Dwarf galaxies put dark matter to the test

U. MICHIGAN (US) — Two satellite dwarf galaxies found 1.1 million and 600,000 light years from Andromeda, the closest spiral galaxy to Earth, may help reveal the nature of dark matter.

Astronomers discovered Andromeda XXVIII and Andromeda XXIX by using a tested star-counting technique on the newest data from the Sloan Digital Sky Survey, which has mapped more than a third of the night sky and follow-up data from the Gemini North Telescope in Hawaii.

Two of the furthest satellite galaxies ever detected, they are 100,000 times fainter than Andromeda, are invisible to the naked eye, and can barely be seen even with large telescopes. The research is reported in the Nov. 20 edition of Astrophysical Journal.

The astronomers set out looking for dwarf galaxies around Andromeda to help them understand how matter relates to dark matter, an invisible substance that doesn’t emit or reflect light, but is believed to make up most of the universe’s mass.

Astronomers believe it exists because they can detect its gravitational effects on visible matter. With its gravity, dark matter may be responsible for organizing visible matter into galaxies.

“These faint, dwarf, relatively nearby galaxies are a real battleground in trying to understand how dark matter acts at small scales,” says Eric Bell, associate professor of astronomy at University of Michigan. “The stakes are high.”

The prevailing hypothesis is that visible galaxies are all nestled in beds of dark matter, and each bed of dark matter has a galaxy in it. For a given volume of universe, the predictions match observations of large galaxies.

“But it seems to break down when we get to smaller galaxies,” explains astronomy PhD student Colin Slater. “The models predict far more dark matter halos than we observe galaxies. We don’t know if it’s because we’re not seeing all of the galaxies or because our predictions are wrong.”

“The exciting answer,” Bell says, “would be that there just aren’t that many dark matter halos. This is part of the grand effort to test that paradigm.”

The research is funded in part by the National Science Foundation.

More news from University of Michigan: www.ns.umich.edu/