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Measuring radial flows in disc galaxies

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Yi He, Filippo Fraternali, Pavel E. Mancera Piña, Enrico M. Di Teodoro

Astrophysics of Galaxies · Submitted 8 Oct 2026 · 82 references

Pith's verdict: A solid methodological advance for measuring radial flows in Hi discs, but the central claim of reduced bias over sequential fitting has only been validated on piecewise-linear warps—the kind that match the model's assumptions.

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arXiv ID2610.11588 (version 1)
Pith IDHF66RLJN, Pith's permanent record of this paper and its review
CitationEnrico M. Di Teodoro, Filippo Fraternali, Pavel E. Mancera Pi\~na, Yi He. "Measuring radial flows in disc galaxies." arXiv:2610.11588 (2026). https://pith.science/paper/HF66RLJN
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The paper

The paper claims

Simultaneously fitting the warp geometry and radial velocity yields a more accurate and less biased recovery of radial flows in disc galaxies, especially at inclinations below 60 degrees.

The reading

Gas accretion models predict that disc galaxies need fresh gas inflow to sustain star formation, but direct detection of radial flows is hindered by their weak signatures and confusion with disc warps. This paper presents a method that fits the warp geometry (position angle variations) and radial velocity simultaneously in the 3D tilted-ring kinematic model, rather than sequentially. Tests on realistic mock galaxies and TNG50 simulations show the method recovers radial motions more accurately, with systematic bias reduced especially for inclinations below 60 degrees. Applied to 12 Milky Way-like TNG50 galaxies, the method finds no global coherent radial flow, only local non-axisymmetric motions.

What carries the argument

The key is the two-stage 3D tilted-ring fitting workflow using 3DBarolo: in Step 3, after fixing the rotation curve and dispersion from a prior regularisation step, both the position angle (PA) and radial velocity ($V_{\rm rad}$) are fitted simultaneously using a $|\sin \theta|$ weighting scheme that emphasises minor-axis regions. The PA profile from this step is then regularised with a Bézier curve in Step 4 to produce the final $V_{\rm rad}$. This simultaneous treatment, plus a bootstrap error estimation, is designed to break the degeneracy between warps and radial flows.

Extended reading notes

Core claim

The central discovery is that simultaneously fitting the position angle (warp) profile and the radial velocity in a tilted-ring 3D kinematic fit significantly reduces the degeneracy between these two effects. When the warp is fit first with radial velocity set to zero, part of the radial flow signal can be absorbed into the warp parameters, leading to systematic underestimation of $V_{\rm rad}$, particularly at low and intermediate inclinations ($\leq 60^\circ$). By freeing both PA and $V_{\rm rad}$ in the same fitting step with appropriate weighting, the method recovers the true radial velocity profile with minimal bias. The method is validated on a grid of 500 mock galaxies with known warps and radial motions, and on synthetic HI datacubes from 12 symmetric TNG50 galaxies, achieving typical $V_{\rm rad}$ recovery accuracy of $\sim 3$ km/s and PA accuracy of $\sim 1^\circ$.

Load-bearing premise

The tilted-ring model assumes the gas disc can be described by a set of concentric rings, each with a single position angle and inclination, which may not capture complex 3D warps (e.g., S-shaped or U-shaped bending) or vertical flaring that could affect the recovered radial velocity.

Editorial extensions

If this is right

  • Existing measurements of radial flows that used sequential fitting may underestimate inflow rates, especially in galaxies with known warps and inclinations below $60^\circ$.
  • The method can be applied to existing HI surveys (e.g., HALOGAS, MHONGOOSE) to obtain unbiased radial flow measurements, directly testing accretion theories.
  • For galaxies with at least 15 independent resolution elements across the disc, the method recovers $V_{\rm rad}$ and PA warps with RMSE $< 4$ km/s and $< 1.2^\circ$, respectively.
  • The TNG50 result suggests that Milky Way-like galaxies in simulations do not exhibit coherent global radial flows, in contrast to some FIRE simulations, motivating further simulation comparisons.

Reading between the lines

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

  • The method could be extended to other emission lines (H$\alpha$, CO) to trace radial flows in molecular or ionised gas, but the smaller spatial extent of these tracers relative to HI may limit the radial baseline available for fitting.
  • A key test would be to apply this method to galaxies where independent evidence for radial inflow exists (e.g., from metallicity gradients or direct molecular gas detection) and check consistency, providing a direct observational falsification.
  • The absence of global radial flows in TNG50 Milky Way analogues, if confirmed with larger samples, would challenge cold-mode accretion scenarios that predict inward gas transport from the outer disc.
Original abstract

