It is difficult to determine the shape of our galaxy. So hard that it was only in the last century that we learned that the Milky Way is just one galaxy among billions. So it’s no wonder that despite all our modern telescopes and spacecraft, we’re still mapping the shape of our galaxy. And one of the more interesting discoveries is that the Milky Way is warped. One explanation is that our galaxy went through collisions, but a new study says it’s caused by dark matter.
The first evidence that the Milky Way is not a purely flat disk came from the Gaia spacecraft. it has mapped the positions and motions of more than a billion stars, and from this we have some idea of the outer structure of our galaxies. For one thing, the Milky Way appears to be expanding outward more than we thought, and the edge appears to have plus a wavy structure. Further analysis also shows deformation towards the outer edge of the galactic disk. The general consensus is that they are caused by ancient galactic collisions such as collision with the Sagittarius dwarf galaxy about 6 billion years ago. This new study instead argues that warped galaxies like ours are caused by a tilted dark matter halo.
We know that most galaxies, including our own, are surrounded by a massive halo of dark matter. Most of the matter in the galaxy is contained in this halo, so the halo can affect the structure of galaxies over time. If the halo is tilted relative to the plane of the galaxy, the gravitational effects of the halo could distort the galactic disk. The question is whether it is significant or common enough to cause the structure we see in the Milky Way.
The authors begin by looking at data from the IllustrisTNG TNG50 simulation run. These are supercomputer simulations of galactic evolution that include cosmological evolution and dark matter, as well as detailed magnetohydrodynamic interactions. From these data, the authors show that the inner region of the dark matter halo can be significantly tilted relative to the galactic plane, and that this can be caused by both galactic collisions and near misses between galaxies. The orientation of the halo can persist for billions of years, long enough to cause a galactic warp.
The team then looked at an archetypal galaxy in the TNG50 simulation that is similar in size and age to the Milky Way, including a dark matter halo named. Based on 6 billion years of simulated evolution, the final result showed striking similarities between the observed Milky Way and the model galaxy.
Astronomers have observed other galaxies with a warped plane, and there is some evidence that up to half of the spiral galaxies in the universe are warped to some degree. This suggests the influence of long-term and ordinary gravitational interactions between the galaxy and its twisted halo. Of course, this is just one study, so more observations and simulations will be needed to understand the details of how many galaxies like the Milky Way there are.
Link: Han, Jiwon Jesse, et al. “Oblique dark haloes are common, long-lived, and can distort galactic disks.” arXiv preprint arXiv:2309.07208 (2023).
Link: Poggio, Eloisa et al. “Evidence of a dynamically evolving galactic warp.” Astronomy of nature 4.6 (2020): 590-596.