Dwarf galaxies near the Milky Way are thought to be its satellites, as is the Earth orbiting the Sun. The new Gaia data released in late 2020 indicate that they have much higher energies and angular moments than known Milky Way satellites, which means that most dwarf galaxies are new in the Galaxy environment.
This is the conclusion of the international team from the Observatoire de Paris – PSL au department Galaxies, étoiles, physique et instrumentation – GEPI, from the National Astronomical Observatory of the Chinese Academy of Sciences (NAOC) and from the Leibniz Institute for Astrophysics at. Potsdam (AIP).
Published online on November 24, 2021 in The Astrophysical Journal, the study questions many ideas about our galaxy and its environment.
For decades, dwarf galaxies surrounding the Milky Way have been thought to be satellites. They are considered our constant companions for many billions of years. Now the Gaia results are so accurate that the above hypothesis can be tested and the result came to a full surprise.
The research team has utilized the most accurate astrometric data available to date. The movements of the dwarf galaxies in the sky have been revealed with unprecedented precision following the release of the third catalog Gaia in December 2020.
Based on these results, the researchers calculated the three-dimensional velocities of each of the 40 dwarf galaxies discovered by Gaia. So from these velocities and the dwarf galaxy locations, they calculated their orbits and deduced the fundamental amounts of their orbital energies and their angular moment (moment of rotation).
Surprisingly, these latter amounts are much larger compared to the other populations in the Galaxy environment, either giant stars or clusters of stars called globular clusters. The inhabitants of the Galaxy fringe have angular energies and momentum that decrease with time, due to the various energy losses to which they are exposed when in orbit. The longer they are in orbit, the weaker their energies and angular momentum.
Many giant stars surrounding the galaxy are the result of a previous collision that formed the Milky Way eight to ten billion years ago. Other stars lie in a gigantic stellar stream, which corresponds to the fall and destruction of Sagittarius’ dwarf galaxy in the Milky Way four to five billion years ago.
The study concludes that many of the dwarf galaxies that live on the edge of the galaxy have reached them much more recently, just a few billion years ago. This is because they have much greater energies and angular moments than giant stars, including those from Sagittarius. Then they came only a few billion years ago, which is equivalent to the time to complete a single orbit. In cosmic terms, these dwarf galaxies have just entered the edge of the Milky Way for the first time.
There are two major consequences of this discovery. The first is that this common “whim” of dwarf galaxies in the halo of the galaxy coincides with the Magellanic clouds. As a result, very few dwarf galaxies are long-term satellites of the Milky Way. The second relates to the presence of dark matter in dwarf galaxies. If dwarf galaxies were satellites orbiting the Milky Way for many billions of years, dark matter would be needed to protect them from the enormous tidal forces in our galaxy. If dwarf galaxies have just entered the Milky Way’s surroundings, dark matter is no longer required, and we only need to reassess whether they are in balance or rather in the process of destruction.