Hubble image of DF2
The Hubble telescope spent 40 90-minute cycles making this deep image of NGC 1052-DF2, the first galaxies to be discovered without dark matter.
Peter van Dokkum et al. / Nature 2022

In 2018 and 2019, a team led by Peter van Dokkum (Yale University) reported the discovery of two dwarf galaxies (called DF2 and DF4) that appeared to be devoid of dark matter. This announcement baffled the astronomer, because it is believed that dark matter provides the gravitational pull that ordinary matter needs to form galaxies. Galaxies deficient in dark matter do not appear to follow this evidence.

Heat debate ensued, with others suggesting that distances to galaxies might have been incorrectly measured. But amid the controversy, what relatively no one has noticed is how similar DF2 and DF4 really are. They are similar in size, luster, morphology, and rapid dispersion, and both contain a peculiar array of very luminous globular clusters. Crucially, assuming the distances are correct, they are also believed to lie in the same vicinity: the region around a bright elliptical galaxy known as NGC 1052.

Twins born in galaxy collision

on May 19 temper nature, van Dokkum’s team asks if DF2 and DF4 are more than just doppelgangers. Could they be twins born of the same cosmic mother? “I started with this assumption that it is very unlikely that these galaxies have nothing to do with each other,” van Dokkum says. “Both are unique in the universe, except for the twin who live in the same group.”

Early in the analysis, the researchers recorded that the two galaxies were escaping at high speeds. Using line-of-sight locations, current velocities, and the estimated lifespan of the DF2, the team traced their locations in time to a common origin. Then, taking into account the distinctive characteristics of the galaxies, the team developed a scenario for their formation, based on Previous computer simulation.

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Diagram showing how a single event could create galaxies lacking dark matter
A scenario of how DF2, DF4 and other potentially dark matter-deficient galaxies originated in a single event about 8 billion years ago. A gas-rich galaxy on an unbound orbit (Progenitor 1) collides with a satellite galaxy of NGC 1052 (Progenitor 2), leaving dark remnants (possibly RCP 32 and DF7), DF2 and DF4, and three to seven other dark-matter-free galaxies.
P. van Dokkum (Yale University)

In this scenario, a galaxy belonging to NGC 1052 collided with another unrelated galaxy about 8 billion years ago. The stars and dark matter slid behind each other, weakly interacting through gravity. But the high-speed collision compressed and slowed the galactic gas.

“The gas expanded into a whole bunch of lumps that then collapsed, under the influence of its own gravity, and formed new galaxies without dark matter, because the dark matter was lost,” explains van Dokkum. Under this origin story, other dark matter-deficient galaxies can be seen in the vicinity, forming a line between DF2 and DF4 like a string of pearls. And what about the remnants of the two galaxies that are dominated by dark matter? It may lie at the ends of the thread.

The team searched the catalog of galaxies around NGC 1052 and found 11 galaxies lined up as expected, including DF2 and DF4. At the far ends, beyond DF2 and DF4, are two strange galaxies: RCP 32 and DF7. These may be missing traces from the original collision.

Picture of small circles of light on a black background
A survey image of NGC 1052, highlighting 11 galaxies that form a chain of galactic pearls that may have formed after a single collision event about 8 billion years ago. Also highlighted was DF9, a galaxy that appears to be on the chain but was not part of the sample.
P. van Dokkum (Yale University)

Although van Dokkum acknowledges that the ancestral identity of RCP 32 and DF7 is somewhat speculative, he feels that the observed string of pearls shows that these galaxies, with the exception of a few potential interlopers, are at least somehow related and may share an origin subscriber.

Cosmic alignment or coincidence?

“The new results provide an interesting and elegant scenario to explain the low/free dark matter content of DF2 and DF4,” says Pavel Mancera Piña (ASTRON, University of Groningen, The Netherlands), who was not involved in the study. Find out more Pi Six galaxies are missing candidates for dark matter, albeit with different characteristics than DF2 and DF4. “I think we’ll be able to say something with more certainty once we have the distance, radial velocities and total mass of more galaxies close to the DF2 and DF4 paths.”

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Michelle Collins (University of Surrey, UK) is similarly cautious. “For me, it’s a nice idea, but with a number of important issues,” she says. “One is that they assume that these 11 systems in line are all related to NGC 1052.”

According to their positions in the sky, the line of 11 dwarf galaxies is located near three other large galaxies. Spiral NGC 1035, for example, is directly on the path but much closer to us than NGC 1052. Given that another group has claimed that NGC 1035 is pulling on DF4, Collins would have liked to see additional analysis that includes the potential impact of these other large galaxies “to convince people that they are It’s not just an opportunity to overlap the line of sight, kind of like a constellation.”

What Collins, Mancera Peña and van Dokkum agree on is that the collision scenario proposed to explain how DF2 and DF4 form can be easily tested in principle. Van Dokkum’s team plans to direct an army of powerful telescopes into these 11 galaxies to settle the problem once and for all.

Ground-based instruments such as the Very Large Telescope in Chile and the Keck Telescope in Mauna Kea, Hawaii, will measure the radial velocities of galaxies. Hubble will take snapshots of each of the 11 galaxies to ascertain if they are all home to luminous globular clusters like DF2 and DF4, and van Dokkum has applied for extra time on the venerable space telescope to measure galactic distances as well.

Finally, van Dokkum hopes to have time on the James Webb Space Telescope to measure the masses of galaxies RCP 32 and DF7 at the far ends of the galactic string of pearls: “This is kind of the holy grail because we may then find a place for dark matter in this whole system.”

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