Stability of rotation axis of galactic disks in HESTIA suite

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.