Mature before their time: in the youthful universe, some galaxies were already old

Science News, March 1, 2003 by Ron Cowen

This winter has been one of the hottest on record for cosmologists. A flurry of new reports suggests that a surprising number of galaxies grew up in a hurry, appearing old and massive even when the universe was still very young. If this portrait of precocious galaxies is confirmed by larger studies, astronomers may have to revise the accepted view of galaxy formation. The provocative reports started pouring in just before Christmas. In mid-December, scientists announced in a press release that they had found a group of distant galaxies that were already senior citizens, chockablock with elderly, red stars a mere 2 billion years after the Big Bang. The same team found another surprise: Some of those galaxies were nearly as large as the largest galaxies in the universe today.

On Jan. 7, another team posted an online report asserting it had found the oldest, and therefore most distant, galaxy known. If confirmed, the study indicates that some galaxies were in place and forming stars at a prolific rate when the universe, now 13.7 billion years old, was just an 800-million-year-old whippersnapper.

The next galaxy-shaking event occurred on Jan. 9, when astronomers reported in Seattle at a meeting of the American Astronomical Society that they had found the farthest known quasar. This quasar is so distant that the light it emitted 13 billion years ago, when the universe was so young that galactic structures were still forming, has only now reached Earth. Quasars shine because they're powered by a supermassive black hole. The light of the newly discovered quasar is so bright that it almost certainly was fueled by a supermassive black hole that already had coalesced and weighed several billion times the mass of the sun.

In the Jan. 23 Nature, other researchers reported evidence that such black holes indeed formed early in the history of the universe and were already devouring matter voraciously a mere billion years after the Big Bang (SN: 1/25/03, p. 31).

Finally, on Feb. 11, cosmologists unveiled at a NASA press briefing in Washington, D.C., what maybe the piece de resistance. When David N. Spergel of Princeton University and his collaborators used a NASA satellite to study the cosmic microwave background, the radiation left over from the Big Bang, the team found something surprising. An analysis of the radiation revealed that the universe had already managed to make a plethora of stars--which had enough collective energy to ionize all the hydrogen in the cosmos--just 200 million years after the Big Bang (SN: 2/15/03, p. 99). That's several hundred million years earlier than many astronomers had estimated.

This early start in stellar mass production--and the formulation of galaxies that housed those stars--may explain why some galaxies appear old and massive when the universe was still quite young, Spergel says. However, astronomers caution, it's still uncertain how much of the chapter on early galaxy formation will need to be rewritten.

SEEING RED Astronomers have known for more than a decade that a few rare galaxies, which arose in unusually dense regions of the universe, managed to acquire a large amount of mass in a short amount of time. "A simple way to understand their early formation is that [these galaxies] are embedded within even bigger overdense regions in the same way that the tallest mountain peaks are usually sitting on the shoulders of a broader mountain range," explains Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "Because their environment is denser than average, they collapse earlier than the collapse time that is typical of objects of the same mass in the rest of the universe," he says. For these galaxies, in other words, the cosmic clock began ticking earlier.

That scenario might seem to jibe with the standard model of galaxy formation. In the model, the vast majority of galaxies are relatively late bloomers, taking many billions of years to pack on mass either by pulling in gas from the surrounding intergalactic medium or merging with neighboring galaxies. In regions of the universe that started out particularly dense, this mass-gathering action could begin sooner than elsewhere. But the standard model still can't easily account for a large number of mature or massive galaxies in the early universe.

More of these galaxies have recently popped into view because astronomers now can peer more easily into the universe at infrared wavelengths, which are invisible to the human eye. Infrared observations are critical because the galaxies that cosmologists typically want to study lie billions of light-years from Earth.

The light from distant galaxies is not only extremely faint. Because of the expansion of the universe, the light from these galaxies is also shifted to substantially longer wavelengths. For extremely distant galaxies, the visible light they originally radiated has shifted into infrared wavelengths. Viewing the visible and infrared light emitted by galaxies is crucial for determining their mass. The old stars that make up the bulk of a galaxy's mass radiate most of their light at visible and infrared wavelengths.