First Galaxies Found at Edge of Universe

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.

“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.

NOTE: An image to accompany this release can be found at
http://cfa-www.harvard.edu/press/pr0304image.html.

Source: Harvard-Smithsonian Center for Astrophysics