Distant, Dusty Galaxies

These newly-found, far-flung systems are forming stars at an astounding rate
New Galaxies
Image: BARGER et al.
BLANK FIELD? SCUBA's record breaking 51-hour exposure of the well-studied field SSA13 reveals several new submillimeter sources.
When it comes to star formation, there's a lot more going on than meets the eye. Last Thursday, researchers announced the discovery of distant, invisible galaxies that radiate as much stellar energy as the entire optical u niverse. The report, authored by Lennox L. Cowie and colleagues both at the University of Hawaii and in Japan, appear in the journal Nature July 16, 1998.

The findings, which are backed by another paper in the same issue by David Hughes and colleagues of the University of Edinburgh, are forcing scientists to reassess theories about how and when stars are born. Earlier surveys, largely restricted to visible galaxies, suggested that star formation peaked when the universe was roughly three quarters of its current age. But the new studies make it clear that very distant--and so even older--galaxies were just as busy.

"The recent submillimeter observations have opened an exciting new era in cosmological exploration comparable to that which occured with the restoration of image quality with the Hubble Space Telescope," said Richard S. Ellis, director of the Institute of Astronomy at the University of Cambridge. "The pioneering deep exposures conducted by groups in Hawaii, the U.K. and Canada have shown the importance of studying galaxies at large look-back times at wavelengths other than simply the traditional optical and infrared regions."

Image: SCUBA
SUPER SCUBA. This radical new bolomoter array, mounted on top of the 15-meter JCMT telescope in Mauna Kea, offers a glimpse at primordial galaxies.

In fairness, such far-off star systems are difficult to detect. Dust absorbs visible starlight and reradiates it at much longer wavelengths. In nearby dusty galaxies, these longer wavelengths still fall within the far-infrared range, at one end of the visible spectrum. In distant ones, though, the reradiated energy is further "red shifted" by the Universe's expansion to even longer wavelengths--those just under a millimeter. (For comparison, visible light is measured in nanometers, a unit 1,000,000 times smaller.) As a result, these sources are described as submillimeter galaxies.

Both teams of astronomers owe their success to a new instrument called SCUBA, for Submillimeter Common-User Bolometer Array, which they mounted on the 15-meter James Clerk Maxwell Telescope (JCMT) on top of Mauna Kea. A bolometer measures the heat emitted from small dust particles in the submillimeter sky, much as a camera measures the light radiated by nearer sources.

SCUBA is far more sensitive than its predecessors thanks to two sets of bolometers: an array of 37 detectors that measure heat emissions at 850 microns in wavelength, and array of 91 detectors that measure emissions at 450 microns. Cowie and colleagues performed deep surveys of two blank regions of the sky, Lockman Hole and the well-studied field SSA13. The group led by Hughes zeroed in on the Hubble Deep Field and found several objects that were not readily picked out on optical images of the same region--confirming that these galaxies are indeed a new population.

The galaxies discovered by the Hawaii-Japan team appear to be forming stars 10 to 100 times faster than typical visible galaxies. And the only known objects they resemble are ultraluminous infrared galaxies, first discovered by IRAS satellite. Because these infrared sources are thought to result from a merger between two gas-rich galaxies, some suggest that the submillimeter galaxies may well represent systems being assembled from smaller, colliding pieces.

Image: HUGHES et al.
OVERLAY. This image, a composite of optical and SCUBA maps of the Hubble Deep Field, shows previously undected submillimeter galaxies (circled).

Clearly these results are just the beginning. Before SCUBA, bolometer measurements were restricted to known and bizarre objects. And earlier SCUBA maps were made only of targeted sources that were for various reasons--such as their proximity to high-redshift-rich clusters--easy to detect. A thorough census of the submillimeter sky must be completed before astronomers will have a comprehensive picture of star formation. At least now they have the tool to do it.
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