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Invisible corona
The stars of the Milky Way's main disk span a region
of space about 100,000 light-years across, and the thick central bulge is a
sphere about 25,000 light-years in diameter. To picture this, think of a typical
sci-fi flying saucer. For decades, this was the official picture of our galaxy.
More recently, a handful of studies have added a sprinkling of stars that live
in a sparse halo outside the main disk.
Then early this year, astronomers made a remarkable
announcement: Our galaxy's influence extends as far as 150,000 light-years in
all directions, in the form of an invisible, gaseous sphere called a
corona.
Researchers have known for some years that a corona
existed. But its newfound size is astonishing, says Kenneth Sembach of the Space
Telescope Science
Institute, who made the discovery with a team of
other researchers using NASA's FUSE spacecraft.
"The corona is hot," Sembach told SPACE.com. "It
probably has a temperature in excess of 1 million degrees (Fahrenheit)."
The corona is only sparsely sprinkled with particles,
however, and nearly impossible to detect on its own. Yet when cold,
extragalactic clouds of hydrogen gas fall into the corona, their outer shells
become superheated and thus detectable, Sembach explained. FUSE found several of
these clouds racing into the galaxy at nearly 1 million mph.
Previous studies had detected similar clouds, but
they were closer in. Researchers aren't sure if the clouds are leftovers from
the Milky Way's formation or if they are bits of smaller galaxies that have been
ripped away by our galaxy's greater gravity.
"We are finding that the Milky Way is a much more
complex system than we had perhaps previously imagined," Sembach
says.
Baby pictures
If the current structure of the Milky Way is complex,
then trying to imagine its formation and evolution is downright
perplexing.
Despite all the new efforts to dig up information
about the galaxy's structure and evolution, some pieces of the puzzle are lost
forever. There are no baby pictures of the Milky Way, and in truth no way to be
sure what it looked like at any time prior to the present.
That's where people like Matthew Malkan can help out.
Malkan studies galaxies so far away that their light was generated back when the
galaxies were forming, several billion years ago. By studying scores of such
galaxies in all stages of development, Malkan finds patterns he thinks can be
pieced together to form a representative baby picture of our own
galaxy.
Malkan and other UCLA astronomers recently worked
with researchers at Ohio State to generate one of the deepest galactic surveys
ever done, called the Hercules Deep Field. They combined visible and infrared
images of galaxies in a selected region of the sky to create refined
calculations for how far away each galaxy is, how old it is, and how many stars
it had formed.
In all this data and in other studies, Malkan looks
for clues to how galaxies formed.
"Galaxy collisions and encounters were very important
early on in the universe as a whole," he says. "In the early days of the Milky
Way there must have been a lot of material flinging into galaxies. That probably
had a big impact on how a galaxy evolved."
Some theorists think these collisions tended to make
galaxies shaped more like a football than like the many known flat galaxies with
central bulges and winging spiral arms, galaxies like the Milky Way. It may be,
they say, that flat spiral structures were too fragile to survive all the
collisions.
"That would imply that the bulge part of the Milky
Way, if it's like the rest of the galaxies, probably was here first," Malkan
says. "And the beautiful disk and spiral arms look like they are
younger."
Removing the mystery
The big question about the Milky Way now is whether
the renaissance of study will yield a definitive picture of formation and
evolution.
Skrutskie, the University of Virginia astronomer,
thinks the Milky Way's history is encoded in present-day stars, their types and
their distributions. Future surveys need only obtain a good picture of the
locations and motions "of a significant fraction of stars" in order to provide
the clues needed to pin down how the galaxy was assembled, when mergers took
place, and when bursts of star formation occurred.
"I don't see the Milky Way as particularly mysterious
given our present state of knowledge," he said. "Instead, I look at it as a rich
archeological site that we are just beginning to explore."
But Malkan and his deep sky colleagues will likely
figure huge in any effort to decode our own galaxy.
"There's a lot of reason to be optimistic," Malkan
says. "It's a really powerful synergism to combine" chemical and light studies
of individual stars in the Milky Way with samples of faraway galaxies in various
phases of evolution.
Malkan has been doing the latter for decades. But, he
says, "It's been less than 10 years that we've really been able to look at these
galaxies in their young states. Things have been exploding since
then."
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