The dwarf galaxy Andromeda 29, which University of Michigan astronomers have discovered, is clustered toward the middle of this image, obtained with the Gemini North telescope in Hawaii. Image: Gemini Observstory/AURA/Eric Bell |
In work that could help
advance astronomers’ understanding of dark matter, University of Michigan
researchers have discovered two additional dwarf galaxies that appear to be
satellites of Andromeda, the closest spiral galaxy to Earth.
Eric Bell, an associate
professor in astronomy, and Colin Slater, an astronomy PhD student, found
Andromeda XXVIII and XXIX—that’s 28 and 29. They did it 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. They also used follow-up
data from the Gemini North Telescope in Hawaii.
At 1.1 million and 600,000
light years from Andromeda, these are two of the furthest satellite galaxies
ever detected. Invisible to the naked eye, the galaxies are 100,000 times
fainter than Andromeda, and can barely be seen even with large telescopes.
The findings are published
in the Astrophysical Journal.
These 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 is believed to 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,” Bell
says. “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,” Slater says. “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.” Bell said. “This is
part of the grand effort to test that paradigm.”
The papers are titled,
“Andromeda XXIX: A New Dwarf Spheroidal Galaxy 200 kpc from
Andromeda,” and Andromeda XXVIII: A Dwarf Galaxy more than 350 kpc from
Andromeda.”
