Disk tilts in Milky Way-like galaxies and their link to mergers

summary

Video file (mp4)

The gist

The gist: Disk tilts in Milky Way–like galaxies and their link to mergers shows that 64 of 93 galaxies have experienced a net disc tilt of more than 30◦ over the past 9 Gyr, with type SE events

In short

The study investigated disk reorientation in 93 Milky Way-like galaxies from TNG50 simulations by linking them to mergers. It found that 64 galaxies have a net disc tilt exceeding 30 degrees over nine billion years, driven primarily by minor mergers. These smooth evolution events (Type SE) can last for extended periods, allowing the cold stellar disk to maintain stability and continue growing despite the reorientation.

Key concepts

Disk Tilt Events
These are specific instances where a galaxy's stellar disk changes its orientation relative to its spin axis. The researchers classified them into two types: Type SE events, which happen during smooth evolution without major disruption, and Type RG events, which involve rapid disruption and regrowth.
Minor Mergers
The study focuses on how smaller galaxies merge with the main galaxy to cause disk tilts. These mergers are often the drivers of the tilt, especially in Type SE events where satellites have masses comparable to or less than the main galaxy's stellar mass.
Orbital Configuration
The path and spin direction of a satellite during a merger significantly influence how it tips the main disk. Satellites on tangential orbits with specific angular momentum relative to the disk's spin are particularly effective at driving large tilt angles.
Type SE Events
These events represent smooth evolution of the stellar disk where disruption is minimal. They are characterized by slow tilt rates (below 10 degrees per Gyr) and can persist for long durations, allowing the galaxy to maintain a stable, thin stellar disk throughout the reorientation process.

Terminology used across episodes

This episode discusses

The paper

Disk tilts in Milky Way-like galaxies and their link to mergers · Read on arXiv

Runsheng Cai, Ling Zhu, Jiang Chang, Lan Zhang, Xiangxiang Xue, Xi Kang

Shanghai Astronomical Observatory, Chinese Academy of Sciences · School of Astronomy and Space Sciences, University of Chinese Academy of Sciences · Purple Mountain Observatory, Chinese Academy of Sciences · National Astronomical Observatories, Chinese Academy of Sciences · Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University · Institute for Astronomy the School of Physics, Zhejiang University

We investigated disk reorientation and its relationship to mergers in a sample of 93 Milky Way-mass galaxies drawn from the TNG50 simulations. From this sample, we identified 117 individual disk-tilt events with clearly defined start and end times, and we divided them into two classes based on whether the disk evolves smoothly or undergoes rapid disruption and regrowth during the tilt duration. For each tilt event, we linked the most massive merger that occurs in close temporal association with it. We focus on those tilt events occurring during phases of smooth disk evolution, which typically show tilt rates below 10 degrees per Gyr, persist for 2-9 Gyr, and are generally connected to minor mergers. Roughly 40% of these events reach tilt angles exceeding 30 degrees, most often triggered by satellites whose masses are comparable to or less massive than the galaxy's total stellar mass, merging on tangential orbits with orbital angular momentum that is retrograde or nearly perpendicular to the disk spin. The satellite consistently drives the disk tilt toward alignment with the satellite's orbital spin; satellites that came in on retrograde orbits relative to the disk spin can thus induce large tilt angles and can produce an almost orthogonal configuration between the stellar disk and the satellite's orbital plane as an intermediate phase. Owing to the comparatively slow tilt rates, the stellar disk can keep stable and continue to grow throughout the reorientation process. Consequently, galaxies that experience strong disk tilts can still maintain stellar disks that are as thin as those in systems without significant reorientation.

Transcript

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

Vera: We're looking at the paper: "Disk tilts in Milky Way-like galaxies and their link to mergers".

Jocelyn: The gist:

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

Paper summary: Vera: We're looking at the paper "Disk tilts in Milky Way-like galaxies and their link to mergers" which investigates disk reorientation using nine-three simulated galaxies from TNG50. The authors identify one hundred seventeen individual disk-tilt events, sorting them into smooth evolution type SE and disrupted type RG events. They link these tilts to the most massive merger happening at the same time.

Jocelyn: The central claim is that forty percent of those smooth evolution events result in tilt angles over thirty degrees, usually caused by satellites with masses similar to or smaller than the host galaxy's stellar mass, merging tangentially on retrograde orbits. This satellite's orbit dictates how the disk tilts toward its spin.

Subrahmanyan: From a theoretical standpoint, this suggests that minor mergers are a significant driver of long-term structural evolution in simulated Milky Way-like systems. It implies that the history of accretion events dictates the final geometry of the stellar disk, which is a key input for understanding how dark matter halos assemble and form galaxies.

Vera: The paper also highlights that these smooth evolution events can persist for as long as nine Gyr, with type SE events having a median duration of six Gyr. This persistence shows that this reorientation process isn't just a quick event but an ongoing dynamical process in these systems.

Jocelyn: And the authors found that even with one dominant satellite, almost all discs tip toward aligning with that satellite's orbit spin. Plus, they noted a specific intermediate phase where the disk spin becomes perpendicular to the satellite’s orbital angular momentum when the satellite approaches on a retrograde orbit.

Subrahmanyan: That perpendicular phase is something I find compelling because it suggests that the reorientation isn't always a direct path to alignment; there can be complex, transitional geometries dictated by specific orbital configurations like those described. This provides more nuance to how these structural changes happen in simulation.

Conclusion: Vera: The paper "Disk tilts in Milky Way-like galaxies and their link to mergers" shows that disk tilting is a frequent feature in the history of Milky Way-like galaxies. The authors confirm that minor mergers are responsible for widespread disc tilts, and these tilts can last for extended periods while still allowing the cold, dynamically cold disk to continue growing steadily.

Jocelyn: So what does this mean practically? It means that we see structural reorientation happening in many galaxies not just through huge violent events, but through a steady stream of smaller mergers over billions of years. The vertical orientation is also shown to be a characteristic intermediate stage during this whole reorientation process.

Subrahmanyan: I think the implication for cosmology is that these simulations are giving us a better picture of how local structure evolves dynamically under relatively mild accretion conditions. It suggests that the final geometry of a disk isn't just set by the initial conditions, but by the entire history of minor mergers and their orbital alignments.

Vera: So, to wrap up this paper: disk tilts are common, minor mergers drive them over long periods, and these events can leave disks in interesting intermediate configurations. That’s what we’ve got from this work.

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