New distance measurements from faraway galaxies further strengthen the view that the strongest burst of star formation in the universe occurred about two billion years after the Big Bang.

Reporting in the April 17 issue of the journal Nature, California Institute of Technology astronomers Scott Chapman and Andrew Blain, along with their United Kingdom colleagues Ian Smail and Rob Ivison, provide the redshifts of 10 extremely distant galaxies which strongly suggest that the most luminous galaxies ever detected were produced over a rather short period of time. Astronomers have long known that certain galaxies can be seen about a billion years after the Big Bang, but a relatively recent discovery of a type of extremely luminous galaxy -- one that is very faint in visible light, but much brighter at longer wavelengths -- is the key to the new results.

This type of galaxy was first found in 1997 using a new and much more sensitive camera for observing at submillimeter wavelengths (longer than the wavelengths of visible light that allows us to see, but somewhat shorter than radio waves). The camera was attached to the James Clerk Maxwell Telescope (JCMT), on Mauna Kea in Hawaii.

Submillimeter radiation is produced by warm galactic "dust" -- micron-sized solid particles similar to diesel soot that are interspersed between the stars in galaxies. Based on their unusual spectra, experts have thought it possible that these "submillimeter galaxies" could be found even closer in time to the Big Bang.

Because the JCMT cannot see details of the sky that are as fine as details seen by telescopes operating at visible and radio wavelengths, and because the submillimeter galaxies are very faint, researchers have had a hard time determining the precise locations of the submillimeter galaxies and measuring their distances. Without an accurate distance, it is difficult to tell how much energy such galaxies produce; and with no idea of how powerful they are, it is uncertain how important such galaxies are in the universe.

The new results combine the work of several instruments, including the Very Large Array in New Mexico (the world's most sensitive radio telescope), and one of the 10-meter telescopes at the W. M. Keck Observatory on Mauna Kea, which are the world's largest optical telescopes. These instruments first pinpointed the position of the submillimeter galaxies, and then measured their distances. Today's article in Nature reports the first 10 distances obtained.

The Keck telescope found the faint spectral signature of radiation that is emitted, at a single ultraviolet wavelength of 0.1215 micrometers, by hydrogen gas excited by either a large number of hot, young stars or by the energy released as matter spirals into a black hole at the core of a galaxy. The radiation is detected at a longer, redder wavelength, having been Doppler shifted by the rapid expansion of the universe while the light has been traveling to Earth.

All 10 of the submillimeter galaxies that were detected emitted the light that we see today when the universe was less than half its present age. The most distant produced its light only two billion years after the Big Bang (12 billion years ago). Thus, the submillimeter galaxies are now confirmed to be the most luminous type of galaxies in the universe, several hundred times more luminous than our Milky Way, and 10 trillion times more luminous than the sun.

It is likely that the formation of such extreme objects had to wait for a certain size of a galaxy to grow from an initially almost uniform universe and to become enriched with carbon, silicon, and oxygen from the first stars. The time when the submillimeter galaxies shone brightly can also provide information about how the sizes and makeup of galaxies developed at earlier times.

By detecting these galaxies, the Caltech astronomers have provided an accurate census of the most extreme galaxies in the universe at the peak of their activity and witnessed the most dramatic period of star buildup yet seen in the Milky Way and nearby galaxies. Now that their distances are known accurately, other measurements can be made to investigate the details of their power source, and to find out what galaxies will result when their intense bursts of activity come to an end.