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| These Hubble images show the dim, star-starved dwarf galaxy Leo IV. |
Extremely faint dwarf galaxies spotted ,these ghost-like galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Digital Sky Survey. But astronomers needed NASA's Hubble Space Telescope to help solve the mystery of these star-starved galaxies.
"These galaxies are all ancient and they're all the same age, so you know something came down like a guillotine and turned off the star formation at the same time in these galaxies," said Tom Brown of the Space Telescope Science Institute in Baltimore, Md., the study's leader. "The most likely explanation is reionization."
The discovery could help explain the so-called "missing satellite problem," where only a few dozen dwarf galaxies have been observed around the Milky Way while computer simulations predict that thousands should exist. One possible explanation is that there has been very little, or even no star formation in the smallest of these dwarf galaxies, making them difficult to detect.
Normal dwarf galaxies near the Milky Way contain 10 times more dark matter than the ordinary matter that makes up gas and stars. In ultra-faint dwarf galaxies, dark matter outweighs ordinary matter by at least a factor of 100. "The small galaxies in our study are made up mostly of dark matter because their hydrogen gas was ionized and the stars got turned off," Brown explained.
These mostly dark-matter islands coexisted unseen with our Milky Way for billions of years, until astronomers began finding them in the Sloan survey.
For images and more information about this study and the Hubble Space Telescope, visit:
http://www.nasa.gov/hubble
Cheryl Gundy
Space Telescope Science Institute (STScI)
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