Figure 1
This graph, or spectrum, shows the light from a dusty, distant galaxy
located 11 billion light-years away. The galaxy is invisible to optical
telescopes, but NASA's Spitzer Space Telescope was able to capture the
light from it and dozens of other similar galaxies using heat-seeking
infrared eyes.
Spectra are created when an instrument called a spectrograph spreads
light out into its basic parts, like a prism turning sunlight into a
rainbow. They contain the signatures, or "fingerprints," of molecules
that contribute to an object's light.
In this case, the galaxy's spectrum reveals the fingerprint for silicate
dust (large dip at right), a planetary building block like sand, only
smaller. This particular fingerprint is important because it helped
astronomers determine how far away the galaxy lies, or more specifically,
how much the galaxy's light had stretched, or "redshifted," during its
journey to Spitzer's eyes. Because the universe is expanding, a galaxy's
light will shift toward reddish wavelengths as it moves away from us.
This galaxy was found to have a redshift of 1.95, which means that its
light took about 11 billion years to get here.
The presence of the silicate fingerprint is also significant because it
implies that galaxies were ripe for planetary formation 11 billion years
ago - back to a time when the universe was 3 billion years old. The
universe is currently believed to be 13.5 billion years old. This is the
furthest back in time that silicate dust has been detected around a
galaxy.
These data were taken by Spitzer's infrared spectrograph in July, 2004.