Star halo of the Milky Way turned out to be a “rugby ball”

(ORDO NEWS) — Scientists at the Harvard-Smithsonian Center for Astrophysics have determined the shape of the stellar halo surrounding the Milky Way galaxy.

The halo is a three-axis ellipsoid with three axes of different lengths and is shaped like a rugby ball. Previously, it was thought that it is spherical and has the same length in all directions.

The researchers analyzed data from the ESA Gaia spacecraft and the H3 survey from the ground-based Whipple Observatory in Arizona.

Gaia is used to map the Milky Way’s star distribution in detail, while the purpose of the H3 spectroscopic survey is to study in detail the stellar halo, the visible part of a more complex galactic structure in which astronomers also include the dark matter halo.

The stellar halo, which makes up one percent of the galaxy’s stellar mass, is known to be replete with relic structures that testify to the early history of the Milky Way.

These include, for example, the remnant of a merger with the ancient dwarf galaxy Gaia Enceladus (GSE), which owns a significant mass of stars in the halo.

Combining data from Gaia and H3 on the chemical composition of stars and their speeds, astronomers have identified 5559 stars belonging to the GSE, over a distance of up to 60 kiloparsecs (195.6 thousand light years).

The stellar halo turned out to be an ellipsoid with an axis ratio of 10:8:7, the major axis of which is directed at an angle of 25 degrees above the galactic plane.

In theory, the density of the halo should increase towards the center according to a power law, but scientists have identified two deviations from this trend at a distance of 12 and 28 kiloparsecs (corresponding to 39 and 91 light years) from the galactic center.

According to the model, clusters of stars at these distances correspond to the two most distant points (apocenters) of the decaying orbit of the GSE before it completely merged with the Milky Way 7-10 billion years ago.

According to scientists, the unusual shape of the stellar halo indicates that the dark matter halo is also distorted.

It is assumed that the galactic disk of the Milky Way is surrounded by a giant halo of invisible matter, which plays a key role in the emergence and evolution of galaxies. The tilt of the halo may play a role in astronomers’ ability to detect hypothetical dark matter particles.


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