Milky Way’s stellar halo is elongated
(ORDO NEWS) — The bright plane of our Galaxy is surrounded by an almost invisible halo. It is commonly thought that this is a sphere uniformly filled with dark matter, gas, and rare stars.
However, new observations show that the halo is elongated and tilted away from the plane of the Milky Way. This may be due to the merger of our Galaxy with a dwarf neighbor billions of years ago.
The visible part of our Galaxy forms a flat disk with arms. Surrounding it is a vast , rarefied halo , composed of old stars, hot gas, and dark matter. It is believed that the halo has a symmetrical spherical shape.
However, new work by scientists from the Harvard-Smithsonian Center for Astrophysics refutes these ideas.
Judging by its results, the halo is elongated and tilted relative to the plane of the Milky Way, which was the result of a long and difficult evolution of the Galaxy.
The halo is dominated by invisible dark matter, which accounts for most of the mass of the Galaxy. There are very few stars there, about one percent of the entire stellar population of the Milky Way.
It is the stellar halo that makes it possible to estimate the size and shape of the entire galactic halo.
To examine it, scientists used new astrometric data that were obtained by the Gaia space probe, as well as during the H3 survey conducted by the ground-based MMT telescope.
These observations determined with high precision the positions and movements of hundreds of thousands of stars in the Milky Way’s halo.
Combining data from both sources, astronomers have shown that the halo is far from being as symmetrical and uniform as previously believed: it is elongated and tilted to the plane of the Galaxy.
According to scientists, this shape is more consistent with our ideas about the past and the evolution of the Milky Way.
First of all, it is associated with its collision with the GSE dwarf galaxy, which occurred seven to ten billion years ago. It is the GSE remnants that make up the majority of stars in the Milky Way’s galactic halo.
Merging with it and collapsing, the GSE managed to make two revolutions in its orbit, losing a slightly larger number of stars at apocenters – points that are as far from the center as possible.
This led to the formation of an elongated halo. Its inclination relative to the plane of the Milky Way seems to reflect the angle at which the two galaxies met.
Finally, the fact that over the past billions of years the halo has not acquired a spherical shape and has not shifted to the plane of the Galaxy, scientists attribute to the fact that the invisible halo of dark matter is also tilted and elongated.
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