A recent sky survey has turned up eight new members in our Local Group of galaxies, including a new class of ultra-faint "hobbit" galaxies and what might be the smallest galaxy ever discovered.
The Local Group is a collection of about 40 galaxies, of which the Milky Way and Andromeda are the dominant members. The rest of the galaxies are mostly small satellites known as “dwarf galaxies” that are gravitationally bound to these two galaxies. The Large and Small Magellanic Clouds are two of the Milky Way’s better known dwarf galaxies.
The new galaxies were detected over the past two years as part of the Sloan Digital Sky Survey and presented last week at the 209th meeting of the American Astronomical Society in Seattle.
Seven of the new galaxies are gravitationally bound to the Milky Way, while the eighth appears to float freely in space, beyond our galaxy’s grasp.
The new Local Group members are even smaller and fainter than other known dwarf galaxies, with luminosities ranging from only a thousand to at most a few hundred thousand times that of our Sun.
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“They seem to be much fainter than anyone suspected galaxies could be before,” said study team member Daniel Zucker of Cambridge University. “So rather than dwarf galaxies, we should perhaps call them ‘hobbit galaxies.’”
The dimness could be the result of stellar age, as seven of the new galaxies contain mostly old stars. Of these seven, two are located in the constellation Canes Venatici, one in Bootes, one in Leo, one in Coma Berenices, one in Ursa Major and one in Hercules.
The eighth and most recently spotted galaxy is in many ways the most interesting. Dubbed Leo T, it is located about 1.4 million light-years away from Earth, so far away that it floats freely in space, unperturbed by the Milky Way.
Unlike the other hobbit galaxies, Leo T includes both old and young stars. It also contains large amounts of neutral hydrogen gas — a prime ingredient of star formation — suggesting it is still an active stellar nursery.
Because of its great distance, Leo T is also the dimmest of the new hobbits. “This is basically the smallest, faintest star-forming galaxy known, by orders of magnitude,” Zucker said.
Current galaxy formation theories predict our Milky Way should be surrounded by a swarm of smaller satellite galaxies. But until the new survey, only twelve had been identified. Astronomers have dubbed this issue the “missing satellite problem.”
The new galaxies could go a long way toward solving this problem and might represent just the tip of a cosmic iceberg, the researchers say.
“The Sloan Digital Sky Survey covers only a fifth of the night sky, so there must be many more dwarfs out there,” said study team member Wyn Evans, also of Cambridge University.