Space Forum / Astronomy / February 2005

I found this fascinating.

http://www.cnn.com/2005/TECH/space/02/24/dark.galaxy/index.html

Astronomers: 'Dark' galaxy discovered

(SPACE.com) -- Astronomers have discovered an invisible galaxy that
could be the first of many that will help unravel one of the universe's
greatest mysteries.

The object appears to be made mostly of "dark matter," material of an
unknown nature that can't be seen.

Theorists have long said most of the universe is made of dark matter.
Its presence is required to explain the extra gravitational force that
is observed to hold regular galaxies together and that also binds large
clusters of galaxies.

Theorists also believe knots of dark matter were integral to the
formation of the first stars and galaxies. In the early universe, dark
matter condensed like water droplets on a spider web, the thinking
goes. Regular matter -- mostly hydrogen gas -- was gravitationally
attracted to a dark matter knot, and when the density became great
enough, a star would form, marking the birth of a galaxy.

The theory suggests that pockets of pure dark matter ought to remain
sprinkled across the cosmos. In 2001, a team led by Neil Trentham of
the University of Cambridge predicted the presence of entire dark
galaxies.

The newfound dark galaxy was detected with radio telescopes. Similar
objects could be very common or very rare, said Robert Minchin of
Cardiff University in the UK.

"If they are the missing dark matter halos predicted by galaxy
formation simulations but not found in optical surveys, then there
could be more dark galaxies than ordinary ones," Minchin told
SPACE.com.

In a cluster of galaxies known as Virgo, some 50 million light-years
away, Minchin and colleagues looked for radio-wavelength radiation
coming from hydrogen gas. They found a well of it that contains a
hundred million times the mass of the Sun. It is now named VIRGOHI21.

The well of material rotates too quickly to be explained by the
observed amount of gas. Something else must serve as gravitational
glue.

"From the speed it is spinning, we realized that VIRGOHI21 was a
thousand times more massive than could be accounted for by the observed
hydrogen atoms alone," Minchin said. "If it were an ordinary galaxy,
then it should be quite bright and would be visible with a good amateur
telescope."

The ratio of dark matter to regular matter is at least 500-to-1, which
is higher than I would expect in an ordinary galaxy," Minchin said.
"However, it is very hard to know what to expect with such a unique
object -- it may be that high ratios like this are necessary to keep
the gas from collapsing to form stars."

Long road to discovery
Other potential dark galaxies have been found previously, but closer
observations revealed stars in the mix. Intense visible-light
observations reveal no stars in VIRGOHI21.

The invisible galaxy is thought to lack stars because its density is
not high enough to trigger star birth, the astronomers said.

The discovery was made in 2000 with the University of Manchester's
Lovell Telescope, and the astronomers have worked since then to verify
the work. It was announced today.

"The universe has all sorts of secrets still to reveal to us, but this
shows that we are beginning to understand how to look at it in the
right way," said astronomer Jon Davies of Cardiff University in the UK.
It's a really exciting discovery." Additional radio observations were
made with the Arecibo Observatory in Puerto Rico. Follow-up optical
work was done with the Isaac Newton Telescope in La Palma. Astronomers
from the UK, France, Italy and Australia contributed to the research.
The project is now searching for other possible dark galaxies.

Dark matter makes up about 23 percent of the universe's mass-energy
budget. Normal matter, the stuff of stars, planets and people,
contributes just 4 percent. The rest of the universe is driven by an
even more mysterious thing called dark energy.