Astronomers have discovered two new satellite galaxies of the Milky Way and one could claim the title of the faintest yet found.
In the past two years, the Sloan Digital Sky Survey's measurements of stars stretching across a quarter of the sky have enabled astronomers to find several new companion dwarf galaxies for the Milky Way and its near-twin galaxy, Andromeda.
"The advent of very wide-area surveys means we're certainly moving to a new regime in terms of what we're able to discover," says Daniel Zucker at the University of Cambridge, UK.
Zucker's team located one of the new dwarf galaxies in the constellation Canes Venatici by finding an "over-density" of old, metal-poor stars in the Sloan data. All of the stars have similar chemical abundances and temperature, but they are quite spread out, spanning more than 6000 light years.
The dim galaxy structure is therefore not apparent in photographic images. "You would need a very, very deep exposure to bring out some structure," Zucker says.
The second galaxy, nicknamed "Boo" for its location in the constellation Bootes, was also identified in Sloan data by another Cambridge astronomer, Vasily Belokurov. His team believes the object is a dwarf galaxy with an extremely low luminosity – two to three times fainter than UMajor, the faintest galaxy previously identified.
Belokurov describes Boo as "a really bizarre object", especially because of its observed shape. The galaxy is believed to have been gravitationally disrupted by the forces of the Milky Way. Normally, such disruption leads to very elongated galaxies with stars forming two narrow tails.
But rather than a single pair of tails, Boo appears to have two pairs that form a cross. Data from the Cerro Tololo Inter-American Observatory in Chile have confirmed the galaxy's irregular shape. The team will now collect additional spectroscopic and photometric data to better understand the oddball galaxy.
The recent discoveries bring the number of Milky Way satellite galaxies identified to about a dozen. But theory predicts there should be between 300 and 500 such satellites.
Scientists have proposed various explanations for the discrepancy. Some say the galaxies that have been seen are embedded in much larger dark matter clumps that obscure many of the faintest (see "Mini-galaxies may reveal dark matter stream"). Others have suggested that as-yet undiscovered dwarf galaxies' star formation process has been cut off, making them virtually impossible to see.
"The question is: are we going to just keep finding handfuls of these things and that's it?" Zucker says. "That's certainly not going to bridge the gap between what some of the models predict and what we see."
His hunch is that astronomers will continue to find more faint galaxies, but not hundreds. "Theory and observation are going to have to meet somewhere in between," he says.
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