This image shows a standard prediction for the dark matter distribution
within about 1 million light years of the Milky Way galaxy, which is
expected to be swarming with thousands of small dark matter clumps
called `halos'. The scale of this image is such that the disk of the
Milky Way would reside within the white region at the center. Until
now, there was no observational evidence that dark matter actually
clumps this way, raising concerns that our understanding of the cosmos
was flawed in a fundamental way. Observations of the ultra-faint galaxy
Segue 2 (zoomed image) have revealed that it must reside within such a
tiny dark matter halo, providing possibly the first observational
evidence that dark matter is as clumpy as long predicted. Credit: Garrison-Kimmel, Bullock (UCI)
Kamuela, Hawaii—The least massive galaxy in the known universe
has been measured by UC Irvine scientists, clocking in at just 1,000 or
so stars with a bit of dark matter holding them together.
The findings, made with the W. M. Keck Observatory and published today in The Astrophysical Journal,
offer tantalizing clues about how iron, carbon and other elements key
to human life originally formed. But the size and weight of Segue 2, as
the star body is called, are its most extraordinary aspects.
“Finding a galaxy as tiny as Segue 2 is like discovering an elephant
smaller than a mouse,” said UC Irvine cosmologist James Bullock,
co-author of the paper. Astronomers have been searching for years for
this type of dwarf galaxy, long predicted to be swarming around the
Milky Way. Their inability to find any, he said, “has been a major
puzzle, suggesting that perhaps our theoretical understanding of
structure formation in the universe was flawed in a serious way.”
Segue 2’s presence as a satellite of our home galaxy could be “a
tip-of-the-iceberg observation, with perhaps thousands more very
low-mass systems orbiting just beyond our ability to detect them,” he
added.
“It’s definitely a galaxy, not a star cluster,” said postdoctoral
scholar and lead author Evan Kirby. He explained that the stars are held
together by a globule called a dark matter halo. Without this acting as
galactic glue, the star body wouldn’t qualify as a galaxy.
Segue 2, discovered in 2009 as part of the massive Sloan Digital Sky
Survey, is one of the faintest known galaxies, with light output just
900 times that of the sun. That’s miniscule compared to the Milky Way,
which shines 20 billion times brighter. But despite its tiny size,
researchers using different tools originally thought Segue 2 was far
denser.
“Keck Observatory operates the only telescopes in the world powerful
enough to have made this observation,” Kirby said of the twin, ten-meter
telescopes housed on the summit of Mauna Kea in Hawaii. He determined
the upper weight range of 25 of the major stars in the galaxy and found
that it weighs at least 10 times less than previously estimated.
Fellow authors are Michael Boylan-Kolchin and Manoj Kaplinghat of UC
Irvine, Judith Cohen of the California Institute of Technology and Marla
Geha of Yale University. Funding was provided by the Southern
California Center for Galaxy Evolution (a multicampus research program
of the University of California) and by the National Science Foundation.
The W. M. Keck Observatory operates the largest, most scientifically
productive telescopes on Earth. The two, 10-meter optical/infrared
telescopes on the summit of Mauna Kea on the Island of Hawaii feature a
suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field
spectroscopy and a world-leading laser guide star adaptive optics
system. The Observatory is a private 501(c) 3 non-profit organization
and a scientific partnership of the California Institute of Technology,
the University of California and NASA.
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