Numerous theoretical studies suggest that gas accretion from the intergalactic medium is essential to feed star formation in galaxies throughout cosmic time. However, the mechanism of gas accretion remains poorly understood, as direct evidence is lacking. In some models, gas accretion is expected to take place in the outer discs of galaxies, from where it should be transferred to the inner star-forming discs through radial flows. Finding such flows and quantifying them, therefore, opens the possibility of inferring the detailed properties of the accreting gas. Unfortunately, detecting radial flows is challenging due to the large disparity between radial and rotational velocities. Additionally, the distortions in velocity fields produced by these flows resemble the effects of warped structures, further complicating their quantification. To address this, we have developed a new methodology to measure axisymmetric radial velocities in disc galaxies. Relying on the 3D kinematic fitting software \texttt{$^{\rm 3D}$Barolo}, we have designed a dedicated workflow to measure axisymmetric radial motions in disc galaxies and to obtain robust uncertainty estimates through a bootstrapping approach. We validated this approach using realistic axisymmetric mock galaxies as well as a small sample of `observed' Milky Way-like galaxies drawn from the TNG50 hydrodynamic simulation. These tests demonstrate that our method works well and is capable of overcoming the degeneracy between radial motions and warps. The TNG50 galaxies we analysed have large regions in their discs with non-axisymmetric inflows and outflows, but they show, on average, no global trend of coherent radial flows.

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, 2026-10-09: 1 request, 8,305 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.62 · human participants 0.01 · controlled comparison 0.11 · randomized assignment 0.01 · states quantitative result 0.10 · compares with prior results 0.22 · mentions code or data availability 0.04.

The abstract, sentence by sentence

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

Numerous theoretical studies suggest that gas accretion from the intergalactic medium is essential to feed star formation in galaxies throughout cosmic time. However, the mechanism of gas accretion remains poorly understood, as direct evidence is lacking. In some models, gas accretion is expected to take place in the outer discs of galaxies, from where it should be transferred to the inner star-forming discs through radial flows. Finding such flows and quantifying them, therefore, opens the possibility of inferring the detailed properties of the accreting gas. Unfortunately, detecting radial flows is challenging due to the large disparity between radial and rotational velocities. Additionally, the distortions in velocity fields produced by these flows resemble the effects of warped structures, further complicating their quantification. To address this, we have developed a new methodology to measure axisymmetric radial velocities in disc galaxies. Relying on the 3D kinematic fitting software \texttt{$^{\rm 3D}$Barolo}, we have designed a dedicated workflow to measure axisymmetric radial motions in disc galaxies and to obtain robust uncertainty estimates through a bootstrapping approach. We validated this approach using realistic axisymmetric mock galaxies as well as a small sample of `observed' Milky Way-like galaxies drawn from the TNG50 hydrodynamic simulation. These tests demonstrate that our method works well and is capable of overcoming the degeneracy between radial motions and warps. The TNG50 galaxies we analysed have large regions in their discs with non-axisymmetric inflows and outflows, but they show, on average, no global trend of coherent radial flows.

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 Numerous theoretical studies suggest that gas accretion from the intergalactic medium is essential to feed star formation in galaxies throughout cosmic time. background 1.00 0.03 0.02 hedged possibility 0.96
s1 However, the mechanism of gas accretion remains poorly understood, as direct evidence is lacking. gap 0.99 0.04 0.02 asserted 1.00
s2 In some models, gas accretion is expected to take place in the outer discs of galaxies, from where it should be transferred to the inner star-forming discs through radial flows. background 1.00 0.04 0.02 hedged possibility 0.95
s3 Finding such flows and quantifying them, therefore, opens the possibility of inferring the detailed properties of the accreting gas. background 0.65 0.05 0.04 hedged possibility 0.94
s4 Unfortunately, detecting radial flows is challenging due to the large disparity between radial and rotational velocities. background 0.72 0.04 0.02 asserted 1.00
s5 Additionally, the distortions in velocity fields produced by these flows resemble the effects of warped structures, further complicating their quantification. background 0.79 0.05 0.02 asserted 0.99
s6 To address this, we have developed a new methodology to measure axisymmetric radial velocities in disc galaxies. contribution 0.93 0.97 0.05 asserted 1.00
s7 Relying on the 3D kinematic fitting software \texttt{$^{\rm 3D}$Barolo}, we have designed a dedicated workflow to measure axisymmetric radial motions in disc galaxies and to obtain robust uncertainty estimates through a bootstrapping approach. method 0.98 0.96 0.04 asserted 1.00
s8 We validated this approach using realistic axisymmetric mock galaxies as well as a small sample of `observed' Milky Way-like galaxies drawn from the TNG50 hydrodynamic simulation. method 0.85 0.94 0.04 asserted 0.98
s9 These tests demonstrate that our method works well and is capable of overcoming the degeneracy between radial motions and warps. result 0.77 0.93 0.17 asserted 1.00
s10 The TNG50 galaxies we analysed have large regions in their discs with non-axisymmetric inflows and outflows, but they show, on average, no global trend of coherent radial flows. result 1.00 0.94 0.63 asserted 1.00

What kind of paper the classifier reports this is: method or tool 1.00 · evidence from simulation or computation 0.97.

