the authors read as the kinematic imprint of the bar's non-axisymmetric shape. Supporting evidence includes a mean anisotropy $\sigma_l/\sigma_b = 1.131\pm0.005$ and a normalised covariance that is negative on the $l>0^\circ$ side but essentially zero on the other. If the interpretation holds, the bulge is a bar-driven pseudo-bulge whose velocity-dispersion field is measurably lopsided, giving dynamical models a new, bulge-wide constraint.

What carries the argument

The machinery is a set of absolute proper motions built from two epochs of HST WFC3/UVIS F814W images per field, separated by 3 to 13 years and reduced with the hst1pass software, which performs PSF photometry and astrometry, corrects geometric distortion and the telescope's 'breathing' focus changes, and uses CTE-corrected _flc data. Gaia DR3 stars are cross-matched to place the relative HST motions on an absolute frame, with the OGLE/Uranus catalogue used as a cross-check on calibration errors. From these measurements the paper computes, per field, the mean motions $\langle\mu_l\rangle$ and $\langle\mu_b\rangle$, the intrinsic dispersions $\sigma_l$ and $\sigma_b$ with measurement errors subtracted in quadrature following the Jones (1970) formula, the anisotropy ratio $\sigma_l/\sigma_b$, and the normalised covariance $C_{lb}$, with bootstrap resampling (3,000 steps) for the errors. In the 16 fields with F606W data, colour–magnitude diagrams separate bulge red-clump stars (core-helium-burning giants used as standard candles) and red giants from foreground disc main-sequence stars; fits to the red-clump magnitude and colour provide extinction and reddening corrections, and double-Gaussian fits to the proper-motion distributions quantify the disc subpopulation that causes the observed non-Gaussianity.

Extended reading notes

Core claim

On its own terms, the paper claims that the bulge's proper-motion dispersion field is asymmetric about the longitude of the Galactic centre. Linear fits of $\sigma_l$ and $\sigma_b$ against longitude, done separately for the two sides, give slopes whose ratios between $l>0^\circ$ and $l<0^\circ$ are $0.70\pm0.09$ and $0.75\pm0.07$: the dispersions climb more steeply on the negative-longitude side, a pattern the paper attributes to non-axisymmetric structure such as the bar. Two further statistics reinforce the claim: the velocity distribution is anisotropic, with $\sigma_l/\sigma_b = 1.131 \pm 0.005$ and all fields above 1.0, and the normalised covariance between the two proper-motion components is negative for $l>0^\circ$ ($-0.085\pm0.003$) while essentially zero for $l<0^\circ$ ($0.004\pm0.011$). The paper also establishes that the motion of the bulge centre itself is reproduced: the mean proper motion in latitude matches that of the central black hole Sgr A*, and the longitude mean, extrapolated from its observed latitude trend to the plane, gives $-6.383\pm0.065$ mas/yr against Sgr A*'s $-6.379\pm0.026$ mas/yr. A caveat the paper itself states is that the covariance trend with latitude rests on limited $l<0^\circ$ data and needs further verification.

Load-bearing premise

The load-bearing assumption is that the asymmetry in dispersions and covariance between the two longitude sides is intrinsic to the bulge, not an artifact of foreground disc contamination that differs between the sides: the headline statistics are computed from all stars with $18.5 \le m_{\rm F814W} \le 23$ without colour-based disc removal, and only 16 of the 27 fields have the colour information needed to test that assumption directly.

Editorial extensions

If this is right

  • If the asymmetry is real, the bulge's velocity-dispersion field is measurably lopsided, and the slope ratio between the two longitude sides becomes a quantitative target that bar models must reproduce rather than a free parameter.
  • The agreement between the extrapolated $\langle\mu_l\rangle$ and Sgr A*'s proper motion strengthens the practice of treating Sgr A* as the dynamical anchor of the bulge centre in kinematic studies.
  • The quantified double-Gaussian disc component, offset by about 4 mas/yr in $\mu_l$ and concentrated in the brighter stars, gives future surveys a template for subtracting foreground contamination even where colour information is missing.
  • Matching the roughly 400,000-star absolute proper-motion catalogue against radial-velocity surveys would deliver a three-dimensional velocity-ellipsoid map of the bulge, extending the two-dimensional kinematics presented here.
  • The sign flip of $C_{lb}$ between the two longitude sides, together with its opposite latitude trends, implies the velocity ellipsoid is tilted and oriented differently on the two sides of the bar, a property future dynamical models can test directly.

