Measuring radial flows in disc galaxies

arXiv:2610.11588 · astro-ph.GA · Submitted 2026-10-08 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Measuring radial flows in disc galaxies".

Jocelyn: The gist The authors developed a new methodology to measure axisymmetric radial motions in disc galaxies by simultaneously fitting disc geometry and radial velocity,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So we're looking at this paper called "Measuring radial flows in disc galaxies," and what they're doing here is trying to find a way to measure the actual motion of gas moving radially within a disc galaxy. The main idea is that gas accretion, which feeds star formation, usually comes from the outer parts of a galaxy and has to move inwards through these radial flows.

Jocelyn: Right, and what they're claiming is that they’ve developed a new way to measure these radial velocities by fitting both the disc geometry and the radial velocity at the same time. They say this helps them get around problems where warps confuse what is actually a flow versus just some kind of warp in the disc structure.

Subrahmanyan: From a theoretical side, that's interesting because those accretion scenarios, like cold-mode or hot-mode accretion, predict different amounts of gas transport from the outer to the inner regions, and this paper claims they can measure that transport using these flows.

Vera: Exactly what they’re pointing out is that earlier studies based on two-dimensional maps reached contradictory conclusions about these flows, so this new approach is trying to provide a clearer picture. The paper says their method relies on the software three deeBarolo to do a three-dimensional tilted-ring kinematic modelling of emission line datacubes to get the kinematics and geometry of rotating disc galaxies #pg3 <ref:2610.11588#pg1>.

Jocelyn: And they outline this whole procedure in four steps, starting with an initial ring-by-ring fit for rotation velocity Vrot and velocity dispersion profiles #pg4. Then there’s a second two-stage run to refine the rotation curve and regularize the profile by fitting it with a smooth Bézier curve as a function of radius #pg4.

Subrahmanyan: That step where they regularize the PA profile using that smooth Bézier curve sounds like a necessary regularization technique to handle those geometric uncertainties in the data #pg4. It’s about making sure you get a stable profile before you start looking at the flows themselves.

Vera: And then in Step three they fix those rotation velocity and dispersion values from the previous run while letting the position angle PA and radial velocity Vrad be free using a sin theta weighting scheme #pg4 <ref:2610.11588#pg1>. They regularize that final position angle profile with a Bézier function to get the final radial velocity profile #pg4.

Jocelyn: So, what this means for us is they are trying to quantify those flows more robustly by coupling the geometry and the motion measurement together instead of doing them separately. It’s a clever way to mitigate some of that degeneracy between warps and actual flows #pg2610 point 11588#pg3 <ref:2610.11588#pg1>.

Subrahmanyan: I think the implication here is that we might finally be able to distinguish between the different accretion scenarios they mentioned earlier, because measuring those radial flows directly would give us an observational signature of how gas is being transported inward #pg2610 point 11588#pg2 <ref:2610.11588#pg1>.

Conclusion: Vera: So wrapping up this paper "Measuring radial flows in disc galaxies," the authors developed this four-step fitting strategy using three deeBarolo to measure axisymmetric radial motions by simultaneously fitting disc geometry and radial velocity #pg2610 point 11588#pg4 <ref:2610.11588#pg1,Measuring radial flows in disc galaxies>.

Jocelyn: They tested their method on two families of model datasets, one where a warp structure amplifies the kinematic distortions from radial flows, and another where it suppresses those distortions #pg5. This showed that fitting the warp geometry and radial velocity at the same time significantly reduces bias when you fix the warp first and then try to determine the flow #pg10.

Subrahmanyan: And they also tested it on mock galaxies drawn from the TNG50 hydrodynamic simulation, including twelve Milky Way-like ones #pg5. The findings there showed that while local noncircular motions are present, the population overall doesn't show coherent radial flows within the disc #pg10.

Vera: But even though they didn't find a strong signal of coherent flows in those simulated galaxies, their mass-weighted mean recovered radial velocities were generally consistent with the true values within their standard deviations #pg10. That suggests the method is at least capturing what’s there, even if the overall picture is less dramatic.

Jocelyn: So for someone listening who just looks at galaxy kinematics, this paper suggests that if you can use a method like this that fits geometry and motion together, you might get a more reliable measurement of those inflows #pg3.

Subrahmanyan: And I think the real implication is in connecting it back to the bigger picture; if we can reliably measure these flows, we get direct observational evidence for how gas moves from the outer halo into the galaxy disc to fuel star formation #pg1.

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

Kapteyn Astronomical Institute · Leiden Observatory, Leiden University · Università di Firenze · INAF - Osservatorio Astrofisico di Arcetri

astro-ph.GA

Submitted: 2026-10-08

Updated: 2026-10-08

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 79/100

The gist: The gist The authors developed a new methodology to measure axisymmetric radial motions in disc galaxies by simultaneously fitting disc geometry and radial velocity, which mitigates degeneracies

Key concepts

Axisymmetric Radial Motions
These refer to the movement of gas within a galaxy's disc that is measured in a way that assumes symmetry around the galactic center. The study focuses on measuring these specific motions, which are crucial for understanding how gas moves toward or away from the galaxy over time.
Degeneracy between Warps and Flows
This is a problem where it is hard to tell if observed distortions in a galaxy's shape (warps) are caused by the movement of gas (radial flows) or vice versa. The new method solves this by fitting both structures at the same time, reducing this confusion.
3DBarolo Software
This is a specific computational tool used by the researchers to perform complex kinematic analyses on galaxy data. It allows the researchers to simultaneously fit multiple parameters, such as disc shape and velocity profiles, which is key to their new methodology.

