Spiral galaxies like the Milky Way have a highly interconnected structure. The thin, expansive disk of stars (i.e. the disk that includes the Sun) is surrounded by a thicker disk of older stars, which is then “packed” into the so-called more extensive galactic halo. At the base and at the same time outside of all this structure there is an interesting part of the galaxy that we cannot see and cannot detect in any similar way: the dark matter halo in which the galaxy is located.
Therefore, spiral galaxies are surrounded by a halo of invisible dark matter, which makes up 85% of the mass of the universe. New research now shows that the angle of inclination of these halo shells can affect the structure and spatial orientation of the stellar halo and the oblate galactic disk itself.
Although all phenomena occur in the dark matter halo milky way While it may seem quite far removed from what happens in the thin disk of a star, research by Jeewon Jesse Han of the Harvard-Smithsonian Center for Astrophysics and colleagues has shown a clear connection between these two aspects of galactic structure. In their latest paper, Hahn’s team examined the alignment of the stellar disks of galaxies and the interior regions of dark matter halos, and examined what might be causing the discrepancies between them.
Using known cosmic simulations LPG50 – Part of the IllustrisTNG cosmological simulation suite – the team selected a sample of artificial galaxies that are similar in many respects to the Milky Way and our neighboring Andromeda Galaxy (M31). The simulated galaxies had masses similar to the current mass of the Milky Way and a similar disk shape. They had no other massive galaxies in their “neighborhood,” which is defined here as a radius of 500 kiloparsecs, or 1.6 million light-years.
A diagram of the structure and composition of the Milky Way’s halo (NASA/ESA//A. Feild (STScI))
Dark corona inclination angle relative to the stellar corona inclination angle calculated for Milky Way-like galaxies in the TNG50 simulation. The measured inclination of the Milky Way’s stellar halo is shown in the pink shaded region. The diagram on the right shows the angle of inclination of the Milky Way’s dark halo most likely. (Han et al. / The Astrophysical Journal 2023)
For all 198 Milky Way galaxies, the team measured the angle between the inner dark corona (the part of the halo that lies within 50 kiloparsecs of the stellar disk), the stellar corona, and the galactic disk. Over the 6 billion years of this simulation, the tilt angles between inner dark halos and stellar disks, and between stellar halos and galactic disks, changed. However, the inner dark corona and the stellar corona showed similar behavior, tilting and distorting in the same directions.
The stellar corona of the Milky Way It is currently downhill Relative to its stellar disk at an angle of about 20 to 30 degrees. Given the similar tilt of the dark and stellar halos in the simulation, this means that the Milky Way’s inner dark corona is also tilted relative to the disk at an angle of about 20 degrees. So what is the effect of this slanted dark halo?
Hahn and his team ran a deeper simulation, focusing on the behavior of a single digital counterpart of the Milky Way that experienced a merger with another object 7 billion years ago. This galactic merger tilted the galaxy’s dark matter halo by 50 degrees, and the stellar halo followed suit, driven by the gravitational pull of this dark halo. Over time, due to friction, torque and other factors, the inclination of the galaxy’s stellar and dark atmosphere decreased to approximately 20 degrees.
The effects of the tilt of the dark matter halo are also felt in the thin stellar disk in this scenario: after a few billion years of merger, the warp in this previously flat disk appears in the simulation. The distortion decreased slightly over time, but has remained in its altered form until “today”. We know that the Milky Way, like this simulated artificial galaxy, experienced a merger billions of years ago. She also currently has a deformed disc. Based on the simulation results, Hahn’s team suggests that galaxy mergers that occur during halo tilt cause long-term deformation of galactic disks — which is now observed in more than half of spiral galaxies.
Source: Sky & Telescope
Prepared by: Elizabieta Kolegoska
Image above: The Milky Way’s stellar disk is immersed in an extended, tilted halo (Melissa Weiss / Center for Astrophysics, Harvard & Smithsonian / CC-BY NC 4.0)
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