Reading between the lines

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

  • The steeper dispersion gradient on the $l<0^\circ$ side is plausibly a near-side/far-side projection effect of a rotating bar: stars on the side closer to the Sun sample the bar's gravitational asymmetry differently, so the slope-ratio amplitude could in principle be converted into a constraint on the bar's viewing angle and pattern speed.
  • A testable extension the paper does not run is to push the same two-epoch pipeline through the existing negative-longitude HST fields from the literature, so the two-sided comparison rests on more than the seven fields at $l<0^\circ$ used here.
  • The red-clump calibration assumes the same intrinsic red-clump magnitude and colour in every field; if the fields differ systematically in metallicity along the bar, the extinction-corrected magnitude and colour bins of Section 4.5 could inherit a small bias that a metallicity-aware calibration would remove.
Original abstract

Constraining the Galactic bulge's non-axisymmetric dynamics requires proper motions in deep fields across many sightlines, yet crowding and extinction limit such measurements. Using multi-epoch observations of 27 fields in the southern Galactic bulge (-6$^\circ$ < l < 4$^\circ$, -5$^\circ$ < b < -1$^\circ$) from the ultraviolet-visible (UVIS) channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with time baselines of 3 to 13 years, and combining them with the absolute proper motion reference frame provided by Gaia Data Release 3 (DR3), we measure absolute proper motions for ~ 400,000 stars. Colour-magnitude diagrams are used to separate bulge stars from foreground disc stars, identify red clump stars and correct for interstellar extinction and reddening. Statistical analysis shows that the mean proper motion in Galactic latitude is consistent with that of the central supermassive black hole Sgr A*, while the mean proper motion in Galactic longitude is affected by disc stars but agrees with Sgr A* after extrapolation to the Galactic plane. Proper motion dispersions increase towards the Galactic centre and are asymmetric across Galactic longitude, with a steeper gradient for l < 0$^\circ$, revealing the bulge's non-axisymmetric bar structure. The velocity distribution shows clear anisotropy ($\sigma_l$/$\sigma_b$ = 1.131 $\pm$ 0.005). The normalised covariance is negative in the l > 0$^\circ$ region (~ -0.085) and close to zero in the l < 0$^\circ$ region. The proper motion distributions are non-Gaussian in both Galactic longitude and latitude, mainly due to contamination by foreground disc stars.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-11-02: 1 request, 6,689 input tokens.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.92 · human participants 0.01 · controlled comparison 0.22 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.53 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Constraining the Galactic bulge's non-axisymmetric dynamics requires proper motions in deep fields across many sightlines, yet crowding and extinction limit such measurements. Using multi-epoch observations of 27 fields in the southern Galactic bulge (-6$^\circ$ < l < 4$^\circ$, -5$^\circ$ < b < -1$^\circ$) from the ultraviolet-visible (UVIS) channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with time baselines of 3 to 13 years, and combining them with the absolute proper motion reference frame provided by Gaia Data Release 3 (DR3), we measure absolute proper motions for ~ 400,000 stars. Colour-magnitude diagrams are used to separate bulge stars from foreground disc stars, identify red clump stars and correct for interstellar extinction and reddening. Statistical analysis shows that the mean proper motion in Galactic latitude is consistent with that of the central supermassive black hole Sgr A*, while the mean proper motion in Galactic longitude is affected by disc stars but agrees with Sgr A* after extrapolation to the Galactic plane. Proper motion dispersions increase towards the Galactic centre and are asymmetric across Galactic longitude, with a steeper gradient for l < 0$^\circ$, revealing the bulge's non-axisymmetric bar structure. The velocity distribution shows clear anisotropy ($\sigma_l$/$\sigma_b$ = 1.131 $\pm$ 0.005). The normalised covariance is negative in the l > 0$^\circ$ region (~ -0.085) and close to zero in the l < 0$^\circ$ region. The proper motion distributions are non-Gaussian in both Galactic longitude and latitude, mainly due to contamination by foreground disc stars.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Constraining the Galactic bulge's non-axisymmetric dynamics requires proper motions in deep fields across many sightlines, yet crowding and extinction limit such measurements. background 0.67 0.04 0.03 asserted 1.00
s1 Using multi-epoch observations of 27 fields in the southern Galactic bulge (-6$^\circ$ < l < 4$^\circ$, -5$^\circ$ < b < -1$^\circ$) from the ultraviolet-visible (UVIS) channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with time baselines of 3 to 13 years, and combining them with the absolute proper motion reference frame provided by Gaia Data Release 3 (DR3), we measure absolute proper motions for ~ 400,000 stars. method 0.93 0.97 0.04 asserted 1.00
s2 Colour-magnitude diagrams are used to separate bulge stars from foreground disc stars, identify red clump stars and correct for interstellar extinction and reddening. method 1.00 0.87 0.02 asserted 1.00
s3 Statistical analysis shows that the mean proper motion in Galactic latitude is consistent with that of the central supermassive black hole Sgr A*, while the mean proper motion in Galactic longitude is affected by disc stars but agrees with Sgr A* after extrapolation to the Galactic plane. result 1.00 0.92 0.14 asserted 0.57
s4 Proper motion dispersions increase towards the Galactic centre and are asymmetric across Galactic longitude, with a steeper gradient for l < 0$^\circ$, revealing the bulge's non-axisymmetric bar structure. result 0.86 0.94 0.58 asserted 1.00
s5 The velocity distribution shows clear anisotropy ($\sigma_l$/$\sigma_b$ = 1.131 $\pm$ 0.005). result 1.00 0.94 0.12 asserted 1.00
s6 The normalised covariance is negative in the l > 0$^\circ$ region (~ -0.085) and close to zero in the l < 0$^\circ$ region. result 1.00 0.94 0.06 asserted 1.00
s7 The proper motion distributions are non-Gaussian in both Galactic longitude and latitude, mainly due to contamination by foreground disc stars. result 0.90 0.91 0.05 asserted 1.00