Terminology

Summary

The gist The authors developed a new methodology to measure axisymmetric radial motions in disc galaxies by simultaneously fitting disc geometry and radial velocity, which mitigates degeneracies between warps and flows<ref:2610.11588#pg2>.

Motivation for the Study

Gas accretion is a fundamental driver of galaxy evolution, yet the mechanism remains poorly understood due to the challenge of detecting radial flows against rotational velocities and warped structures<ref:2610.11588#pg3>. The paper addresses this by developing a new methodology to measure axisymmetric radial velocities in disc galaxies using 3DBarolo software<ref:2610.11588#pg3>. This approach is designed to overcome the degeneracy between radial motions and warps, which previously complicated the quantification of these flows<ref:2610.11588#pg3>.

Methodology

The new methodology employs a four-step fitting strategy relying on the 3DBarolo software to simultaneously fit disc geometry and radial velocity<ref:2610.11588#pg4>. The procedure involves:

  1. Step 1: Obtain an initial ring-by-ring fit to obtain robust rotation velocity Vrot and velocity dispersion σ radial profiles<ref:2610.11588#pg4>.

  2. Step 2: Perform a second TWOSTAGE run to refine the rotation curve and velocity dispersion profile, regularizing the PA profile by fitting it with a smooth Bézier curve as a function of radius<ref:2610.11588#pg4>.

  3. Step 3: Proceed to the main radial flow fit where Vrot, σ, and i are fixed to values from Step 2, while the PA and Vrad are left free using the sin θ weighting scheme<ref:2610.11588#pg4>.

  4. Step 4: Regularize the PA profile obtained in Step 3 using a Bézier function to ensure a smooth radial variation, yielding the final Vrad profile<ref:2610.11588#pg4>.

The uncertainty estimation is performed using a bootstrapping approach, where preliminary residuals are computed by subtracting the best-fitting model datacube from the observed datacube<ref:2610.11588#pg4>. This involves generating 50 new mock datasets by randomly permuting spectral channels of the residual cube and refitting them<ref:2610.11588#pg4>.

Validation and Mock Galaxies

The method was validated using two families of model datacubes similar to Wang & Lilly (2023):

**(a) warp-enhancing, where the warp structure amplifies the kinematic distortions produced by radial flows; and (b) warpsuppressing, where the warp structure reduces such distortions<ref:2610.11588#pg5>. The mock galaxies were constructed using GALMOD in 3DBarolo, adopting realistic observational conditions such as a signal-to-noise ratio (S/N) of 10 and a spatial resolution of 30 independent resolution elements<ref:2610.11588#pg5>. The disc kinematics were defined by specific profiles for rotation velocity, velocity dispersion, and surface density<ref:2610.11588#pg5>. The testing included varying the transition radii and exploring different radial gradients in the PA and Vrad profiles<ref:2610.11588#pg5>. The relative signs of ∆RPA and ∆RVrad determined whether the warp structure enhanced or suppressed the distortion in the velocity field<ref:2610.11588#pg5>. The analysis also included testing variations of inclination angle with linear variations in PA as a function of radius<ref:2610.11588#pg5>. The results demonstrated that fitting the warp geometry and radial velocity simultaneously significantly mitigates the bias that can arise when the warp is determined first and then held fixed<ref:2610.11588#pg10>. The method was further tested on 3DBarolo mock galaxies drawn from the TNG50 hydrodynamic simulation, selecting 12 Milky Way-like galaxies to test its performance under realistic conditions<ref:2610.11588#pg5>. The analysis of these TNG50 galaxies indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The results for these simulated galaxies showed that the recovered profiles reproduced the overall inflow signatures seen in the intrinsic velocity fields, and the mass-weighted mean recovered radial velocities were consistent with their corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The results for these simulated galaxies showed that the recovered profiles reproduced the overall inflow signatures seen in the intrinsic velocity fields, and the mass-weighted mean recovered radial velocities were consistent with their corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>. The analysis of TNG50 galaxies also indicated that while local noncircular motions are present, the population as a whole does not exhibit coherent radial flows within the disc<ref:2610.11588#pg10>. The mass-weighted mean recovered radial velocities were generally consistent with the corresponding true values within their standard deviations<ref:2610.11588#pg10>.

Improvements for AI systems

  1. The improved AI system can accurately measure axisymmetric radial motions in disc galaxies by employing a four-step fitting strategy using 3DBarolo software to simultaneously fit the disc geometry and radial velocity, which better disentangle[s] the degeneracy between geometric warps and radial gas flows.

  2. The system can obtain robust uncertainty estimates through a specific bootstrapping approach where it generates a new residual datacube by randomly permuting spectral channels of the residual cube 50 times, yielding realistic uncertainties.

  3. The improved system can perform better in low-inclination systems by fitting the warp geometry and radial velocity simultaneously, as this strategy significantly mitigates the bias that can arise when the warp is determined first and then held fixed.

  4. The system will be capable of assessing observational limitations, specifically noting that the recovery accuracy is comparable for Res = 15 and Res = 30 but explicitly warning that at lower spatial resolutions (Res=4), the recovery starts to exhibit systematic deviations from the one-to-one relation, which should be considered when applying the technique to nearby dwarf galaxies.

  5. The system can distinguish between different physical scenarios by analyzing the weighting scheme; specifically, it can determine that the sin θ scheme is adopted as a default because it enhances the contribution from regions close to the minor axis, and its choice plays a much weaker role than the effect of simultaneously fitting radial motions together with the geometrical parameters.

  6. The improved system can be validated against complex, realistic dynamics by applying its methodology to TNG50 simulated galaxies, successfully recovering radial profiles of Vrot and Vrad that agree well with their intrinsic profiles despite azimuthal variations in the true velocity field.

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