Measuring galaxies by how light shifts

To build 3-D maps of galaxies, astronomers use a familiar celestial-coordinate system for plotting the objects' positions in the sky.

But to give their work that extra dimension - depth - they must rely on redshifts. This is why galaxy-mapping efforts commonly are called redshift surveys.

How do they work? When heated sufficiently, elements such as hydrogen emit light at unique wavelengths. Spectrographs stretch the spectrum of incoming light and pick out these chemical fingerprints. By comparing spectra from hydrogen in a galaxy to spectra from a lab sample of hydrogen, scientists can measure how far the galaxy's fingerprints have shifted toward the red end of the spectrum. A redshift also can be used to calculate how fast the expansion of space is carrying off the galaxy. The farther the galaxy, the longer the redshift and the faster the expansion. But a galaxy's motion also is affected by any nearby galaxies. Astronomers must refine their calculations to separate this "peculiar velocity" from the velocity due to the expanding universe.

Ask astronomers how far into space a redshift survey will go, and they'll likely answer either with a redshift number or with a recession velocity.

And the instruments that take these measurements? Hectospec, the spectrometer destined for the Smithsonian's 6.5-meter telescope on Mt. Hopkins near Tuscon, Ariz., uses 300 tiny fiber-optic lines to pull in the light. The spectrograph's positioning system can re-aim the fiber-optic lines in 5 to 10 minutes, allowing astronomers to gather data on several thousand galaxies a night. During the Center for Astrophysics's "stickman" survey, researchers took a year to gather data on 1,700 galaxies.

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