Strangely light galaxies could imply new model for dark matter

Most of the mass in any given galaxy is invisible, made up of stuff we call dark matter. But in recent years astronomers made the puzzling discovery of two bizarre galaxies with almost no dark matter. Now, a new study has suggested an explanation for that mystery – a larger nearby galaxy is stripping it away.

The idea that the universe contains huge amounts of invisible matter dates back to the 1930s, when astrophysicists observed distant galaxies moving much faster than their visible mass should allow. The team suggested that there must be far more mass than they could see, and this extra stuff was termed “dark matter.”

In the decades since, evidence for dark matter has consistently turned up in astronomical observations, although the particles themselves continue to elude detection. But some of the strongest pieces of evidence supporting the idea have actually been its absence.

In 2018, astronomers announced the surprising discovery of NGC 1052-DF2 (or DF2 for short), a dwarf galaxy with almost no dark matter. Its visible matter was enough to account for the relatively slow movement of its star clusters. The following year another similarly light galaxy turned up, NGC 1052-DF4 (or DF4).

These discoveries could turn out to be the exception that proves the rule. After all, one of the biggest arguments against dark matter is that it’s a fundamental miscalculation, and our models are wrong. But if that was the case, those same errors should apply to all galaxies. Finding examples that lack dark matter implies that it is a physical substance that, occasionally, turns up short.

Now, a new study investigates where the dark matter from those galaxies might have gone. DF2 and DF4 are both satellite galaxies, orbiting a much larger galaxy called NGC 1052 – and the astronomers suggest that galactic tidal forces are stripping mass from the satellites to the larger galaxy.

“It is commonly thought that dark matter dominates the overall mass in a galaxy,” says Hai-Bo Yu, lead researcher on the study. “Observations of NGC 1052-DF2 and -DF4 show, however, that the ratio of their dark matter to their stellar masses is about one, which is 300 times lower than expected. To resolve the discrepancy, we considered that the DF2 and DF4 halos may be losing the majority of their mass through tidal interactions with the massive NGC 1052 galaxy.”

The team simulated the interactions between the three galaxies, using two proposed types of dark matter. The mechanics of this process seem to work as an explanation for DF2 and DF4’s dark matter deficiencies – but there’s an intriguing implication.

Currently the leading model of dark matter suggests it’s “cold” – that is, dark matter particles move slowly and don’t interact with each other very much. But a more recent theory suggests the stuff interacts strongly with itself through a new “dark” force. Both ideas explain the large-scale structure of the cosmos, but have different implications for dark matter within galactic cores.

By modeling the dwarf galaxies with both types of dark matter, the researchers found that tidal stripping could work in both cases, but it does so much more easily with self-interacting dark matter. That’s because those interactions would push more dark matter particles out from the center of the galaxies into the outer regions, where it could be stripped away easier.

“A typical cold dark matter halo remains too massive in the inner regions even after tidal evolution,” says Yu.

But there’s an extra wrinkle to the story too. Another study argued that DF2 isn’t lacking in dark matter at all – the discrepancies were all due to miscalculations in the distance to the galaxy. If that’s true, we could be back to square one.

Whatever the case, the team on the new study plans to investigate the whole NGC 1052 system of galaxies in more detail to hopefully find new clues about this strange stuff.

The research was published in the journal Physical Review Letters.

Source: University of California Riverside