Finding the forest despite the trees: two new galaxies, hidden among, the stars

About fifteen years ago, a group of leading astronomers judged that the time was right for a new map of the sky. The map would tap revolutionary techniques and technology, including electronic cameras that collect light far faster than photographic film; computerized telescopes that aim and capture images supereffectively; and global computer networks that process, analyze, and store massive amounts of data.

Today, this magnum opus of cosmic study, called the Sloan Digital Sky Survey (SDSS), is well on its way to completion. To date, the database released to the public includes more than 8,000 gigabytes of images, 672,000 spectra, and 180 million catalogued objects (to take a look, visit the project's SkyServer Web site at http://cas.sdss. org). The survey has already led to major advances in just about every area of astronomy, including the detection of nearby dwarf stars, distant quasars, and the origins of structure in the universe.

The latest discovery to join the SDSS hit parade comes from a pair of studies led by Beth Willman, an astrophysicist at New York University's Center for Cosmology and Particle Physics. Willman and her colleagues have scoured the Sloan database for signs of small, faint dwarf galaxies hiding in the outskirts of the Milky Way. Now, it appears, they've found two.

You might think that the closer a galaxy lies to our own Milky Way, the easier it would be to see. That's certainly the case for larger galaxies, such as the Andromeda Galaxy or the Magellanic Clouds, which you can see with the naked eye. It's also true for smaller galaxies with bright cores of densely packed stars, such as the dwarf elliptical galaxy known as Messier 32, which can readily be detected with a small telescope. But a dwarf galaxy much closer to the Milky Way than Messier 32, made up mostly of faint (and old) stars, blends right into the foreground of the hundreds of billions of stars in the Milky Way. Furthermore, a nearby dwarf galaxy takes up a much larger patch of sky than a distant one, and so it doesn't stand out as a distinct object.