Smallest galaxy known could confirm how the Universe formed

By James Maynard , Jun 10, 2013 06:43 PM EDT

The smallest galaxy ever seen has just been measured, orbiting the Milky Way. This tiny companion of our own galaxy, named Segue 2, is a collection of only around 1,000 stars. The entire dwarf galaxy is just 900 times brighter than our Sun, or 20 million times dimmer than the Milky Way.

Astronomers measured this tiny collection of stars using the Keck Observatory in Hawaii. By determining the upper weight limit of 25 stars in the galaxy, the team was able to deduce that Segue 2 has only one-tenth the mass previously believed. This leads James Bullock, a cosmologist from the University of California, to believe that there could be many more such dwarf galaxies that are just slightly too dim to have been detected so far.

"Finding a galaxy as tiny as Segue 2 is like discovering an elephant smaller than a mouse," Bullock said.

There are collections of stars around this size called star clusters. Segue 2 is classified as a galaxy and not a cluster is because it is held together by the presence of a dark matter halo, according to Evan Kirby, one of the researchers involved in the study.

This finding might also help theories regarding the formation of the Universe. The model currently favored by most astronomers would tend to produce more of these dwarf galaxies surrounding our own galaxy than we see. This is the smallest such dwarf galaxy known.

The question among astronomers as to why small collections of stars like Segue 2 are so rare "has been a major puzzle, suggesting that perhaps our theoretical understanding of structure formation in the universe was flaws in a serious way," said Bullock.

Segue 2 is so dim that it was not first seen until 2009, when it was detected as part of the Sloan Digital Sky Survey. That eight-year project, beginning in the year 2000, mapped over 930,000 galaxies.

Little was known about Segue 2 because most observatories could not make out any detail in the grouping. The Keck Observatory, a pair of telescopes each almost 33 feet in diameter, made detailed observations of the dwarf galaxy possible.

Researchers also believe that these new measurements of Segue 2 can help uncover more information about how carbon, iron and other elements necessary for life form.

Results of the finding were published in The Astrophysical Journal June 10.

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