A Tiny Galaxy is Born
New detailed images from NASA’s Hubble Space Telescope show a
“late-blooming” galaxy, a small, distorted system of gas and stars that
still appears to be in the process of development, even though most of
its galactic cousins are believed to have started forming billions of
years ago.
Evidence of the galaxy’s youthfulness can be seen in the burst of
newborn stars and its disturbed shape. This evidence indicates that the
galaxy, called POX 186, formed when two smaller clumps of gas and stars
collided less than 100 million years ago (a relatively recent event in
the universe’s 13-billion-year history), triggering more star formation.
Most large galaxies, such as our Milky Way, are thought to have formed
the bulk of their stars billions of years ago.
The Hubble images of POX 186 support theories that all galaxies
originally formed through the assembly of smaller “building blocks” of
gas and stars. These galactic building blocks formed shortly after the
Big Bang, the event that created the universe. Astronomers Michael
Corbin of the Space Telescope Science Institute in Baltimore, Md., and
William Vacca of the Max-Planck Institute for Extraterrestrial Physics
in Garching, Germany, used the telescope‘s Wide Field and Planetary
Camera 2 to study POX 186 in March and June 2000. Their results will
appear in the Dec. 20 issue of the Astrophysical Journal.
“This is a surprising find,” Corbin says. “We didn’t expect to see any
galaxies forming in the nearby universe. POX 186 lies only about 68
million light-years away, which means that it is relatively close to us
in both space and time.”
Adds Vacca: “POX 186 may be giving us a glimpse of the early stages of
the formation process of all galaxies.”
POX 186 is a member of a class of galaxies called blue compact dwarfs
because of its small size and its collection of hot blue stars. [The
term “POX” is derived from the French “prism objectif,” or objective
prism, a device that astronomers place in front of a telescope to
photograph spectra of all objects in its field of view.] POX 186 was
discovered 20 years ago, but ground-based telescopes resolved few
details of the galaxy’s structure because it is so tiny. To probe the
galaxy’s complex structure, astronomers used the sharp vision of the
Hubble telescope. The Hubble pictures reveal that the system is puny by
galaxy standards, measuring only about 900 light-years across, and
containing just 10 million stars. By contrast, our Milky Way is about
100,000 light-years across and contains more than 100 billion stars.
So why did POX 186 lag behind its larger galactic cousins in forming?
Corbin and Vacca find that the young system sits in a region of
comparatively empty space known as a void. Its closest galactic
neighbors are about 30 million light-years away. The two small clumps of
gas and stars that are merging to form POX 186 would have taken longer
to be drawn together by gravity than similar clumps in denser regions of
space. The Hubble data don’t reveal the ages of the stars in the clumps.
Corbin, however, suspects that the oldest stars may be about 1 billion
years old, which is young on the cosmic time scale.
The youthful galaxy’s puny size may support a recent theory of galaxy
formation known as “downsizing,” which proposes that the least massive
galaxies in the universe are the last to form. In clear contrast to POX
186, the most massive galaxies in the universe, known as giant
ellipticals, have a generally spherical structure with few or no young
stars, indicating that they formed many billions of years in the past.
To actually see the formation process of stars in such large galaxies,
astronomers are awaiting the deployment of Hubble’s successor, the James
Webb Space Telescope. This telescope is designed, in part, to study
faint objects whose light left them early in the 13-billion-year history
of the universe.
Although the POX 186 results are tantalizing, Corbin and Vacca realize
that one galaxy is not enough evidence to support the idea that galaxy
formation is an ongoing process. They are proposing to use Hubble to
study nine other blue compact dwarfs for similar evidence of recent
formation.
Electronic image files and additional information are available at
http://hubblesite.org/news/2002/16
EDITOR’S NOTE: This release is the first to go live with a new
system — HubbleSite NewsCenter. The redesigned news web site is an
easy-to-access and all-inclusive collection of Hubble images (dating
back to the telescope‘s launch in 1990), supportive text, video,
illustrations, and more. The News Media Resources within HubbleSite
NewsCenter provide journalists with news advisories, information on
upcoming events, conversion charts, calculators, and background about
the telescope and its discoveries. For future reference, bookmark
http://hubblesite.org/newscenter .
The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).