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. Toy models of a rotating disc with warps and/or radial motions, adopting a default inner position angle PA = 120◦ . The top and bottom rows show the warp–radial motion coupling in the suppressing and enhancing scenarios, respectively. Relatively strong warp amplitudes are adopted here to emphasise the qualitative differences between the two kinematic configurations. Left column: Line-of-sight velocity fields. The grey solid line shows the location of the highest projected velocities (kinematic major axis), while the green line marks the systemic-velocity contour. The black dashed…
Figure 2. 3D tilted-ring modelling of a model galaxy with a total ∆PA = 7.5 deg and a total ∆Vrad = −27 km s−1 . Top panels: left, line-of￾sight velocity fields of the input model galaxy, the best-fitting tilted-ring model obtained with our approach described in Section 2.1, and the corresponding residual velocity field (data–model). Black crosses mark the adopted galaxy centre, green thick contours trace the systemic velocity (∆Vlos = 0), and grey thin lines indicate the kinematic major axis. Right panels show position–velocity (PV) slices extracted along the average major and minor axes.…
Figure 3. Comparison between the recovered kinematic parameters derived with our fitting procedure (Section 2.1) and the input parameters. Top Row: Radial Velocity (Vrad). Bottom Row: Position Angle (PA). Columns (Left to Right): (1) All data points combined; (2) The warp-suppressing configuration; (3) The warp-enhancing configuration. In each subplot, the upper panel shows the scatter plot of Recovered vs. True values, and the lower panel displays the binned residuals (Recovered − True). The data points are colour-coded by galaxy inclination: i = 45◦ (blue circles), i = 60◦ (orange squares),…
Figure 4. Recovery of kinematic parameters for mock galaxies across different S/N values and spatial resolutions; ‘Res’ in the legend indicates the number of independent resolution elements per side of the disc. Colours indicate different S/N values and resolution configurations. The solid lines and shaded bands in the residual pan￾els show the binned median residuals and the 16th–84th percentile range. of both Vrad and the position angle. Overall, these results demon￾strate that our method works well across the tested S/N range and at moderate-to-high spatial resolution, but becomes less…
Figure 5. Recovery of kinematic parameters for mock galaxies with warps in PA and inclination. Columns from left to right: radial velocity (Vrad), position angle (PA), and inclination offset (∆i). In the Vrad and PA panels, points are colour-coded according to three bins of the absolute input inclination offset: |∆i| < 5 ◦ , 5◦ ≤ |∆i| ≤ 10◦ , and 10◦ < |∆i| ≤ 15◦ . In the inclination panel, points are colour-coded by the base input inclination. The dashed black lines indicate the ideal one-to-one relations in the upper panels and zero residuals in the lower panels. Solid lines in the residual…
Figure 6. Examples of three simulated galaxies illustrating the recovery of radial motions. The left panels show the intrinsic radial velocity fields of the gas in the face-on galaxy plane, with the colour bars indicating the radial velocity. The two dashed circles mark the inner and outer boundaries of the radial range over which the radial flow is measured, as defined by the radial extent of the H i disc above a surface density of 1M⊙/pc2 . The middle panels show the recovered (black) and true (blue) radial velocity profiles as a function of radius. The horizontal dashed lines indicate Vrad…
Figure 7. Comparison of Vrad (left) and PA (right) recovered with our method (orange) and with the approach of Di Teodoro & Peek (2021) (blue). The dashed black lines mark the ideal one-to-one relation (upper panels) and zero residual (lower panels). Solid lines and shaded regions in the residual panels indicate the binned median residuals and the 16th–84th range. gle set of geometrical and kinematic parameters. Consequently, the recovered radial velocity should be interpreted as the ring￾averaged radial component rather than a full 2D description of local non-circular motions. The method is…
Figure 8. Radial velocity profiles of the simulated TNG50 galaxies. The horizontal axis shows the radius normalised by the H i size, R/RHI, and the vertical axis shows the radial velocity Vrad. The dashed blue lines represent the profiles of individual galaxies, while the solid blue line shows the median profile of the sample. The shaded region indicates the 16–84 percentile range among the galaxies. The dashed black horizontal line marks Vrad = 0. Although our sample of simulated TNG50 galaxies consists of systems selected for their highly symmetric large-scale kine￾matics, the radial…

Figures reproduced from arXiv:2610.11588 by the authors.

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