Hubble telescope reveals mysterious 'ghost galaxies' of ancient universe

The team is actually studying Hubble observations of six faint dwarf galaxies, but only completed its analysis of the Hercules, Leo IV and Ursa Major objects. The Hubble observations are follow-up looks at data from the Sloan Digital Sky Survey, which identified about a dozen of the ultra-faint galaxies.

The three galaxies observed in Brown's study are irregular objects that coalesced about 100 million years before reionization began. They are only 2,000 light-years wide, smaller than the dwarf galaxies seen today near the Milky Way. They are all between 330,000 and 490,000 light-years from Earth.

The process of reionization may have stripped the galactic dwarfs of the vital gas required to build more stars and grow into larger galaxies, the researchers said. With little active star formation, such dwarf galaxies could be all but invisible to astronomers trying to understand why so few of the objects have been found, when theories predict that thousands should be visible, they added.

There is one more oddity about the faint ancient dwarf galaxies. They appear to have 100 times as much dark matter as normal visible matter, the researchers said. That's substantially more dark matter than the younger, brighter dwarf galaxies seen near the Milky Way, which typically have about 10 times as much dark matter as normal matter, they added.

"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 said.

The research is detailed in the July 1 edition of the Astrophysical Journal Letters. Brown and his team used the Advanced Camera for Surveys aboard Hubble to obtain the new dwarf galaxy views.

The Hubble Space Telescope has been peering deep into the universe since its launch in 1990. The space observatory's mission overseen by NASA and the European Space Agency.

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