Great, darkly strange galaxy found hidden on the other side of the Milky Way | Science

A stealthy giant surrounds our galaxy. Astronomers have discovered a dwarf galaxy, called Antlia 2, which is one-third the size of the Milky Way itself. As big as Big Magellan Cloud, the galaxy's biggest companion, Antlia 2 has escaped detection so far because it is 10,000 times weaker. Such a strange beast challenges models of galaxy formation and dark matter, invisible elements that help to gather galaxies.

"It's a very strange and very exciting object because we do not yet know how to interpret all of its properties," says Andrey Kravtsov of the University of Chicago, Illinois, who did not participate in the work.

The galaxy was discovered from data from the Gaia satellite of the European Space Agency, a space telescope measuring the motions and properties of more than a billion stars in and around the Milky Way . Gabriel Torrealba, a postdoc in astronomy at the Academia Sinica in Taipei, decided to screen the RR Lyrae's star data. These old stars, often found in dwarf galaxies, glow with a throbbing blue light that flashes at a rate that indicates their inherent brightness, allowing researchers to determine their distance.

"RR RR Lyrae are so rare at these distances that even if you see two RRs, you wonder why they are together," says Vasily Belokurov, astronomer at the University of Cambridge in the UK and a collaborator of the discovery . When the team found three, about 420,000 light-years away, it was an "overwhelming signal" from a large group of stars at this time. place, said Belokurov. But since the stars of RR Lyrae are on the other side of the Milky Way disk and its veil of stars and gas obstructs it, it was not easy to find their companions.

Gaia's data helped the team see beyond the stars in the foreground. The objects in the Milky Way disk are close enough for Gaia to measure their parallax: a change in their apparent position as the Earth moves around the sun. More distant stars seem to be fixed in the same place. After removing the stars carrying parallax, the researchers focused on more than 100 giant red stars moving together in the constellation Antlia, they report in an article posted on the pre-print server arXiv this week. Giants mark a sprawling companion galaxy 100 times less mbadive than anything of the same size, with far fewer stars.

To explain such a diffuse galaxy, Belokurov suggests that at the beginning of the Antlia 2 story, many young stars exploded under the name of violent supernovae. This would have thrown gas and dust out of the galaxy, weakening its gravity so that it swells. Shea Garrison-Kimmel, an astrophysicist at the California Institute of Technology in Pasadena, adds to the credibility of this idea an abundance of heavy elements scattered in the guts of stars blowing up. Antlia 2 could also have lost matter as the stars were driven by gravitational tidal forces as it gravitated around the greater Milky Way.

Nevertheless, its disproportionate size is difficult to explain. Galaxies are thought to have formed when the gravity of huge mbades of dark matter has sufficiently absorbed ordinary matter to fuel the birth of stars. The team believes that Antlia 2 could be born from a type of dark matter more fluid and faster than current models.

For Garrison-Kimmel, an example is not enough to say that the dark matter in Antlia 2 is different from that of the Milky Way and its other satellites. "There is nothing in this galaxy that shouts that we need to rethink dark matter," he says. "But if there are a lot, we may have to step back and ask what's going on."

It could happen now that astronomers know how to find these great elusive companions. "I think this object is a precursor," says Kravtsov. "A taste of things to come."