Cosmological simulations include dozens of prescriptions to describe the 13.8-billion-year evolution of the Universe, including numerical recipes for dark energy (so-called lambda), weakly interacting cold dark matter, gas accretion onto primordial galaxies, star formation and evolution, and feedback from quasars and supernovae. The outcome of these lambda cold dark matter simulations reproduce many of the observed features of the real universe. However, the models predict that dozens of small dwarf satellite galaxies should orbit medium-sized galaxies like our Milky Way and Andromeda in random orientations, but new research suggests most satellite galaxies orbit their host galaxies aligned along a single plane.
Vast Polar Structure
“So this means that we are missing something,” said Marcel Pawlowski, a Schwarzschild Fellow at the Leibniz-Institute for Astrophysics about the finding that smaller systems of stars should be more or less randomly scattered around their anchoring galaxies and should move in all directions. Yet massive elliptical galaxy Centaurus A (Hubble image above) is the third documented example, behind the Milky Way and Andromeda, of a “vast polar structure” in which satellite dwarfs co-rotate around a central galactic mass in what Pawlowski calls “preferentially oriented alignment.”
“Either the simulations lack some important ingredient, or the underlying model is wrong. This research may be seen as support for looking into alternative models.”
Validity of Cosmological Models and Simulations Questioned
“The significance of this finding is that it calls into question the validity of certain cosmological models and simulations as explanations for the distribution of host and satellite galaxies in the universe,” said co-author Pawlowski.
An international team of astronomers has determined that Centaurus A.13 million light-years from Earth, is accompanied by a number of dwarf satellite galaxies orbiting the main body in a narrow disk. In a paper published in Science in 2018, the researchers noted that this is the first time such a galactic arrangement has been observed outside the Local Group, home to the Milky Way.
The difficulty of studying the movements of dwarf satellites around their hosts varies according to the target galaxy group. It’s relatively easy for the Milky Way. “You get proper motions,” Pawlowski said. “You take a picture now, wait three years or more, and then take another picture to see how the stars have moved; that gives you the tangential velocity.”
11 Milky Way Satellite Galaxies Measured
Using this technique, scientists have measurements for 11 Milky Way satellite galaxies, eight of which are orbiting in a tight disk perpendicular to the spiral galaxy’s plane. There are probably other satellites in the system that can’t be seen from Earth because they’re blocked by the Milky Way’s dusty disk.
Andromeda provides observers on Earth a view of the full distribution of satellites around the galaxy’s sprawling spiral. An earlier study found 27 dwarf galaxies, 15 arranged in a narrow plane. And Andromeda offers another advantage, according to Pawlowski: “Because you see the galaxy almost edge-on, you can look at the line-of-sight velocities of its satellites to see the ones that are approaching and those that are receding, so it very clearly presents as a rotating disk.”
Centaurus’s Satellites –“Sleeping in the Archives”
Centaurus A is much farther away, and its satellite companions are faint, making it more difficult to accurately measure distances and velocities to determine movements and distributions. But “sleeping in the archives,” Pawlowski said, were data on 16 of Centaurus A’s satellites.
“We could do the same game as with Andromeda, where we look at the line-of-sight velocities,” Pawlowski said. “And again we see that half of them are red-shifted, meaning they are receding from us, and the other half are blue-shifted, which tells us they are approaching.”
The researchers were able to demonstrate that 14 of the 16 Centaurus A satellite galaxies follow a common motion pattern and rotate along the plane around the main galaxy – contradicting frequently used cosmological models and simulations suggesting that only about 0.5 percent of satellite galaxy systems in the nearby universe should exhibit this pattern.
“Something Unexpected is Going On in that Satellite Galaxy” ––New Findings
“The original study was based on archival data and contained a sample of only 16 satellite galaxies around Centaurus A.” Pawlowski wrote in an email to The Daily Galaxy. “While that was sufficient to identify a substantial mismatch, there are many more satellites known in that system. We were able to obtain time with the fantastic MUSE instrument on ESO’s Very Large Telescope to measure spectroscopic velocities for another 12 satellites. This new data confirms the strong kinematic correlation we identified originally, and was published early this year in Müller et al. (2021).”
“So, on the observational side the tension with cosmological expectations remains in place, even though the new data could in principle have shown the opposite,” Pawlowski notes.”I believe that’s a very reassuring confirmation of our original findings, but an alarming result for the cosmological model. Something unexpected is indeed going on in that satellite galaxy system.
“There has also been progress in the theoretical side since 2018, so we wondered whether that might help resolve the issue,” Pawlowski adds in his email. “In our new study, we thus now compare to the state of the art IllustrisTNG simulation, which implements a number of improvements over previous simulations. Yet, we again confirm a similar degree of tension, systems with as an extreme degree of correlation among their satellite galaxies as observed for Centaurus A remain rare in these simulated universes. Only one in a few hundred should show similar structures. Additionally, as before there does not seem to be a significant difference between simulations that model the complexities of baryonic physics, and those that merely focus on the dark matter distribution. The problem posed by these planes of satellite galaxies seems quite fundamental, as little tweaks in the details of simulations are insufficient to address it.”
“Beyond work on this specific system, Pawlowski observed, our findings have since motivated a number of studies investigating other satellite galaxy systems. Intriguingly, several of these find hints of similar structures, for example around NGC 253 and NGC 2750 as well as in a broader statistical study of over 100 satellite systems.”
Pawlowski concludes in his email: “There has of course also been a lot of progress on the study of the satellite galaxy planes in the Local Group, which were known to pose a problem for the cosmological expectation already before our Centaurus A work.”
Image credit top of page: Centaurus A, also known as NGC 5128, is known for its dramatic dusty lanes of dark material. Hubble’s observations, using its most advanced instrument, the Wide Field Camera 3, are the most detailed ever made of this galaxy. As well as features in the visible spectrum, this composite shows ultraviolet light, which comes from young stars, and near-infrared light, which lets us glimpse some of the detail otherwise obscured by the dust. NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: R. O’Connell (University of Virginia) and the WFC3 Scientific Oversight Committee.