A 'Ghost Galaxy' May Have Given the Milky Way Its Signature Swirl

The Milky Way's edge is still wobbling due to a collision with a dwarf galaxy loaded with dark matter.
Artists representation of Milky Way. Image: NASA​
Artist representation of Milky Way. Image: NASA

A newly discovered dwarf galaxy on the fringes of the Milky Way may be responsible for the characteristic ripples on the outer disk of our galaxy, according to a new study.

The research has major implications for understanding the evolution of the Milky Way, and may shed light on the enigmatic nature of dark matter, a theorized type of particle that dominates the universe.

This “ghost” galaxy, called Antlia 2, was first spotted in November 2018 by Gaia, a European Space Agency spacecraft tasked with mapping out the universe in three dimensions.


Despite its large mass, Antlia 2 remained undetected for so long because it is about 10,000 times as dim and diffuse as comparably large satellite galaxies. It was also obscured behind the Milky Way’s central bulge, orbiting our galaxy at a distance of about 425,000 light years from Earth.

But though direct observational evidence of Antlia 2 was not obtained until last year, one scientist has had a decade-long hunch that it was there. Sukanya Chakrabarti, an astrophysicist at the Rochester Institute of Technology, predicted in 2009 that an object packed with dark matter was causing tidal effects at the edge of the Milky Way.

“The basic idea behind our method is actually pretty similar to the way Neptune was discovered,” Chakrabati told Motherboard in a phone call. She noted that the ghostly tug of both Neptune and Antlia 2 on their surroundings tipped scientists off to a possible presence.

“It is basic Newtonian dynamics, though it’s more complicated on galaxy scales,” she said.

Over the past decade, Chakrabarti and her colleagues published a few other papers that backed up the hypothesis of an undiscovered galaxy ruffling up the edge of the Milky Way. They looked for the galaxy themselves, but found only tantalizing evidence of debris from the possible object.

So when Gaia posted its observations of Antlia 2 in late 2018, Chakrabarti was met with a flooded inbox of messages alerting her to the discovery.


“On first glance, it definitely looked to us like Antlia 2 matched the prediction in 2009,” she recalled. “At that point, I felt encouraged. I even remember having a conversation with my PhD thesis advisor and asking: Do you think this is the one? The dwarf galaxy?”

Chakrabarti has now led new research that builds on Gaia’s observations, which she presented last week at the 234th meeting of the American Astronomical Society in St. Louis, Missouri. The study has been submitted to The Astrophysical Journal and a preprint version is available on arxiv.

By studying the new data at the Kavli Institute for Theoretical Physics at UC Santa Barbara, Chakrabarti and her colleagues estimated the backward trajectory of the dwarf galaxy over the past three billion years.

The team’s simulations showed that Antlia 2 likely shot through the Milky Way multiple times, passing close to the core. These dramatic collisions sculpted the distinctive ripple pattern that is now observable on the Milky Way’s edge.

The researchers also calculated the future trajectory of Antlia 2, which can be checked against the next batch of Gaia data due out in 2020. This will either validate or complicate the hypothesis that the dwarf galaxy is the main driver of the Milky Way’s outer disk perturbations.

In addition to understanding our galaxy’s history, Antlia 2 could shed light on dark matter, which is paradoxically the most elusive yet plentiful type of matter known in the universe.


“We believe dark matter exists, but we don’t know what a dark matter particle is,” Chakrabarti explained. “Different dark matter particle theories have different ramifications for dark matter in a dwarf galaxy.”

Antlia 2 is shaping up to be the perfect natural environment to test out those theories. It is exceptionally faint because it does not contain many stars, but that darkness also offers an upside.

Galaxies with lots of active stars also produce lots of stellar explosions, or supernovae. Those cataclysmic events can alter the distribution of dark matter density inside them, making it harder for scientists to determine which theories about dark matter particles line up with observational evidence.

Dark matter in Antlia 2, in contrast, has not been significantly twisted by supernovae and therefore its density profiles should be relatively pristine.

Antlia 2 is “a pretty clean laboratory where you can look, and try to understand, the effects of how different dark matter particles change the density profile,” Chakrabarti said.

As a result, future observations of this exotic object could help scientists compare the expected behavior of dark matter with a real observable galaxy packed with this mysterious material.