Welcome to the early universe, less than a billion years after the big bang: It's a violent place where the rate of star birth has surged with amazing speed to baby boom proportions, but the young stars are clustered in galaxies so small and weak that they may be unable to save their inner monsters—black holes—from starvation.
Five billion years later, the galaxies have grown into hulks by accreting or colliding with one another, and they now harbor an impressive population of super-massive, gluttonous black holes. But the galactic growth rate is trailing off. In another billion years, the star birthrate will drop even more abruptly to about one-tenth the rate of the earlier boom.
The era of major cosmic construction has subsided, and the universe is only about half its present age of 13.7 billion years. (The sun and Earth will not be born for another few billion years.)
These rare snapshots of the growing universe are some of the initial findings, released Thursday, from the broadest in-depth survey ever conducted of the evolution of the largest cosmic building blocks—galaxies.
Among the survey's intriguing discoveries are seven mystery objects that could be the most distant black holes ever detected.
This is the first time that the cosmic tale of how galaxies build
themselves has been traced reliably to such early time, said Mauro
Giavalisco, leader of the Hubble Space Telescope's part of the
survey. He compared it to seeing a human at the age of 5 or 6.
really are inching back to see the toddler and the infant at
delivery—though we can't see that quite yet.
The international effort, known as GOODS, for Great Observatories
Origins Deep Survey, combines NASA's major observing platforms in
space with the world's largest ground telescopes. The effort,
Giavalisco said, so far has taken several years, more than 100
pretty much the largest mirrors on the planet.
It has generated at least 15 papers, which are undergoing peer review
for a special issue of the Astrophysical Journal Letters.
The survey's two images—of the Northern and Southern skies—show a total of about 50,000 galaxies across a wide range of distances, ages and growth stages.
The findings to date come primarily from the Hubble telescope and the Chandra X-ray Observatory. NASA's Space Infrared Telescope Facility (SIRTF) is to join the effort after its launch, scheduled for August.
This combination of instruments enables astronomers to examine the same patch of sky at different wavelengths and to reveal a variety of properties in sharper detail and with higher sensitivity over greater distances than ever before, the astronomers said.
The combined capabilities are
incredible, said Mario Livio, of
the Space Telescope Science Institute in Baltimore.
We can see how
galaxies and their central monsters build themselves.
Chandra's survey portrait marks the deepest penetration into space by an X-ray image. It focused on the high-energy sparks produced by black holes as they gobble up nearby gas and stars. Millions or billions of times the mass of the sun, black holes are sites of extreme gravitational collapse so dense that not even light can escape their pull.
Less than two decades ago, they were still hypothetical beasts. Now scientists not only have confirmed their existence, but also are collecting increasingly sophisticated data on their many variations and their role as the central engines of galaxies. There is a cottage industry in black hole census-taking.
Chandra found 1,500 actively feeding super-massive black holes within
a patch of sky over half as big as the full moon looks from
Earth. Extending that to the whole universe, researchers estimate
there are 300 million active black holes (and presumably others that
are quiescent—i.e., not currently
But in the early universe, the new data show,
black holes are more
rare than we thought, said Niel Brandt of Pennsylvania State
University, who leads the X-ray side of the GOODS survey.
One possible explanation is that early generations of massive stars
exploded as supernovas and blew away the black-hole food supply. At
that point in the life of the cosmos, Brandt said, galaxies were such
shreddy little things that perhaps they couldn't hold on to
their gas in the face of the fierce supernova forces as well as their
larger successors would.
The seven mysterious black hole candidates Chandra detected in X-rays
are completely invisible in the optical [wavelengths] with
Hubble, Brandt said.
Either they are the most distant black
holes ever detected, or they are less distant black holes with the
thickest cloak of dust known.
Whatever they are, they are something
The Hubble's new advanced camera looked at 60 times the area of
sky the observatory surveyed in its celebrated Deep Field observations
of 1995, Livio said. While the cosmic portrait that emerged from the
first was comparable to a snapshot of, say, Iowa,
now you can look
at the whole United States.
The large sample size of the new survey made it clear that these objects existed at such an early time, he said. Previous evidence was sketchier, less systematic.
The observations show not only that the galaxies grew
companies, by mergers and acquisitions, he said,
but also that
a galaxy may change its whole nature.
The findings support
bottom-up models of galactic evolution and
fit with the idea that the process is linked with dark matter, the
unseen stuff that accounts for much of the mass in the universe. In
theory, galaxies were born in the early universe when dark matter
pooled into gravitational puddles, which then attracted conventional
gases that in turn condensed into star clusters and, eventually,
small, ragged galaxies.
These galaxies merged, collided with or accreted other small galaxies, growing gradually into the giant elliptical and spiral conglomerations of billions of stars seen today. Astronomers estimate the universe contains 100 billion galaxies.
One of the survey's surprises is that when the universe was still
a mere slip of a thing, just about 7 percent of its current age, it
was already forming stars at a rate almost equal to its peak rate a
billion years later, Livio said.
It's like the
movie—'Fast and Furious.' . . . So if we want to look at
the first star formation, we have to look even further back.