Gaia detects stars traveling between galaxies
Gaia detects stars traveling between galaxies
While searching for hypervelocity stars that escape from the Milky Way with the latest set of data from the ESA Gaia mission, a team of astronomers discovered by surprise how a series of stars traveled into our Galaxy, perhaps from a different galaxy. .
In April, ESA's astrometric satellite published an unprecedented catalog of more than a billion stars. Astronomers around the world have been working tirelessly in recent months with this extraordinary data set, scrutinizing the properties and movements of the stars of our Galaxy and beyond with unprecedented precision, which has led to a multitude of new and interesting studies.
The Milky Way contains more than one hundred billion stars. The majority is in a disc with a dense and bulky center, in the middle of which there is a supermassive black hole. The rest extends through a much larger spherical halo.
The stars circulate in the Milky Way at hundreds of kilometers per second, and their movements contain huge amounts of information about the past of the Galaxy. The fastest stars are the so-called "hypervelocity stars". It is believed that they are born near the galactic center, from which they escape towards the limits of the Milky Way due to their interaction with the black hole.
So far only a small number of hypervelocity stars have been discovered, so the second Gaia data catalog offers a unique opportunity to search for more stars of this type.
Movement of stars in the Milky Way. (Photo: ESA (artist's impression and composition), Marchetti et al 2018 (star positions and trajectories), NASA / ESA / Hubble (background galaxies), CC BY-SA 3.0 IGO)
As soon as the new data set was published, several groups of astronomers entered it in search of hypervelocity stars. Among them, three scientists from the University of Leiden (Netherlands), to whom the catalog was a great surprise.
Gaia has measured the positions, paralages (which indicate their distance) and two-dimensional movements in the plane of the sky of one thousand three hundred million stars. And for a subset of seven million of the brightest stars, it has also measured how fast they are moving away from us.
"Among those seven million Gaia stars with full measurements of three-dimensional speeds, we found twenty that traveled fast enough to end up escaping the Milky Way," explains Elena Maria Rossi, author of the new study.
Elena and her colleagues, who had discovered several hypervelocity stars last year in an exploratory study based on Gaia's first data catalog, were pleasantly surprised, as they hoped to find at best one star that escaped from the Galaxy between the Seven million. But there is still more.
"Instead of getting away from the galactic center, most of the hypervelocity stars detected seem to approach him," adds Tommaso Marchetti, co-author of the study.
"It could be stars from another galaxy, which are crossing the Milky Way."
It is possible that these intergalactic intruders are originally from the Large Magellanic Cloud, a relatively small galaxy that orbits the Milky Way, although they could also come from an even more distant galaxy. If that is the case, they carry the imprint of their place of origin, and their study at much closer distances than their progenitor galaxy can offer unique information about the nature of the stars of other galaxies, similar to what happens when studying Martian material brought to our planet by meteorites.
"Stars can accelerate at high speeds when they interact with a supermassive black hole," says Elena.
"Thus, the presence of these stars could be a sign of this type of black holes in nearby galaxies. But the stars could also have been part of a binary system, and have been thrown into the Milky Way when its companion exploded in the form of a supernova. In any case, studying them would allow us to know more about this type of processes in neighboring galaxies. "
Another explanation is that the newly identified stars could be native to the halo of our Galaxy, and would have been accelerated and displaced inward by the interaction with one of the dwarf galaxies that fell into the Milky Way during its formation. Having additional information about the age and composition of the stars could help astronomers clarify their origin.
"A star in the halo of the Milky Way is probably quite old and consists mostly of hydrogen, while the stars of other galaxies could contain a lot of heavier elements," says Tommaso.
"Observing the colors of the stars gives us more information about their composition."
New data will help clarify the nature and origin of these stars, and the team will use ground-based telescopes to learn more about them. Meanwhile, Gaia will continue to observe the entire sky, including the stars analyzed in this study.
In addition to investigating the nature of these possible stellar intruders, the team is also diving into the data set of Gaia's second launch in search of more hypervelocity stars, although their hopes are also placed in the future. There are at least two other Gaia data catalogs planned for the 2020s and each of them will provide new data and more accurate information about a larger set of stars.
"In the future we expect to have full three-dimensional velocity measurements of up to one hundred and fifty million stars," said Anthony Brown, co-author of the study and chair of the executive committee of the Gaia Data Processing and Analysis Consortium (DPACE).
"This will help locate hundreds or thousands of hypervelocity stars, understand their origin much better and use them to investigate the environment of the galactic center and the history of our Galaxy," he adds.
"This fantastic find shows that Gaia is a whole machine that paves the way for new and unexpected discoveries about our Galaxy", concludes Timo Prusti, scientist of the Gaia project of the ESA. (Source: ESA)
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