Stability of rotation axis of galactic disks in HESTIA suite

arXiv:2610.30922 · astro-ph.GA · Submitted 2026-10-22 · Read on arXiv

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

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Stability of rotation axis of galactic disks in HESTIA suite".

Jocelyn: Here's the gist of the paper. In approximately 1/4 of Milky Way or M31 analogues in the HESTIA suite of simulations there is a significant, > 45◦, change in the axis of rotation orientation of the gaseous disk during the last 6 Gyr<ref:2610.30922#pg8>.

Vera: Large Orientation Change in Some MW and M31 Analogues Simulations from the HESTIA project are made in a box with Lbox = 100 Mpc/h side based on the reconstruction of initial conditions from the measured distribution of velocities of the galaxies around us<ref:2610.30922#pg3>.

Jocelyn: The set of models contains 14 realizations in which other, unmeasured parameters are varied<ref:2610.30922#pg3>.

Vera: The analysis considers only the last 38 snapshots from the simulations which corresponds to the last six Gyr (redshifts 0.

Paper summary: Vera: To summarize this paper, they’re using the HESTIA suite simulations to look at how long galaxy disk spin directions stay stable. The core idea is that for these galaxies, most of them—about twenty-one out of twenty-eight in their sample—keep the direction of their gaseous component spin consistent over those six billion years <ref:2610.30922#pg3>.

Jocelyn: That means past six billion years in the Local Group evolution, as simulated here, was a relatively quiet phase for disk orientation.

Subrahmanyan: It sets a baseline for us. If most disks are stable, it suggests that the large-scale structure around us isn't constantly violently reorienting everything on this timescale.

Vera: They also found that all the galaxies that actually did show a significant change in their rotation axis direction had very low angular momentum amplitudes about six billion years ago.

Jocelyn: That’s a key piece of evidence, linking the orientation flip to a specific state of angular momentum in the past.

Subrahmanyan: If we can link disk orientation flips to low initial spin, it helps us understand what kind of galaxy seeds are most susceptible to these kinds of recent structural changes.

Vera: The mechanism they point toward is satellite accretion—individual galaxies falling into the main one and causing that tilt.

Jocelyn: They found that this accretion causes the change in the L tilt, and in those cases, the satellite itself must have carried a lot of orbital angular momentum when it approached pericenter.

Subrahmanyan: That’s an important detail for theoretical work; it means the incoming satellite isn't just a passive object; it has significant momentum that drives this reorientation.

Vera: Now, they also looked at how resolution affects these results, and in some cases, changing the simulation resolution by eight times—making particles smaller—didn't change the orientation of about five out of six main galaxies over that time period.

Jocelyn: That suggests we can get a reasonably stable picture even with different particle resolutions in this regime.

Conclusion: Vera: So, looking at the title, "Stability of rotation axis of galactic disks in HESTIA suite," it really boils down to how long these disks keep pointing the same way. The authors found that most of these simulated galaxies are stable, which implies a quiet period for disk evolution in the Local Group over the last six billion years.

Jocelyn: It’s about finding those specific, rare events where a galaxy's orientation gets significantly tilted past forty-five degrees in that timeframe.

Subrahmanyan: What this means for us is that when we look at observations of real galaxies, if we see a disk that has flipped its axis recently, we should be thinking about recent interactions or accretion events rather than some slow, steady evolution.

Vera: The authors also highlighted that the mechanism they detect—satellite accretion causing the flip—can help us figure out why some large-scale structure elements don't always correlate cleanly with rotation axes in our observations.

Jocelyn: Because if satellites can flip a galaxy’s orientation, it complicates trying to link its current spin to something bigger, like the surrounding dark matter halo or filaments.

Subrahmanyan: It suggests that the environment and local accretion history are more important drivers of these disk orientations than just the initial conditions of the large structure itself.

Naira R. Arakelyan, Sergey V. Pilipenko, Stefan Gottlöober, Noam I. Libeskind, Gustavo Yepes, Yehuda Hoffman

Astro Space Center of P.N. Lebedev Physical Institute, Moscow, Russia · Leibniz-Institut für Astrophysik Potsdam (AIP) · Departamento de Fúrsica Teórica, Módulo 15, Facultad de Ciencias, Universidad Autona de Madrid · Centro de Investigacin Avanzada en Fúsica Fundamental (CIAFF), Facultad de Ciencias, Universidad Autona de Madrid · Racah Institute of Physics, Hebrew University

astro-ph.GA

Submitted: 2026-10-22

Updated: 2026-10-22

Comments: 8 pages, 4 figures

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

The gist: The gist: In approximately 1/4 of Milky Way or M31 analogues in the HESTIA suite of simulations there is a significant, > 45◦, change in the axis of rotation orientation of the gaseous disk during

