Carnegie Mellon University
November 13, 2018

Gaia Spots a ‘Ghost’ Galaxy Next Door

Jocelyn Duffy
  • Carnegie Mellon University
  • 412-268-9982
Sarah Collins
  • Cambridge University

An international team of researchers, including Carnegie Mellon University physicists Sergey Koposov and Matthew Walker, have used data from the European Space Agency’s Gaia satellite to discover an enormous galaxy lurking in the outskirts of the Milky Way. Antlia 2 (Ant 2), named after the constellation in which it appears, had previously escaped detection due to its astonishingly low density and perfectly chosen hiding place behind the Milky Way’s disk.

“This is a ghost of a galaxy,” said Gabriel Torrealba, a physicist at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan and the lead author of the discovery paper. “Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the unprecedented quality of the Gaia data." 

Ant 2 is a dwarf galaxy that is a satellite of the Milky Way. Dwarf galaxies are among the first structures to have emerged in the early universe. Ant 2 is unlike any of the other known dwarf satellites of the Milky Way. It is much larger in size and gives off much less light, meaning it is either too large for its luminosity or too dim for its size. 

One possible reason for the unusual properties of Ant 2 is the gravitational interaction between the Milky Way and the dwarf galaxy, which exerts strong tidal forces on the dwarf galaxy. 

“The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the galactic tides,” said Koposov, assistant professor of physics and member of the McWilliams Center for Cosmology at Carnegie Mellon. “What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow."

Ant 2’s existence could also indicate that researchers may need to revise their theories of dark matter. The current, widely accepted cold dark matter theory says that dark matter gathers tightly in the center of galaxies. Ant 2’s properties indicate that its dark matter would be more diffuse throughout the dwarf galaxy, indicating that its dark matter particles wouldn’t cluster.

“Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball,” said Walker, assistant professor of physics and member of the McWilliams Center for Cosmology at Carnegie Mellon. “We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.” 

The researchers will continue to search the Gaia data for similar galaxies.