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Public Affairs Office
Harvard-Smithsonian Center for Astrophysics Cambridge, Massachusetts

For more information, contact:

David A. Aguilar, Director of Public Affairs Harvard-Smithsonian Center for
Astrophysics 617-495-7462

Christine Pulliam, Public Affairs Specialist Harvard-Smithsonian Center for
Astrophysics 617-495-7463

For Release: Tuesday, June 03, 2008 01:00:00 PM

Release No.: 2008-13

Milky Way's Inner Beauty Revealed

Saint Louis, MO -- We live in the Milky Way galaxy -- a disk-shaped
collection of about 400 billion stars including the Sun. Many of those
stars and much of the dense gas between the stars concentrate into large
arms that spiral outward from the galactic center.

Astronomers have worked for decades to map the Milky Way and its spiral
arms. They have just discovered a new spiral arm on the far side of the
galactic center from Earth, which is a virtual twin of a known arm on the
near side of the galactic center. The Milky Way therefore shows a beautiful
symmetry, with two matching spiral arms lazily spinning near the galaxy's
center.

Tom Dame of the Harvard-Smithsonian Center for Astrophysics (CfA) presented
this finding today in a press conference at the 212th American Astronomical
Society meeting.

"Our galaxy isn't as messy as many thought. What we have found is
evidence of some balance and order, like the yin and yang of Chinese
philosophy," said Dame.

The Milky Way has other, larger spiral arms farther out in the galactic
disk. This study focuses on the two arms closest to the center of the
galaxy,

A 50-year puzzle solved

Fifty years ago, radio astronomers found an unusual spiral arm about 10,000
light-years from the center of the Milky Way along our line of sight. They
called it the expanding 3-kiloparsec (3-kpc) arm since 3 kpc equals 10,000
light-years, and it was found to be expanding away from the center at more
than 50 kilometer per second. The expanding 3-kpc arm contains about 10
million sun's worth of gas, mostly hydrogen atoms and molecules.

They suspected a similar arm might exist on the far side of the Milky Way,
since many other galaxies tend to be symmetrical. But for decades, they
were unable to find any evidence for a far-side counterpart of the
expanding 3-kpc arm. One reason is that the galactic center is so crowded,
with many different hydrogen gas clouds overlapping the same patch of sky.

"Studying the galactic center is like listening to a conversation in
the middle of a crowded cocktail party. There's lots of noise," said
Dame's CfA colleague Patrick Thaddeus.

Dame and Thaddeus analyzed data obtained using a 1.2-meter-diameter
millimeter-wave telescope located at Cerro Tololo Inter-American
Observatory in Chile.

When Dame processed the data to highlight large-scale structures very close
to the plane of the galaxy, a new feature jumped out at him. The data
clearly showed a spiral arm exactly where the far 3-kpc arm should be, with
properties like radius, expansion velocity, mass, and brightness that were
mirror images of the near 3-kpc arm.

Spiral arms and galactic bar linked

Dame and Thaddeus suspect that the 3-kpc spiral arms are linked to the
galactic bar. At the center of the Milky Way, billions of stars inhabit an
elongated conglomeration called a stellar bar, which extends for several
thousand light-years on either side of the galactic center. As that central
bar rotates, it produces large-scale shock waves that likely sculpt the
3-kpc spiral arms and power their outward motions. In fact, theorists
believe that those spiral arms connect to the ends of the bar.

"The 3-kiloparsec arms are a natural result of the stellar bar,"
explained Thaddeus. "We expected that the bar should drive symmetric
structure. Now, we have proof that it does."

Dame and Thaddeus confirmed their discovery using 21-centimeter radio
measurements of atomic hydrogen collected by colleagues in Australia. Next,
they plan to apply for observing time on the Robert C. Byrd Green Bank
Telescope to examine the 3-kpc spiral arms in more detail. They also want
to extend their observations to greater galactic longitudes as seen from
Earth.

"We want to see how far we can chase these arms -- hopefully to their
origins at the ends of the central bar," said Dame.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

[NOTE: Images supporting this release are available at
http://cfa-www.harvard.edu/press/2008/pr200813_images.html ]
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