By Adam Mann
The circulation of our galaxy is a huge giant. Astronomers have discovered a dwarf galaxy called Antlia 2, which is one-third of the size of the mallard itself. As large as the Big Magellanic Cloud, the largest companion of the galaxy, Antlia 2 has bypassed the detection so far, as it is 10,000 times weaker. Such a strange beast causes models of galaxy formation and dark matter, the unseen things that help pull the galaxies together.
"It is a very strange and interesting thing because we do not yet know how to interpret all its properties," says Andrey Kravtsov of the University of Chicago, Illinois, who was not involved in this work.
The galaxy was discovered with data from Gaia's European Space Agency, a space telescope that measures the movements and properties of over 1 billion stars in and around the Milky Way. Gabriel Torrealba, astronomer post-post at the Sinica Academy in Taipei, decided to analyze the data for RR Lyrae stars. These old stars, often found in dwarf galaxies, shine with a pulsing blue light that pulses at a rate that signals their inherent luminosity, allowing scientists to fix their distance.
"RR Lyrae are so rare at these distances that even if you see two, you ask why they are together," says Vasily Belokurov, an astronomer at Cambridge University in the United Kingdom and a collaborator of the discovery. When the team found three, about 420,000 light-years away, it was "an overwhelming signal" of a large group of stars in that location, Belokurov says. But because the RR Lyrae stars are on the farthest side of the Milky Way disc and the veil obstructed by stars and gas, finding their comrades was not easy.
Gaia's data helped the team to see beyond the stars. Objects in the Milky Way disc are close enough for Gaia to measure parallax: a change in their apparent position as the Earth moves around the Sun. More distant stars appear fixed in one place. After removing parallax-bearing stars, the researchers turned to over 100 giant red stars moving together in Antlia's constellation, according to a paper posted on the pre-print arXiv servers this week. The giants mark an obsolete companion galaxy, 100 times less massive than any of similar sizes, with much smaller stars.
To explain such a diffuse galaxy, Belokurov suggests that at the beginning of Antl. 2's history many youngsters exploded as violent supernovae. It would have blown gas and dust out of the galaxy, weakening gravity so it would swell. An abundance of heavy elements that are scattered from the courage of explosive stars adds credibility to this idea, says Shea Garrison-Kimmel, an astrophysicist at the California Institute of Technology in Pasadena. Antlia 2 could also have lost the matter as the stars were pulled by tidal gravity forces while orbiting around the larger Milky Way.
Even so, the disproportionate size is hard to explain. It is believed that galaxies formed when the gravity of enormous clusters of dark matter attracted enough ordinary matter to feed the birth of stars. The team speculates that Antlia 2 could have been born of a more fluorescent and faster type of dark matter than the hypothesis of the current models.
For Garrison-Kimmel, one example is not enough to say that the dark matter in Antlia 2 is different from the Milky Way and its other satellites. "There is nothing in this galaxy that calls me to rethink the dark matter," he says. "But if there are many, then it would be good to take a step back and ask what is going on."
That can happen now that astronomers know how to find these great, elusive comrades. "I think this object is a whistleblower," says Kravtsov. "A taste of things to come."