Key concepts

Axis of Rotation Measurement
To measure the rotation axis, researchers tracked the angular momentum vector ($\vec{L}$) of gas particles within a sphere around the galaxy center. This measurement is crucial for determining if the disk's spin direction has changed over time.
L Tilt
The 'L tilt' is a measure of how much the rotation axis orientation has changed over six billion years. A change greater than 45 degrees is considered significant because it implies the new rotation vector ($\vec{L}'$) aligns better with a different axis.
Satellite Accretion
The primary mechanism proposed for these large orientation changes is the accretion of individual satellites. When a massive satellite approaches its orbit, it carries significant angular momentum, which can directly cause the host galaxy's rotation axis to flip or tilt.
Angular Momentum Amplitude
The amplitude of angular momentum refers to the magnitude of $\vec{L}$. The study found that galaxies showing a significant orientation change had low angular momentum amplitudes six billion years ago, supporting the quiet phase hypothesis.

Summary

The gist: In approximately 1/4 of Milky Way or M31 analogues in the HESTIA suite of simulations there is a significant, > 45◦, change in the axis of rotation orientation of the gaseous disk during the last 6 Gyr<ref:2610.30922#pg8>.

Large Orientation Change in Some MW and M31 Analogues

Simulations from the HESTIA project are made in a box with Lbox = 100 Mpc/h side based on the reconstruction of initial conditions from the measured distribution of velocities of the galaxies around us<ref:2610.30922#pg3>. The set of models contains 14 realizations in which other, unmeasured parameters are varied<ref:2610.30922#pg3>. The analysis considers only the last 38 snapshots from the simulations which corresponds to the last six Gyr (redshifts 0.

Axis of Rotation Measurement

To define the axis of rotation one must choose particle type and select a region to measure angular momentum<ref:2610.30922#pg4>. For the former, the choice of gas particles is motivated by the physics of disk galaxies: the gaseous disk is maintained in equilibrium by its rotation<ref:2610.30922#pg4>. The angular momentum vector L⃗ of all gas particles (actually, pseudo gas particles defined by the AHF halo finder based on the gas distribution in the AREPO Voronoi cells) found in a sphere around the galaxy center given by AHF (we do not select only gravitationally bound particles) is measured<ref:2610.30922#pg4>. The sphere radius is chosen to be 20 kpc; we varied it from 5 to 40 kpc and verified that our results do not change significantly<ref:2610.30922#pg4>.

Causes of Orientation Change

The authors track gas mass accretion histories (MAHs) of the two main LG members, absolute value of the angular momentum, distance and angular position of the second LG member, distances, masses and angular momenta of the three most massive satellites<ref:2610.30922#pg5>. They found that all galaxies that have significantly changed their orientation have one common property: they all had low angular momentum, at least 2 times below the median value, at the time we start our analysis (6 Gyr ago)<ref:2610.30922#pg5>. The MAHs of galaxies, measured at several radii, do not show significant changes except the rapid growth when a satellite crosses the corresponding shell<ref:2610.30922#pg6>. The distance between the two main galaxies also changes steadily by no more than 15% during this interval, which cannot explain rapid and significant L⃗ changes by tidal effects<ref:2610.30922#pg6>.

Satellite Accretion as a Mechanism

The authors found that in 6 out of 7 such cases the change is likely caused by the accretion of individual satellites, while in one case such a connection is not obvious<ref:2610.30922#pg2>. The analysis shows that the L tilt change is directly related to the first or second massive satellite approaching to its orbit pericenter<ref:2610.30922#pg6>. In all of these cases the satellite itself carries large orbital angular momentum (larger than the momentum of the galaxy in the beginning of our time interval)<ref:2610.30922#pg6>. The satellites lose their orbital angular momentum and mass during this approach<ref:2610.30922#pg6>.

Resolution Effects and Conclusion

The HESTIA suite contains 3 realizations computed with the effective resolution 81293 inside the zoom region, (i.e., all particle masses 8 times smaller than in the 14 realizations we used above)<ref:2610.30922#pg7>. From those 3 realizations, in 5 out of 6 main galaxies their gaseous disk axis of rotation differs from the lower resolution version by no more than 20◦ during last 6 Gyr<ref:2610.30922#pg7>. The galaxies which experienced such a change in their angular momentum direction all had low angular momenta 6 Gyr ago<ref:2610.30922#pg5>. While our findings support the idea that satellite accretion is the main cause of the late changes in the angular momentum, in one of the realizations we could not confirm the direct role of satellites<ref:2610.30922#pg8>. The change of orientation also was accompanied by the growth of the amplitude of angular momentum<ref:2610.30922#pg5>.