What kind of paper the classifier reports this is: empirical study 0.98 · evidence from observation or experiment 0.85.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Proper motion comparison between HST and Gaia DR3 (top) and OGLE/Uranus (bottom) after absolute proper motion calibration. The proper motions are in right ascension (𝛼) and declination (𝛿). 𝜎abs is the standard error of absolute proper motion calibration calculated by equation (2). N stands for the number of stars eventually used for the calibration (red), while the grey points are excluded due to the 4𝜎Δ𝜇 clipping. The black lines indicate the 1 : 1 relation. The left 2 panels show data from field 01 (𝑙 = 1.256◦ , 𝑏 = −2.651◦ ), and the right 2 panels show data from field 18 (𝑙 =…
Figure 2. CMD for Field 15 (𝑙 = 1.16◦ , 𝑏 = −2.73◦ ). This CMD is con￾structed from the Epoch 1 data of this field and contains 100,190 stars. The red data points in the upper right are mainly bulge RC/RGB stars, while the blue data points in the upper left are mainly foreground disc MS stars. The black dashed lines indicate the colour and magnitude ranges of RC/RGB stars and foreground MS stars, respectively. 3 STATISTICAL ANALYSES 3.1 Mean proper motion, dispersion, anisotropy, and covariance After obtaining the absolute proper motions of stars in each field, we transform the proper motion
Figure 3. The selected regions in the CMD for the fits of RC star magnitudes (black solid line) and colours (black dash-dotted line). The two panels show the distributions and fitted curves for colour (top) and magnitude (right) within the selected regions. The grey dashed lines indicate the fitted magnitude and colour of the RC stars. 3.2 Error estimation To estimate the errors of the above statistics, we adopt the bootstrap method. The most critical parameter in the bootstrap method is the number of resampling iterations (steps). If too few steps are chosen, the error estimates may not…
Figure 4. Distributions of the mean proper motions ⟨𝜇𝑙 ⟩ and ⟨𝜇𝑏 ⟩ as functions of 𝑙 and 𝑏. The purple dashed lines show the average values of the mean proper motions, and the grey dash-dotted lines show the proper motion of Sgr A* measured by Reid & Brunthaler (2004). In the upper-right panels, the purple solid line and shaded region show the linear fit and 1𝜎 error band. 6 4 2 0 2 4 6 l (°) 2.5 3.0 3.5 l ( m a s y r 1 ) 5 4 3 2 1 b (°) 2.5 3.0 3.5 l ( m a s y r 1 ) Koz owski et al. (2006) 6 4 2 0 2 4 6 l (°) 2.0 2.5 3.0 b ( m a s y r 1 ) Koz owski et al. (2006) 5 4 3 2 1 b (°) 2.0 2.5 3.0 b…
