Several dwarf galaxies orbiting around a nearby elliptical galaxy called Centaurus A are rotating in synchrony around their host, to astronomers’ surprise. This is inconsistent with more than 99% of comparable galaxies in simulations.
Centaurus A. Image credit: NASA / ESA / Hubble Space Telescope / ESO / Robert Gendler & Roberto Colombari.
Current theories of galaxy formation, based on standard cosmological ingredients such as dark matter, predict that small dwarf satellite galaxies should orbit in random positions and directions around their closest large galaxy.
Dr. Oliver Müller from the University of Basel and his colleagues from Australia, Germany and the United States analyzed the satellite dwarf galaxies around Centaurus A (also known as NGC 5128), a large elliptical galaxy about 13 million light-years away.
They found that Centaurus A’s satellite galaxies are not only arranged in a single plane, but the plane also rotates coherently: satellite galaxies on one side of the host galaxy are approaching, while those on the other side are receding.
Of the 16 satellite galaxies for which the scientists have data, 14 followed this organized movement – and yet the probability of such a scenario is a mere 0.5% based on simulations.
Published in the journal Science, the team’s results hint that widely accepted models of cosmology and/or galaxy formation are lacking, or at least misconstruing crucial components.
“Astronomers had previously observed planes of dwarf galaxies whirling around our Milky Way Galaxy and the neighboring Andromeda Galaxy,” said co-author Dr. Helmut Jerjen, of the Australian National University.
“The latest finding challenged a long-held theory among cosmologists and theoreticians that there were thousands of dwarf galaxies in all directions around these large galaxies like bees swarming around a hive.”
“Cold dark matter theory made astronomers believe that the best studied galaxies in the Universe — the Milky Way and Andromeda — are the odd ones out,” he said.
“It seems that our Milky Way and Andromeda are normal galaxies after all, and spinning pancake-like systems of satellite galaxies are more common than scientists expected.”
Dr. Jerjen added dwarf galaxies were distributed in planes that were almost perpendicular to the disks of the Milky Way, Andromeda and Centaurus A.
“Even the best cosmological simulations struggle to explain the phenomenon of these small galaxies revolving in one direction around host galaxies.”
“It was likely most large galaxies in the Universe have had a close encounter or merged with another galaxy at least once in their life,” Dr. Müller said.
“Co-rotating dwarf galaxy systems could have formed during such interactions. In this scenario dwarf galaxies should be devoid of dark matter.”
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Oliver Müller et al. 2018. A whirling plane of satellite galaxies around Centaurus A challenges cold dark matter cosmology. Science 359 (6375): 534-537; doi: 10.1126/science.aao1858
