By Emily Carlson,

Using a new sky survey instrument called WHAM, astronomers have
detected a faint gas that spreads into the far reaches of the galaxy
and sometimes forms distinct strands that stretch halfway across the

The findings were reported here today, June 4, at the annual meeting
of the American Astronomical Society.

UW-Madison astronomer Gregory Madsen, along with colleagues Ronald
Reynolds and L. Matthew Haffner, studied the Milky Way’s interstellar
medium, or the “stuff” between the stars; they found that much of it
contains a diffuse hydrogen gas that’s been ionized, or stripped of
its electrons, by the ultraviolet light produced by stars.

Since 1939, when Swedish-born American astronomer Bengt Stršmgren,
proposed the idea, astronomers have suspected that ionized hydrogen
existed only in regions called “Stršmgren Spheres” that formed
around massive, hot O stars found in the mid-plane of the Milky
Way — primarily because instruments have been unable to detect it
elsewhere. But, as Madsen explains, ionized hydrogen can emit a
faint light that most instruments can’t discern.

The Wisconsin H-Alpha Mapper or WHAM, on the other hand, is a
specialized spectrometer that can observe faint emissions of light
in the night sky. Designed by UW-Madison’s Reynolds for the sole
purpose of studying the interstellar medium, WHAM uses the latest
technology in spectroscopy — the study of wavelengths, or color,
of light — to detect diffuse optical emissions in interstellar
gas clouds. The instrument is funded by the National Science
Foundation and is currently located at the Kitt Peak National
Observatory near Tucson, Ariz.

“Until WHAM, diffuse ionized gas couldn’t be seen,” says Reynolds.
“The big surprise is that we see it everywhere — not only spread
between the stars, but extending thousands of light years above
the Milky Way.”

Observations from WHAM, the UW-Madison astronomers conclude, show
that about 90 percent of ionized hydrogen gas is found throughout
the galaxy, not just confined inside Stršmgren Spheres.

“We used to think that stars and the energy they produced only
influenced the region right around them,” Reynolds adds. “Now we
see they’re influencing the Milky Way far beyond what people had
ever thought.” Madsen points out that astronomers investigating
other galaxies have also observed widespread ionized hydrogen.

But the group has also found that not all this ionized hydrogen
gas is diffuse throughout the galaxy — some of it forms into
distinct strands which stretch up to 3,000 light years above the
Milky Way’s disk, where the stars concentrate. These strands,
also called filaments, are slightly brighter than the diffuse
ionized hydrogen surrounding them; they resemble jet contrails
arching across the sky.

Detailed analysis by Madsen of the largest of these filaments
reveals that the temperature and ionization states of the gas are
similar to those of the diffuse gas surrounding it. Madsen says
analyses of other filaments show a clear connection between their
gas and the gas surrounding O stars, suggesting that stars’
ultraviolet light does reach far into the rest of the galaxy.

“What WHAM has shown in a dramatic way is that the O stars buried
in the mid-plane of the galaxy are somehow able to ionize gas over
almost the entire Milky Way,” Madsen says. “How exactly the stars’
ultraviolet light can travel such long distances remains a mystery.”

While the mechanism by which ionized hydrogen fills the Milky Way
remains unclear, the group suggests that these filaments, as well
as the diffuse background of ionized hydrogen, should be
incorporated into existing models and theories about the galaxy
and its evolution. Knowledge of the pervasive gas, the researchers
add, also provides an opportunity to understand the relationship
between star formation and ionization and the existence of “super
bubbles,” or regions where stars have exploded and left behind an
area void of any gas whatsoever.

“WHAM has provided a totally new view of the Milky Way,” Reynolds
says. “It’s like putting on a new pair of glasses, looking up at
the sky and seeing something you’ve never seen before. We haven’t
quite figured out what these huge networks of filaments mean, but
we’re very excited to know they’re there.”