How it works

The critical level for tilt is chosen because if the rotation axis is changed by more than 45 degrees, one can find that the new L⃗ ′ is more aligned with some other axis: (L⃗ ′, ⃗a′) > (L⃗ ′, ⃗a1) where ⃗a′ ⊥ ⃗a1<ref:2610.30922#pg4>. The propagation from large to small radii is interpreted as an indication of a satellite with large angular momentum crossing shells while approaching to the center<ref:2610.30922#pg6>.

Conclusions

Our investigations of the analogues of MW and M31 in the HESTIA suite of simulations have shown that 21 out of the 28 HESTIA MW and M31 analogues retain the direction of their (gaseous component) spins during the last 6 Gyr<ref:2610.30922#pg8>. This implies that past 6 Gyr of LG evolution is a quiet phase<ref:2610.30922#pg8>. The galaxies which experienced such a change in their angular momentum direction all had low angular momenta 6 Gyr ago<ref:2610.30922#pg5>.

Acknowledgments

The authors thank Alexander Knebe for useful comments<ref:2610.30922#pg8>. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (www.gauss-centre.eu) for funding HESTIA project by providing computing time on the GCS Supercomputer SuperMUC at Leibniz Supercomputing Centre (www.lrz.de)<ref:2610.30922#pg8>. The GY would like to thank the Spanish Agencia Estatal de Investigaci´on of the Ministerio de Ciencia e Innovaci´on (AEI/MICINN) for financial support under the project grant PID2024-156100NB-C21<ref:2610.30922#pg8>. YH acknowledges the support of the Israeli Science Foundation (grant 1450/24)<ref:2610.30922#pg8>.

How it works

The paper details how to measure the orientation change of L⃗ as the angular distance θ(t) between the angular momentum vector at z = 0 (t = t0) and at any other given time<ref:2610.30922#pg4>. This angle measures from 0 to 180 degrees<ref:2610.30922#pg4>. The evolution of this angle, which we call the “L tilt”, over the last 6 Gyr is shown in Fig. 1 for all 14 realizations and for both main LG members<ref:2610.30922#pg4>. The critical level of the tilt is chosen because if the rotation axis is changed by more than 45 degrees, one can find that the new L⃗ ′ is more aligned with some other axis: (L⃗ ′, ⃗a′) > (L⃗ ′, ⃗a1) where ⃗a′ ⊥ ⃗a1<ref:2610.30922#pg4>. The evolution of the angle, which we call the “L tilt”, over the last 6 Gyr is shown in Fig. 1 for all 14 realizations and for both main LG members<ref:2610.30922#pg4>. The critical level of the tilt is chosen because if the rotation axis is changed by more than 45 degrees, one can find that the new L⃗ ′ is more aligned with some other axis: (L⃗ ′, ⃗a′) > (L⃗ ′, ⃗a1) where ⃗a′ ⊥ ⃗a1<ref:2610.30922#pg4>. The evolution of this angle, which we call the “L tilt”, over the last 6 Gyr is shown in Fig. 1 for all 14 realizations and for both main LG members<ref:2610.30922.

Improvements for AI systems

  1. Improved orbital stability prediction for satellite systems: The AI can now predict changes in gravitational potential that can distort the satellite orbits by modeling disk axis drift, as this is a consequence of disk axis drift will cause the change of gravitational potential which can distort the satellite orbits.

  2. Enhanced correlation analysis between structure and rotation: The system can better identify or rule out correlations between a galaxy's rotation axis and large-scale structure elements because it understands that satellite accretion can flip galaxy orientation, [which] can explain the difficulties in identifying correlations between the rotation axis and the orientation of large-scale structure elements.

  3. Satellite accretion mechanism classification: The AI can classify whether an observed change in a galaxy's rotation axis is likely caused by satellite accretion by comparing it to models where 6 out of 7 such cases the change is likely caused by the accretion of individual satellites, while in one case such a connection is not obvious.

  4. Angular momentum growth phase identification: The system can determine if a galaxy has undergone a quiet phase of evolution in the LG by analyzing whether it had angular momentum amplitudes below the sample median 6 Gyr ago, supporting this conclusion when orientation changes are observed.

  5. Satellite mass and angular momentum tracking: The AI can track the specific impact of satellites by identifying events where the L tilt change is directly related to the first or second massive satellite approaching to its orbit pericenter and quantifying how the satellite's properties, such as their gaseous masses at the beginning of our analysis vary between 4 x 109 Msun and 10 10 Msun.

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