Taken from http://www.newscientist.com/news/news.jsp?id=ns99994959

10:15 09 May 04

The map of the Milky Way is being redrawn, following the discovery of another
arm of the galaxy.

The structure consists of an arc of hydrogen gas 77,000 light years long and a
few thousand light years thick running along the galaxy's outermost edge.

"We see it over a huge area of sky," says Naomi McClure-Griffiths of the
Australia National Telescope Facility in Epping, New South Wales, who led the
team that made the discovery.

Redrawing the genetic map - takes you an image that I can't post the link to.

Astronomers are shocked that the feature has been overlooked until now. "I was
absolutely flabbergasted, it was quite clearly seen in some of the previous
surveys but it was never pointed out or given a name," says Tom Dame at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Horizon to zenith

Scientists have known for 50 years that the Milky Way has a spiral structure,
and careful observations have identified four main arms that swirl out from
the galactic centre as well a number of smaller arcs between them. The new
feature is a massive arc that sweeps around outside the other arms. If it were
visible in the night sky on Earth it would reach approximately from the
horizon to the zenith.

McClure-Griffiths and colleagues made their discovery while mapping the
distribution of hydrogen gas within the Milky Way, a project known as the
Southern Galactic Plane Survey.

Most of the Milky Way is obscured by clouds of dust. But hydrogen emits radio
waves which pass through the interstellar dust clouds, so when
McClure-Griffiths saw an unexpected ridge of bright emission in the radio map
she wondered whether it might be a new feature of the galaxy.

To test this hunch, she developed a simple computer model of a spiral galaxy
to show how hydrogen gas should be distributed. This model closely reproduced
the known distribution of hydrogen in the Milky Way, she says in a report to
be published in Astrophysical Journal Letters. The new arc also fitted the
model, convincing the team that the feature was real.

Further evidence that the feature is part of the Milky Way is that the arm is
rotating with the rest of the galaxy, says Robert Benjamin, an astronomer at
the University of Wisconsin in Whitewater.

Galactic collision

The arc could be a tendril that once joined up with another spiral arm,
McClure-Griffiths says. It would not be unusual for a mid-sized galaxy like
ours to have arms that extend so far - the Andromeda galaxy, which is similar
to ours, has long gaseous arms. Another possibility is that the gas was drawn
out of the Milky Way in a collision with a dwarf galaxy early in its
evolution.

The next step will be to characterise the make-up of the arm in more detail.
In 2003, a group of astronomers, including Brian Yanny of Fermilab in Batavia,
Illinois, found a streak of stars along the rim of the Milky Way 60,000 light
years from the middle, near to the new arc of gas.

Whether these stars are part of the new arm is not clear, however. Their
composition suggests that they came from a small invading galaxy that was torn
apart as it brushed past the Milky Way, says Yanny.

In this case, the stars' alignment with the arm could be mere coincidence.
However, he is planning to run simulations to determine whether such a brush
may have created the new feature on the Milky Way map.

Benjamin says that, in the last few years, surveys of the sky by robotic
telescopes have inundated astronomers with information about the Milky Way.
That is why he thinks the arm "probably won't be the last major galactic
structure to be discovered".

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For more related stories search the print edition Archive
http://archive.newscientist.com/

Weblinks

McClure-Griffiths et al article (PDF)
http://arxiv.org/PS_cache/astro-ph/pdf/0404/0404448.pdf

Southern Galactic Plane Survey
http://www.astro.umn.edu/~naomi/sgps.html

Milky Way images, NASA
http://adc.gsfc.nasa.gov/mw/mmw_images.html

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