Hubble Reveals Massive Disk Galaxies in the Early Universe (Forwarded)

Institute for Astronomy
University of Hawaii at Manoa
Manoa, Hawaii

Contacts:

Dr. Elizabeth McGrath
University of California, Santa Cruz
1-808-255-1970

Dr. Alan Stockton
Institute for Astronomy
University of Hawaii at Manoa
Honolulu, Hawaii 96822
1-808-956-7995

Mrs. Karen Rehbock
Assistant to the Director
Institute for Astronomy
University of Hawaii at Manoa
1-808-956-6829

Embargoed until Tuesday, January 8, 2008 1:00 p.m. CST (9:00 a.m. HST)

Hubble Reveals Massive Disk Galaxies in the Early Universe

Some of the first massive galaxies in the Universe formed when huge gas
clouds rapidly collapsed, according to Elizabeth McGrath of the University
of California, Santa Cruz, Alan Stockton of the University of Hawaii, and
their collaborators. This discovery, which is based on new Hubble Space
Telescope images, challenges the commonly held idea that all of the
earliest massive galaxies formed when smaller galaxies merged. It is being
presented this week at a meeting of the American Astronomical Society in
Austin, TX.

The standard theory of galaxy formation predicts that the most massive
galaxies in the Universe take a long time to grow, accumulating mass
through the coalescence of many smaller galaxies in a process that
continues until relatively recent times. To test this theory, McGrath,
Stockton, and their collaborators searched for the oldest, most massive
galaxies they could find and used clues from their shape and structure to
deduce how they may have formed. High-resolution images from the Hubble
Space Telescope revealed galaxies more massive than our own Milky Way
Galaxy that existed when the Universe was very young -- only one-fifth its
current age. It is believed that such galaxies are the distant ancestors
of the most massive galaxies in the Universe today.

"We expected these galaxies to look similar to the football-shaped
elliptical galaxies that we see at the centers of dense groups of galaxies
today, where mergers are common. We were quite surprised to find that many
of them appear instead to be flattened, rotating disks of ordered
material," said McGrath.

Disk galaxies are pancake or saucer-shaped, and their stars orbit in
circles around the center of the galaxy, much like the planets in our
solar system orbit around the sun, or like a Frisbee spins as it moves
through the air. This type of galaxy is more likely to have formed from a
single massive cloud that collapses under its own gravity into a flattened
disk rather than through violent collisions of previously formed galaxies.
Computer simulations of the latter scenario predict that collisions would
destroy disks and send stars from each merging galaxy into more chaotic,
three-dimensional orbits, producing football-shaped, or elliptical
galaxies. The most massive galaxies in the Universe today all appear to be
elliptical in shape, and therefore can be quite naturally explained
through the merger hypothesis. The existence of massive disk galaxies in
the early Universe, however, challenges this perspective.

In total, McGrath and her collaborators observed seven of what are likely
some of the first massive galaxies to form in the Universe. Of these, four
had shapes dominated by disk-like profiles. By age-dating the galaxies
from studying properties of the stars within them, McGrath's team
discovered that these disk structures have remained in pristine condition
for over one billion years. Even so, it seems inevitable that eventually
these galaxies will merge with others and be reformed into the more
elliptical-shaped massive, old galaxies that are familiar to us in the
nearby Universe.

Other researchers who participated in this study include Gabriela Canalizo
at the University of California, Riverside, Masanori Iye at the National
Astronomical Observatory of Japan, and Toshinori Maihara at Kyoto
University. The results are part of McGrath's Ph.D. dissertation, which
she recently completed at the University of Hawaii.

Founded in 1967, the Institute for Astronomy at the University of Hawaii
at Manoa conducts research into galaxies, cosmology, stars, planets, and
the sun. Its faculty and staff are also involved in astronomy education,
deep space missions, and in the development and management of the
observatories on Haleakala and Mauna Kea.

IMAGE CAPTION:
[http://www.ifa.hawaii.edu//info/pres...sk_color.5.jpg
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The Hubble Space Telescope captured this massive saucer-shaped galaxy more
than 9 billion light-years distant from us. Its age implies it was formed
when the Universe was very young. Meanwhile, the thin disk-like structure
has remained intact since birth, apparently unharmed by destructive galaxy
collisions for over one billion years. Previously, theories of galaxy
formation predicted such massive galaxies could only be formed by frequent
galaxy collisions that increase mass and reorganize matter into
three-dimensional football-shaped objects. This galaxy and others like it
may have formed instead from the collapse of a single massive gas cloud.
Eventually, it will merge with neighboring galaxies to form one of the
most massive galaxies in the Universe.