Out they go, two-by-two

Astronomers have discovered that sometimes pairs of stars are expelled from their host galaxies, after which they drift for billions of years in interstellar space.

In a paper published in The Astrophysical Journal, researchers led by Xiangyu Jin of McGill University in Montreal, Canada, report using images gained by NASA’s Chandra X-ray Observatory to chart the rather bleak history of 30 binary star sets expelled from the NGC 1399 and NGC 1404, two of the largest galaxies in the Fornax cluster, about 60 million light-years from Earth.

“It’s like a guest that’s asked to leave a party with a rowdy friend,” says Jin.  “The companion star in this situation is dragged out of the galaxy simply because it’s in orbit with the star that went supernova.”

Binary star systems account for perhaps as many as four-fifths of the stars in the universe. They can comprise any type of star, but in the case of the expelled sets the researchers say it is highly likely that one of them is a super-dense neutron star, created after an earlier massive incarnation went nova and collapsed in on itself. 

Under certain circumstances, Jin and colleagues explain, the forces generated by supernova explosion are not symmetrical. In these cases, the uneven recoil can blast the newly formed neutron star out of the galaxy – pushing its gravitationally bound binary partner with it. 

The expelled binaries were detected by Chandra as it searched for x-ray light – produced because friction in the disc of gas that accretes around a neutron star bumps the temperature up to many million degrees.

“Rather than being tethered to a particular galaxy, these pairs of stars now exist in the space between galaxies or are on their way out of their home galaxy,” explains co-author Meicun Hou, from Nanjing University in China.

The researchers also found 150 additional sources of x-rays beyond the outer limits of the two Fornax galaxies, but these, they suggest, do not represent expelled binaries.

Instead, they may be the signature of stars that were pulled away from other galaxies by the immense gravitational pull of NGC 1399 or NGC 140, or the lonely remnants of past galaxies destroyed by titanic collisions.

“This is like the end of a party, where the people attending head off in different directions, and only the hosts are left behind,” says co-author Zhenlin Zhu. 

“In the case of Fornax, the extreme case is that the original galaxies don’t really exist anymore.”

The research is available in full on the Cornell University preprint site, arXiv.