A false-color image of the Smith Cloud made with data from the Green Bank Telescope
(GBT). New analysis indicates that it is wrapped in a dark matter halo.
Credit: NRAO/AUI/NSF
Like a bullet wrapped in a full metal jacket, a high-velocity hydrogen
cloud hurtling toward the Milky Way appears to be encased in a shell of
dark matter, according to a new analysis of data from the National
Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT).
Astronomers believe that without this protective shell, the
high-velocity cloud (HVC) known as the Smith Cloud would have
disintegrated long ago when it first collided with the disk of our
Galaxy.
If confirmed by further observations, a halo of dark
matter could mean that the Smith Cloud is actually a failed dwarf
galaxy, an object that has all the right stuff to form a true galaxy,
just not enough to produce stars.
“The Smith Cloud is really
one of a kind. It’s fast, quite extensive, and close enough to study in
detail,” said Matthew Nichols with the Sauverny Observatory in
Switzerland and principal author on a paper accepted for publication in
the Monthly Notices of the Royal Astronomical Society. “It’s also
a bit of a mystery; an object like this simply shouldn’t survive a trip
through the Milky Way, but all the evidence points to the fact that it
did.”
Previous studies of the Smith Cloud revealed that it
first passed through our Galaxy many millions of years ago. By
reexamining and carefully modeling the cloud, astronomers now believe
that the Smith Cloud contains and is actually wrapped in a substantial
“halo” of dark matter -- the gravitationally significant yet invisible
stuff that makes up roughly 80 percent of all the matter in the
Universe.
“Based on the currently predicted orbit, we show that
a dark matter free cloud would be unlikely to survive this disk
crossing,” observed Jay Lockman, an astronomer at the National Radio
Astronomy Observatory in Green Bank, West Virginia, and one of the
coauthors on the paper. “While a cloud with dark matter easily survives
the passage and produces an object that looks like the Smith Cloud
today.”
The Milky Way is swarmed by hundreds of high-velocity
clouds, which are made up primarily of hydrogen gas that is too rarefied
to form stars in any detectable amount. The only way to observe these
objects, therefore, is with exquisitely sensitive radio telescopes like
the GBT, which can detect the faint emission of neutral hydrogen. If it
were visible with the naked eye, the Smith Cloud would cover almost as
much sky as the constellation Orion.
Most high-velocity clouds
share a common origin with the Milky Way, either as the leftover
building blocks of galaxy formation or as clumps of material launched by
supernovas in the disk of the galaxy. A rare few, however, are
interlopers from farther off in space with their own distinct pedigree. A
halo of dark matter would strengthen the case for the Smith Cloud being
one of these rare exceptions.
Currently, the Smith Cloud is
about 8,000 light-years away from the disk of our Galaxy. It is moving
toward the Milky Way at more than 150 miles per second and is predicted
to impact again in approximately 30 million years.
“If confirmed
to have dark matter this would in effect be a failed galaxy,” said
Nichols. “Such a discovery would begin to show the lower limit of how
small a galaxy could be.” The researchers believe this could also
improve our understanding of the Milky Way's earliest star formation.
***
Contact:
Charles E. Blue
Public Information Officer
(434) 296-0314
email: cblue@nrao.edu
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
