Two of NASA’s Great Observatories, bolstered by the largest ground-based
telescopes around the world, are beginning to harvest new clues to the
origin and evolution of galaxies. It’s a bit like finding a family
scrapbook containing snapshots that capture the lives of family members
from infancy through adolescence to adulthood.

“This is the first time the cosmic tale of how galaxies build themselves
has been traced reliably to such early times in the universe’s life,”
said Mauro Giavalisco, head of the Hubble Space Telescope (HST) portion
of the survey, and research astronomer at the Space Telescope Science
Institute (STScI) in Baltimore.

The HST has joined forces with the Chandra X-ray Observatory to survey a
relatively broad swath of sky encompassing tens of thousands of galaxies
stretching far back into time. The Space Infrared Telescope Facility
(SIRTF), scheduled for launch in August, will soon join this
unprecedented survey. Called the Great Observatories Origins Deep Survey
(GOODS), astronomers are studying galaxy formation and evolution over a
wide range of distances and ages. The project is tracing the assembly
history of galaxies, the evolution of their stellar populations, and the
gusher of energy from star formation and active nuclei powered by
immense black holes.

HST astronomers report the sizes of galaxies clearly increase
continuously from the time the universe was about 1 billion years old to
an age of 6 billion years. This is approximately half the current age of
the universe, 13.7 billion years. GOODS astronomers also find the star
birth rate rose mildly, by about a factor of three, between the time the
universe was about one billion years old and 1.5 billion years old, and
remained high until about 7 billion years ago, when it quickly dropped
to one-tenth the earlier “baby boomer” rate. This is further evidence
major galaxy building trailed off when the universe was about half its
current age.

This increase in galaxy size is consistent with “bottom-up” models,
where galaxies grow hierarchically, through mergers and accretion of
smaller satellite galaxies. This is also consistent with the idea the
sizes of galaxies match hand-in-glove to a certain fraction of the sizes
of their dark-matter halos. Dark matter is an invisible form of mass
that comprises most of the matter in the universe. The theory is dark
matter essentially pooled into gravitational “puddles” in the early
universe, then collected normal gas that quickly contracted to build
star clusters and small galaxies. These dwarf galaxies merged
piece-by-piece over billions of years to build the immense spiral and
elliptical galaxies we see today.

The Chandra observations amounted to a “high-energy core sample” of the
early universe, allowing us to “study the history of black holes over
almost the entire age of the universe,” said Niel Brandt of Penn State
University, a co-investigator on the Chandra GOODS team, who studied the
X-ray results. One of the fascinating findings in this deepest X-ray
image ever taken is the discovery of mysterious black holes, which have
no optical counterparts.

“We found seven mysterious sources that are completely invisible in the
optical with Hubble,” said Anton Koekemoer of the STScI, a
co-investigator on both the Hubble and Chandra GOODS teams, who compared
the X-ray and optical results. “Either they are the most distant black
holes ever detected, or they are less distant black holes that are the
most dust enshrouded known, a surprising result as well.”

When comparing the HST and Chandra fields, astronomers also found active
black holes in distant, relatively small galaxies were rarer than
expected. This may be due to the effects of early generations of massive
stars that exploded as supernovae, evacuating galactic gas and thus
reducing the supply of gas needed to feed a super massive black hole.

These and other results from the GOODS project will be published in a
special issue of the Astrophysical Journal Letters, entirely devoted to
the team’s results. The Chandra results are found in papers led by
Koekemoer and Stefano Cristiani of the Trieste Astronomical Observatory.
Hubble’s findings came from papers led by Giavalisco, Mark Dickinson,
and Harry Ferguson of the STScI.

Electronic images and additional information are available on the
Internet at: