Since 2008 the international PANDAS (Pan-Andromeda Archaeological Survey) collaboration has been availing itself of the felicitous proximity of the Andromeda galaxy to examine relics of its formation (see Physics Today, November 2009, page 15). Only 2.5 million light-years (ly) away, Andromeda is a large galaxy very much like our own, encompassing 1012 stars within its iconic spiral-armed disk, 105 ly in diameter. The team searches the much larger, spherical halo of stellar and dark matter that envelops Andromeda with a wide-field camera on the Canada-France-Hawaii Telescope atop Hawaii’s Mauna Kea.

An important PANDAS goal has been the completion of an unbiased census of dwarf satellite galaxies within Andromeda’s halo. That task addresses the troubling “missing-satellite problem.” Dark-matter computer simulations of large-galaxy formation predict many more dwarf satellite galaxies than observers have been finding.

The PANDAS detection threshold is encouraging; at Andromeda’s distance, the team can see...

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