Dwarf Galaxies Fail To Match Expectations

NASA, Holland Ford (JHU), the ACS Science Team and ESA. A long tidal tail results from a collision between galaxies. A theory that similar events explain the Milky Way's satellite galaxies has profound implications for cosmology if proven right

The debate about the nature of dwarf galaxies has been renewed with claims that the characteristics of our smaller neighbors do not fit the dominant model of galactic formation. While the argument sounds esoteric there could be profound consequences for cosmology more generally if the latest paper proves correct.

The lambda cold dark matter model predicts that galaxies should form in halos of dark matter, with a wide distribution and effectively random motion.

"But what astronomers see is different," says Dr Marcel Pawlowski of Case Western Reserve University. "We see the satellite galaxies are in a huge disk and moving in the same direction within this disk, like the planets in our solar system moving in a thin plane in one direction around the sun. That's unexpected and could be a real problem."

The Milk Way has what is called the Magellanic Plane, which takes in almost all the known dwarf galaxies and star clusters. Satellite galaxies can be hard to find when they are hidden by the galactic center or other dense gas clouds so our knowledge may be incomplete, but a similar pattern has been observed around Andromeda, where half the satellite galaxies form the “Great Plane”.

Whether this is a problem or not for theories of galactic formation has been a subject for debate for several years.

Pawlowski is one of those who has previously suggested there distribution is not in keeping with theoretical models, having previously claimed, “The probability to find the observed clustering of streams is only 0.3%

Publishing in Monthly Notices of the Royal Astronomical Society Pawlowski and 13 co-authors have rejected three recent papers that concluded observations and theory align. "When we compared simulations using their data to what is observed by astronomers, we found a very substantial mismatch," Pawlowski says. Out of thousands of simulations of galaxies like the Milky Way, only one trial produced something that matched what is observed.

"But we also have Andromeda," Pawlowski said. "The chance to have two galaxies with such huge disks of satellite galaxies is less than one in 100,000."

If the standard formation model is wrong, some very important ideas could come into question. "The standard model contains various putative ingredients— such as dark matter and dark energy —which were introduced because the model wasn't consistent with observations," said Dr Benoit Famaey, of the University of Strasbourg,

Famaey and Pawlowski advocate an older, but now largely discarded, theory of the origins of satellite dwarf galaxies. They proposed that arrays of satellites occur when two large galaxies collided, ripping material from each and throwing it to substantial distances where it formed “tidal dwarf galaxies”. "Standard galaxies must contain dark matter, but tidal galaxies cannot contain dark matter,” says Bonn University's Professor Pavel Kroupa, another of the paper's authors. "There's a very serious conflict, and the repercussion is we do not seem to have the correct theory of gravity."

If the paper's authors are correct a major revision of dark energy and matter – thought to dominate the mass of the universe compared to the 5% we can see – will be required. Such a major challenge will require plenty more evidence behind it to gain widespread acceptance. However, co-author Professor  David Merritt of Rochester Institute of Technology says, "When you have a clear contradiction like this, you ought to focus on it. This is how progress in science is made."

Marcel Pawlowski. This simulation of the effects of two galaxies colliding suggests an explanation for the distribution of satellite galaxies around our own.

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