First Milky Ways found at edge of Universe
Posted: January 22, 2003

With their giant telescopes pointed toward the heavens, astronomers look back in time to when young galaxies were just beginning to coalesce and when the first generations of stars were forming -- stars without planets in a realm dominated by hydrogen and helium. One key question that has puzzled astronomers for decades is: When did the first stars and galaxies form after the Big Bang occurred?

The answer -- very quickly! Astronomers Rennan Barkana (Tel Aviv University) and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) have found the first direct evidence that galaxies as large as the Milky Way already had formed when the Universe was less than a billion years old.

Simulated structure of the early universe. The Milky-Way sized galaxies discovered by Rennan Barkana and Avi Loeb would be the bright points at the vertices of these galactic filaments. Credit: Volker Springel, Max Planck Institute for Astrophysics and Harvard-Smithsonian Center for Astrophysics, and Lars Hernquist, Harvard-Smithsonian Center for Astrophysics
"In some ways, it's surprising that such large galaxies formed so quickly. Most galaxies in the early Universe were only one-hundredth that size," said Loeb. "But our model, combined with observations by other researchers, provides clear evidence that massive galaxies existed within a relatively short time after the Big Bang."

Intriguingly, the large galaxies discovered by Barkana and Loeb are still around today. Over billions of years, they continued to consume smaller galaxies, like a cosmic software corporation absorbing many smaller companies. These galactic cannibals have grown from the seeds that existed in a billion-year-old Universe to become monstrous giant elliptical galaxies, resting in the centers of galaxy clusters.

Distant lighthouses
To learn about the early Universe, astronomers study the most distant objects -- quasars whose light has traveled for billions of years to reach the Earth. Quasars (short for quasi-stellar objects) are the brightest known astronomical objects. Their great luminosities are believed to be powered by supermassive black holes. A black hole acts as a quasar's central "engine," gulping down huge amounts of gas and blasting enormous quantities of radiation into space, creating a beacon visible for billions of light-years.

Studies of nearby galaxies have shown that a black hole's mass tends to be correlated with the mass of its host galaxy. That is, big galaxies have big black holes while little galaxies have little black holes. Astronomers expected that the same would be true of the more distant black holes in the early Universe, but they had no evidence to prove it. Barkana and Loeb have provided that evidence.

In studying the spectra of quasars -- the intensity of their light at different wavelengths, or colors -- astronomers had recorded a curious feature which did not attract their attention. Certain quasars showed a "double-horn" profile in their spectra. Barkana and Loeb created a computer model that explained the spectral feature as being the result of absorption by hydrogen gas.

Intergalactic hydrogen falling into a quasar's host galaxy absorbs some of the quasar's light. This infall can be used to measure the host galaxy's mass. Barkana and Loeb found that the two quasars they examined, for which detailed spectra were available, lie in galaxies about as massive as the Milky Way.

"This is the first time that the mass of an early galaxy has been directly measured," said Barkana.

Tip of a cosmic iceberg

According to the widely accepted hierarchical model of galaxy formation, the first structures to form in the early Universe were small protogalaxies containing the mass of only a few thousand Suns. Over billions of years, protogalaxies collided to form the larger galaxies we see today. This process takes time, so it is intriguing that relatively large, Milky-Way-sized galaxies could have formed in less than a billion years.

"What we've found is the tip of the iceberg," said Loeb. "We studied the brightest quasars and found them to be in the most massive galaxies existing at that time. Many smaller galaxies also were around, containing only about one-hundredth the mass of the Milky Way. We don't see those baby galaxies because, even if they contain quasars, they would be fainter and more difficult to see."

Loeb also points out that, while the masses of the bright quasars' host galaxies were similar to the Milky Way, there also is an important difference. "The Milky Way has a small black hole at its center, containing only about three million solar masses. These early galaxies, even though they've had less time to form, contain black holes of up to one billion solar masses."

So far, Barkana and Loeb have applied their model to two high-redshift quasars for which high-resolution spectra were available. (Redshift is a measure of how fast an object is receding from us due to the expansion of the universe. Higher redshifts indicate greater recessional speeds and hence greater distances.) High-resolution spectral observations of additional quasars are needed to confirm their model.

This research is being reported in the January 23, 2003 issue of the journal Nature.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into six research divisions study the origin, evolution, and ultimate fate of the universe.

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