Astronomers find first 'dark galaxy'

10:15 20 October 03

Astronomers have found the first "dark galaxy" - a black cloud of
hydrogen gas and exotic particles, devoid of stars. The gloomy galaxy
lurks two million light years from Earth.

Joshua Simon, Timothy Robishaw and Leo Blitz of the University of
California, Berkeley, observed a cloud of hydrogen gas called HVC
127-41-330 using the Arecibo radio telescope in Puerto Rico.

It appears to be rotating so fast it would fall apart unless it contains
a strong, hidden source of gravity. The researchers therefore argue that
the cloud must be at least 80 per cent dark matter, the hypothetical
invisible substance whose gravity is supposed to explain why many
objects in the cosmos move as fast as they do.

If they are right, this could resolve a problem in dark matter theory.
In our local group of galaxies, we know of only about 35 dwarf galaxies,
but simulations of galaxy formation using dark matter suggest there
should be about 500.

If most of these dwarfs are dark galaxies with no stars, that would
explain why we have missed them until now. The reason HVC 127-41-330 and
its kind are dark may be because they have too small. Without enough
mass, their gravitational forces would be too weak to cram gas together
densely enough to form any stars.

NewScientist
http://www.newscientist.com/news/news.jsp?id=ns99994272


Astronomers date Universe's 'cosmic jerk'

17:04 13 October 03

The point when the repulsive force of dark energy overwhelmed gravity
and started the accelerating expansion of the Universe that continues
today has been revealed.

"It happened five billion years ago," says Adam Riess, an astronomer at
the Space Telescope Science Institute in Baltimore, Maryland. "That was
when the Universe stopped slowing down and began to accelerate,
experiencing a cosmic jerk."

Astronomers discovered that the Universe is expanding at an accelerating
rate in 1998 by studying exploding stars known as Type 1a supernovae.
These supernova always emit the same amount of energy, so their
brightness indicates how far away they are.

Because the Universe is expanding, the light from the supernovae shifts
towards the red end of the spectrum. The 1998 observations revealed that
light from such supernovae appeared dimmer than their red shifts
predicted - showing that the expansion of the Universe was accelerating.

But there was always the worry that other factors, like intergalactic
dust, could make the objects seem less bright. Now, Reiss's findings
have set aside these doubts.

Read the rest at NewScientist
http://www.newscientist.com/news/news.jsp?id=ns99994264

--
Kind Regards,
Robert Karl Stonjek.

The Preprint "Dark Matter in Dwarf Galaxies: The First Dark Galaxy?"
is available at
http://www.arxiv.org/abs/astro-ph/0310192

Title and Abstract:-

Satellites and Tidal Streams
ASP Conference Series, Vol. **VOLUME***, 2003

F. Prada, D. Mart´inez-Delgado, and T. Mahoney, eds.
Dark Matter in Dwarf Galaxies: The First Dark Galaxy?
Joshua D. Simon, Timothy Robishaw, and Leo Blitz
Department of Astronomy, University of California at Berkeley, 601
Campbell Hall, CA 94720

Abstract. We present new H i observations of the high-velocity cloud
(HVC) that we resolved near the Local Group dwarf galaxy LGS 3.
The cloud is rotating, with an implied mass that makes it dark matter-
dominated no matter what its distance from the Milky Way is. Our new,
high-sensitivity Arecibo observations demonstrate that the faint H i
fea-
tures that we previously described as tidal tails are indeed real and do
connect to the main body of the HVC. Thus, these observations are con-
sistent with our original hypothesis of a tidal interaction between the
HVC
and LGS 3. We suggest that the HVC may be one of the missing dark
matter satellites in the Local Group that are seen in Cold Dark Matter
numerical simulations but have not yet been identified observationally.



--
Kind Regards,
Robert Karl Stonjek.