Galactic Encounters: What TNG50 Reveals About the Milky Way’s Dance with Sagittarius
The Sagittarius Dwarf Galaxy (Sgr) is slowly being torn apart by the Milky Way’s gravity, and scientists have long debated how this ongoing interaction has affected our galaxy’s disc. A new study led by Marcin Semczuk takes a fresh approach: instead of modeling the Milky Way directly, the team used the TNG50 cosmological simulation to find galaxies similar to the Milky Way and Sagittarius, then analyzed whether similar interactions caused noticeable effects.
Method: Finding Galactic Look-Alikes
To do this, the authors searched through a set of simulated galaxies for pairs where a satellite galaxy, comparable in size to Sagittarius, passed close (within 50 kiloparsecs) to a larger galaxy on a mostly polar orbit. After applying several filters to exclude noisy or unrepresentative cases, they ended up with a sample of 52 Milky Way-like galaxies that experienced 105 such interactions.
Vertical Motion: Mostly Unchanged
One main focus of the paper is whether these encounters disrupt the vertical motions of stars in the galaxy discs, a phenomenon known as vertical kinematic perturbation. Using a mathematical tool called Fourier analysis, they measured how much the stars were moving up and down in a wave-like pattern before and after each interaction. Surprisingly, in about 90% of cases, there was no significant change. The vertical motions were already present before the encounter, and most of the galaxy discs seemed unaffected. Only when the discs were initially "cold", meaning stars had lower random vertical motion, did the passage of a Sagittarius-like satellite seem to make a clear difference.
Star Formation: Only in Quiet Galaxies
The authors also investigated whether such encounters could spark new waves of star formation. This idea was previously suggested as a possible explanation for bursts of star formation in our own galaxy’s history. They found that increased star formation happened only when the galaxy had very little ongoing star formation to begin with. In most of those cases, the galaxy's star-forming gas disc had already been disturbed, likely by strong black hole (AGN) feedback modeled in the TNG50 simulation. While this isn’t an exact match to the Milky Way, which still has a relatively intact gas disc, it hints that our galaxy’s current low star formation rate could make it more sensitive to even modest galactic encounters.
A Commonly Unbalanced State?
Altogether, the study provides statistical context for the Milky Way-Sagittarius interaction. It suggests that most such flybys do not strongly disrupt galaxies like ours, unless certain conditions are met, such as the presence of a cold disc or a previously quiet star formation environment. Interestingly, many of the simulated Milky Way analogues were already in an unbalanced state before the interaction, pointing to a broader question: is vertical disequilibrium common in disc galaxies, or is it a feature specific to simulations like TNG50?
Implications for Our Galaxy
While the study stops short of drawing firm conclusions about the Milky Way itself, it shows that similar interactions are not guaranteed to cause dramatic effects. Future research using different simulations or observations of real galaxies will help determine just how typical our Galaxy’s story is.
Source: Semczuk
