Study: Filaments of gas pervade Milky Way

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 sky.

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 Strömgren, proposed the idea, astronomers have suspected that ionized hydrogen existed only in regions called “Strömgren 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 Strömgren 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.”