The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/95_05.txt

Text File | 1996-01-12 | 5KB | 119 lines

FOR RELEASE: Immediately

CONTACT:  Ray Villard, [phone]
      Space Telescope Science Institute

      Lori Stiles, [phone]
      University of Arizona, News Services

PRESS RELEASE NO.:  STScI-PR95-05


HUBBLE IDENTIFIES HUGE CLOUDS OF
INTERGALACTIC GAS


Astronomers using NASA's Hubble Space Telescope have discovered
evidence that clouds of hydrogen found between galaxies at distances
of billions of light-years from Earth are at least ten times larger than
previously thought -- at least one million light-years in diameter --
and may have a remarkable sheet-like structure.

Theorists will have to rethink explanations of how such large clouds
exist, according to astronomers.

The new Hubble results shed light on the properties of these mysterious
clouds, the true nature of which remains elusive more than 25 years
after their discovery.  Understanding this intergalactic material might
give important clues to the nature of dark matter and processes
occurring in the early universe, including galaxy formation.

The observations were conducted by Nadine Dinshaw, a graduate
student at the University of Arizona Steward Observatory, Craig
Foltz of the University of Arizona/Smithsonian Institution Multiple
Mirror Telescope Observatory, Christopher Impey of the University
of Arizona Steward Observatory, Ray Weymann of the Observatories
of the Carnegie Institutions of Washington, and Simon Morris of
the Dominion Astronomical Observatory, Victoria, British Columbia.

The intergalactic gas clouds are so diffuse they cannot be observed
directly.  The only signatures of their existence are the imprints they
leave on the light from more distant background objects.
Astronomers have used these signatures to detect the presence of
the clouds and to measure some of their physical properties, but
have been unable to discover information on cloud sizes and shapes.

"This information is crucial to any attempt to distinguish between the
several theoretical explanations for the sites and mechanisms which
produce the clouds," says Craig Foltz of the University of Arizona,
Tucson, a member of the Hubble team who made the discovery.

Previous explanations have been that the clouds are the halos of
primordial clumps of dark matter that they are the very outer halos of
normal galaxies, or that they are produced by shock waves resulting
from explosive galaxy formation.

"These Hubble results do not explain the details of how the clouds
are produced, but they directly imply that the clouds are so large that
none of the popular scenarios provides an entirely adequate
explanation," Foltz says.

The team used Hubble's Faint Object Spectrograph at ultraviolet
wavelengths to observe a pair of quasars -- extremely luminous
and distant objects -- separated by an angle on the sky about
one-twentieth the diameter of the full Moon.  The clouds are detected
by the dark absorption lines that they produce in the spectra of the
light from the quasars.  The quasars act like two flashlights seen from
a distance of five billion to 10 billion light-years, shining through an
intergalactic "forest" where the "trees" are the clouds.

A problem with such studies in the past has been in finding two
quasars that are appropriately paired in the sky, roughly equidistant
from the Earth and about equal in brightness.  The team used the HST
to study a pair of quasars bright at ultraviolet wavelengths but
unsuitable for ground-based observations, since ultraviolet
observations are impossible to make from Earth.

In the HST observations, several matching absorption lines were
seen in the spectra of both quasars, implying that the clouds are at
least large enough to cover the lines of sight to both quasars, or at
least a million light-years in diameter, roughly ten times larger than
previously thought.  (By comparison, the luminous disk of our Milky
Way Galaxy is about 100,000 light-years in diameter.)  Detailed
analysis of the observations allowed the team to estimate that the
actual size of the clouds may be as much as twice as large, or two
million light-years in diameter, and that they may take the form of
huge filaments, sheets or flattened disks of material.

"These results were absolutely unexpected," Dinshaw says of the
discovery.  "We were just hoping to get upper limits on the size of
these clouds.  We never expected to see so many matches in the
absorption lines (in the spectra of each of the quasars in the pair).
We never expected the sizes of these clouds would be so large."

The team plans to confirm their discovery with more Hubble
observations of this pair of quasars as well as other close pairs of
quasars.  They also have undertaken an extensive ground-based
observing program to attempt to better understand both the
properties of the clouds and their relationship to normal galaxies.

Their results appear in the January 19, 1995 issue of the science
journal Nature.


                      * * * * * *

The Space Telescope Science Institute 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).