Figure 5. Distributions of the proper motion dispersions 𝜎𝑙 and 𝜎𝑏 as functions of 𝑙 and 𝑏, along with linear fits. The purple solid lines and shaded regions indicate the linear fits and their 1𝜎 error bands. In the left panels, the fits for 𝑙 > 0 ◦ and 𝑙 < 0 ◦ are performed separately. The outlying point with the largest 𝜎𝑙 is excluded from the 𝜎𝑙 fits. The grey solid lines and shaded regions show the trends of proper motion dispersions obtained by Kozłowski et al. (2006) near Baade’s Window. MNRAS 000, 1–14 (2026)
Figure 6. Distributions of 𝜎𝑙/𝜎𝑏 (top panels) and the normalised covariance 𝐶𝑙𝑏 (bottom panels) of bulge stellar proper motion dispersions as functions of Galactic coordinates 𝑙 and 𝑏. In the top panels, the purple dashed lines show the mean value, with the outlying largest 𝜎𝑙/𝜎𝑏 point excluded. The grey solid lines and shaded regions show the trends from Kozłowski et al. (2006). In the bottom panels, the data are separated into 𝑙 > 0 ◦ (purple) and 𝑙 < 0 ◦ (magenta), with their respective mean values (dashed lines); the lower-right panel shows linear fits to the two groups (solid lines and…
Figure 7. Absolute proper motion distributions of disc stars (upper panels) and bulge stars (lower panels) in Field 14 (𝑙 = 1.19◦ , 𝑏 = −2.78◦ ). The disc stars are the blue bright stars in the upper-left branch of the CMD (260 in total). The remaining stars are classified as bulge stars (61,205 in total). Each distribution is fitted with a double Gaussian. The two red dashed lines indicate the two individual Gaussians, and the black solid line shows the combined fit. The 𝜇𝑙 of Sgr A* (Reid & Brunthaler 2004) is marked with a vertical grey dashed line. 𝐶𝑙𝑏 between the two sides increases with…
Figure 8. Extinction-corrected magnitude 𝑚′ F814W (defined at the end of Section 3.4) ranges: 𝑚′ F814W < 20 (blue circles), 20 ≤ 𝑚′ F814W < 22 (green squares), 𝑚′ F814W ≥ 22 (red triangles). Fields 26 and 27 (marked with borders) are not corrected for extinction due to the small number of bright stars. Data points with fewer than 100 stars are not included. other has a mean 𝜇𝑙 shifted positively away from Sgr A* (i.e., disc stars). Since disc motion is mainly along the Galactic plane, 𝜇𝑙 is more affected by disc contamination, while 𝜇𝑏 is less affected, which explains why the two Gaussian me
Figure 9. Reddening-corrected colour 𝐶′ F606W−F814W (defined at the end of Section 3.4) ranges: 0.5 ≤ 𝐶′ F606W−F814W < 1.0 (blue circles), 1.0 ≤ 𝐶′ F606W−F814W < 1.5 (red squares), 1.5 ≤ 𝐶′ F606W−F814W < 2.0 (dark red triangles). Fields 26 and 27 (marked with borders) are not corrected for red￾dening due to the small number of bright stars. Data points with fewer than 100 stars are not included. populations. In Baade’s Window, Marchetti et al. (2022) also found a double Gaussian structure, and fitted bulge and residual foreground components with mean 𝜇𝑙 values of −6.525 and −1.781 mas yr−1 ,

Figures reproduced from arXiv:2610.41395 by the authors.

References

Discussion

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