Centaurus A’s satellites rotate together

New observations confirm what astronomers have seen elsewhere … and challenge current dark matter models.
By | Published: February 1, 2018 | Last updated on May 18, 2023
Centaurus A (NGC 5128) is an active galaxy that likely resulted from the recent merger of an elliptical with a spiral. Astronomers have now shown that 14 of 16 surveyed satellite galaxies rotate around the parent galaxy coherently in a plane.
The force of gravity locks smaller things in place around big ones. This holds true for galaxies, including our own, which maintain systems of satellite galaxies (for example, our Large and Small Magellanic Clouds). While the current standard cosmological model used by astronomers to describe and understand the universe states that these satellites should be randomly distributed and orbit around their parent galaxy in a disorderly fashion, new observations have just shown, for the third time, that this is not the case.

The results, published today in Science, show that the satellite galaxies surrounding Centaurus A, a well-known and relatively nearby galaxy 13 million light-years away, are orbiting not only in a single plane, but in a nice, orderly fashion as well. Such observations confirm what astronomers have already seen around our own Milky Way and the Andromeda Galaxy, but fly in the face of the standard cosmological model, which say that such systems should be rare — as in, 0.5 percent of galaxies. But at three for three, astronomers have seen it in 100 percent of systems, instead.

The Andromeda Galaxy, along with two of its satellites: M110 can be seen below the plane, and M32 as the small, round object just above it.
Adam Evans
Of course, outliers that don’t follow the model’s predictions should exist. And three systems is still an extremely small number — but even so few observations now show that satellites are much more likely to orbit in an orderly fashion than believed. It’s looking like the standard cosmological model, which states that galaxies are largely made up of dark matter, may need some tweaks. “Coherent movement seems to be a universal phenomenon that demands new explanations,” said Oliver Müller of the University of Basel in Switzerland, and lead author of the study, in a press release.

Centaurus A is an ideal object around which to study satellite galaxies because its satellites appear arranged neatly on a plane that is perpendicular to the galaxy itself, and edge-on when we see it from Earth. That means any Doppler shifting of the light received from the satellite galaxies due to their motion is from their motion around the galaxy’s center, rather than overall motion toward or away from us. Out of 16 satellite galaxies observed, 14 are rotating together around the center of the galaxy. This is consistent with the findings when astronomers observe satellites around the Milky Way and Andromeda.

The 3-D spatial distribution of the CenA subgroup of satellite galaxies around Centaurus A.

O. Müller et al., Science (2018)

The ultimate conclusion is that it’s common for satellite galaxies to move together — not randomly, as dark matter models predict. But while these results put added force behind a blow to our current understanding of dark matter, they don’t necessarily mean that dark matter is no more. What they do mean, however, is that our understanding of the way dark matter interacts with the universe may not be completely correct — which makes sense, given that we’ve yet to detect it directly. Challenges to current models are the best way to hone and improve those models, pushing them to better match the universe as we make new discoveries and uncover new pieces of the puzzle.

Homepage Slider Image of Centaurus A: Christian Wolf & SkyMapper Team/Australian National University