Missing dark matter mystery reveals a galaxy entering death spiral
Astronomers are a step closer to solving a cosmic mystery, thanks to new Hubble data. Observations from the space telescope show a dwarf galaxy previously found to be missing most of its dark matter is being stripped of this matter by a larger nearby galaxy, and it's beginning to tear the smaller galaxy apart.
Dark matter has been a feature of cosmology since the 1930s, when the idea was first proposed. The speed at which stars and galaxies move is supposed to be proportional to their mass – but in 1933, Swiss astrophysicist Fritz Zwicky observed clusters moving much faster than they “should” have been able to, based on the mass of their visible matter. He hypothesized that there had to be huge amounts of invisible matter out there, adding to the overall mass.
In the decades since, the idea has been backed up in countless observations – essentially every galaxy studied is moving faster than its visible matter allows. That is until 2018, when galaxy NGC1052-DF2 was the first discovered to be moving like it had much less dark matter. This surprising find was followed the next year by a similarly light galaxy called NGC1052-DF4.
That may sound like a blow to the dark matter hypothesis, but it’s actually the exception that proves the rule. The main argument against dark matter is that it’s simply an error in our calculations, but if that was the case the same error should apply across the board. The absence of dark matter in two galaxies shows that it is a quantifiable substance that can be present in higher or lower amounts.
The question then is, what happened to all the dark matter in these galaxies? An international team of astronomers has studied DF4 closer and now believe they have an idea where it went – it was stripped away through the gravitational influence of a larger galaxy called NGC 1035. Simulations recently suggested it was possible, but now that’s been backed up by observations.
In the new study, the team performed deep optical imaging of DF4 using the Hubble Space Telescope, examining its light and the distribution of groups of stars called globular clusters. The layout of these clusters suggests that they’re in the early stages of being pulled out of their home galaxy, and the light analysis reveals evidence of “tidal tails” – stars that are already on their way out.
Since dark matter is spread out more diffusely than regular matter, it would be slurped out of a galaxy first. Once it’s gone, stars begin to follow the same fate.
“This result is a good indicator that, while the dark matter of the galaxy was evaporated from the system, the stars are only now starting to suffer the disruption mechanism,” says Ignacio Trujillo, an author of the study. “In time, NGC1052-DF4 will be cannibalized by the large system around NGC1035, with at least some of their stars floating free in deep space.”
The research was published in the Astrophysical Journal.