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Image of the Hubble Deep Field South taken by the MUSE instrument. Different symbols indicate different object types and distances.

Image of the Hubble Deep Field South taken by the MUSE instrument. Different symbols indicate different object types and distances.

ESO/MUSE consortium/R. Bacon

New instrument peers even deeper than Hubble

Over 10 days in December 1995, the Hubble Space Telescope took 342 images of the same tiny patch of sky in the constellation Ursa Major. The resulting data set, the Hubble Deep Field, revolutionized the study of the early universe by revealing the profusion of galaxies in that faint and distant era when the first galaxies were forming.

Now, in a demonstration of how astronomy has moved on in the past 20 years, a team of European astronomers has produced a similar deep field observation in just 27 hours and already revealed more than Hubble was able to do, as they report online today in Astronomy & Astrophysics. The new technology may lead to insights into galaxy evolution, says team leader Roland Bacon of the Astrophysics Research Center of Lyon in France.

The team used an instrument called the Multi Unit Spectroscopic Explorer (MUSE), attached to one of the four telescopes that make up the European Southern Observatory’s Very Large Telescope (VLT) at Cerro Paranal, Chile. Soon after MUSE was commissioned last year, a deep field observation was one of its first targets. “This was a major design driver for the instrument,” says Bacon, who is MUSE’s principal investigator. Because the original Hubble Deep Field can't be observed from Chile, the team focused on a second data set compiled from 995 images, the Hubble Deep Field South (HDF-S), which Hubble collected during September and October 1998.

MUSE is an integral field spectrograph, a new type of instrument which, instead of just recording the intensity of light in each pixel of the image, provides a full spectrum for each pixel. (In MUSE’s case, 24 spectrographs yield spectra for every one of 400 million pixels.) “It’s a whole new class of instrument … a new way of probing the [distant] universe,” says astronomer Gerry Gilmore of the University of Cambridge in the United Kingdom, who was not involved in the observation.

The extra data for each pixel allow astronomers to immediately calculate an object’s distance and composition and, for many galaxies, their internal motions. Certain very faint objects that only show up in a narrow range of wavelengths—usually swamped by other colors in a normal image—are easy to pick out. Previously, astronomers had to go back with other instruments to obtain spectra for each object one at a time.

In MUSE’s observation of HDF-S, the team was able to easily measure the distances to 189 of the galaxies in the image—10 times more than had been measured before. MUSE also detected more than 20 very faint objects that Hubble did not pick up at all. “You have all this extra information for the whole image immediately. Even stuff you didn’t know was there,” Gilmore says.

Bacon hopes in the future they will be able to use MUSE to directly image the cosmic web, the largest scale structure of the universe. Surveys have detected elements of the web—long filaments and sheets containing thousands of galaxies—separated by almost empty voids. In computer simulations, the cosmic web looks like foam. “We should be able, with MUSE’s exceptional sensitivity, to detect the faint glow of web filaments,” Bacon says. “It would be fantastic to really see it.”