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Catching a Virus

An innovative set of spectrographs will observe thousands of galaxies each night

Hill and MacQueen

HETDEX project scientists Gary Hill (left) and Phillip MacQueen stand beside the prototype VIRUS unit, which is attached to McDonald Observatory's Harlan J. Smith Telescope for testing. [Martin Harris]

Ambitious projects require ambitious tools, and few scientific projects are more ambitious than a quest to understand dark energy. Yet "ambitious" doesn't have to mean huge, complicated, or expensive. For HETDEX (Hobby-Eberly Telescope Dark Energy Experiment), University of Texas astronomers have designed an innovative set of instruments that will be powerful, but that will take advantage of existing technologies to hold down costs and the time required to bring them online.

The instruments are known as VIRUS — Visible Integral-Field Replicable Unit Spectrographs. Each one will use bundles of fiber optics to gather light from distant galaxies, and a spectrograph to break the light into its individual wavelengths or colors.

This information will reveal each galaxy's distance and how fast it is moving away from us, allowing HETDEX astronomers to produce a three-dimensional map of a large volume of space. The map will tell the astronomers how fast the universe was expanding at different times in its history, which will help reveal the role that dark energy has played at different epochs.

HETDEX will consist of 150 VIRUS units attached to the Hobby-Eberly Telescope (HET), which is being upgraded to view a larger section of the night sky. Splitting the spectrographs into 150 separate boxes will make it possible to build the units far more quickly and for less money than a single large spectrograph with the same capabilities, allowing HETDEX to begin observing the universe years earlier.

Light will strike HET's primary mirror, reflect to a series of smaller mirrors at the top of the telescope that sharpen the view, then be directed into the VIRUS units.

Each unit will contain a bundle of about 230 optical fibers, like those that carry telephone calls and computer data. The entire array will consist of 34,000 fibers, each of which will look at a tiny piece of the sky. If a distant galaxy lines up in a fiber's field of view, VIRUS will capture the galaxy's spectrum.

VIRUS instrument

Four images of the fiber bundles that carry light to the VIRUS spectrograph. [HETDEX/McDonald Observatory]

Astronomers expect that during any single observation, fewer than one percent of the fibers will be pointing at galaxies. Even so, the experiment will take data on several hundred galaxies during each observation, which will last around 20 minutes. After an observation is completed, the telescope will be moved slightly to view the next patch of sky, providing several dozen observations per night. HETDEX will conduct about 140 nights of observing, providing spectra for at least one million galaxies.

Target Galaxies
HETDEX will target "lyman-alpha" galaxies, which produce strong spectral lines at specific wavelengths. These are young, vigorous galaxies that are giving birth to many new stars. The energy from these young stars "excites" the clouds of hydrogen gas around them, producing a bright glow. Although lyman-alpha lines are produced in ultraviolet wavelengths, the redshift stretches them to longer wavelengths, which can be observed with ground-based telescopes.

Spectroscopy: A Powerful Tool

HETDEX will use one of astronomy's most powerful tools, called spectroscopy, to learn details about its target galaxies.

A spectrograph splits the light from an astronomical object into its individual wavelengths or colors. The intensity of each wavelength varies, and that is the key to spectroscopy's power.

Each chemical element produces its own unique "fingerprint," which shows up as an increase or decrease in the intensity of light at specific wavelengths, producing a series of bright or dark lines in an object's spectrum. From these fingerprints, astronomers can measure the chemical composition of a galaxy, star, or other object.

An object's motion toward or away from us produces a shift in the wavelength of its light through an effect called the Doppler shift.

We can observe the Doppler shift in everyday life through sound. When a train is moving toward you, the sound from its horn is compressed because the distance between you and the horn is shrinking, so the pitch of the horn goes up. As the train passes, though, the sound waves are stretched because the distance is increasing, so the pitch goes down.

The same thing happens with astronomical objects. If an object is moving toward us, its light waves are compressed a little, so the wavelengths of its component elements are shifted toward the blue end of the spectrum. But if an object is moving away from its, its light waves are stretched out, producing a shift toward the red end of the spectrum.

This "redshift" is a powerful tool for analyzing galaxies, because all but a handful of galaxies are moving away from us as a result of the expansion of the universe. The greater the redshift, the faster the galaxy is moving away from us. And there is a direct correlation between a galaxy's motion away from us and its distance: faster galaxies are farther away. So by measuring a galaxy's redshift, astronomers can measure both its speed and its distance, which in turn tells them how fast the universe was expanding at different times in its history.

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