13 Feb Journey Leads Astronomer to New Galactic Structure
MADISON – For Ron Reynolds, an early peek at what would turn out to be a major new feature of the Milky Way came through a hole in the ceiling.
As a physics post-doc at the University of Wisconsin-Madison in the late 1970s, Reynolds sawed a hole in the ceiling of an office of the university’s Physical Science Laboratory. Pointing a specially built spectrometer through the roof of the building, Reynolds obtained his first clear view of a faint glowing gas that permeates the space between the stars.
“We had to get away from Madison, and at that time it was still pretty dark out there,” Reynolds recalls of the moonless nights he would walk the rural Stoughton, Wis., complex, switching off lights to keep stray light from swamping his readings of the galaxy.
What Reynolds was probing in those early days, and later at specially designed facilities at UW-Madison’s Pine Bluff Observatory west of Madison and atop Kitt Peak near Tucson, Ariz., were the mysteries of interstellar space, the gaps between the stars occupied by what Reynolds describes as the ‘galaxy’s atmosphere,’ a chemical soup of elements that plays a critical role in the life cycle of stars.
“The problem we have is that we’re not looking at stars. The light we’re interested in comes from between the stars,” says Reynolds, now a UW-Madison professor of astronomy. “So you’re looking at a big patch of the sky. We need to do that just to get enough light to detect these photons.”
Reynolds got his start exploring the interstellar medium in the late 1960s under the direction of his faculty advisor, UW-Madison physics Professor Frank Scherb, who, with physics colleague Fred Roesler, showed Reynolds how to build a spectrometer sensitive enough to detect the faint light from interstellar space.
The photons or light particles Reynolds was looking for were believed to come from somewhere between the stars. Hints that ionized hydrogen gas – gas atoms stripped of electrons – might permeate the heavens emerged with the discovery of pulsars in the 1960s. The pulses of energy emitted by the dense, rapidly spinning pulsars span much of the electromagnetic spectrum. But a phenomenon known as ‘pulse dispersion,’ where the low-energy photons of a pulse tended to arrive at detectors on Earth after the high-energy photons, suggested that the pulse was traveling through a medium rich in free electrons, Reynolds explains.
“At that time, trace elements like carbon were thought to be the only ionized material in the interstellar medium. No one expected to see ionized hydrogen out in the middle of nowhere.”
But ionized hydrogen was indeed what Reynolds had detected, and he found it nearly everywhere he looked in the Milky Way. “It’s all over the sky, but it is brightest in the plane of the galaxy.”
What Reynolds had uncovered, in fact, was a major but hidden feature of our galaxy. This gas, now frequently called the ‘Reynolds layer’ by astrophysicists, is an enormous swirling mass of hydrogen gas. It glows a faint red because it has absorbed much of the ionizing power of all the stars in our galaxy and it envelops the Milky Way’s disk of stars in a structure that is roughly six thousand light years thick and 75,000 light years in diameter.
“This gas rotates with the stars,” Reynolds explains, and is an important component of the feedback loop in the great cycle of star birth and death.
“The galaxy is a dynamic, evolving beast. There are processes of stars forming and dying and, in the process, using the hydrogen to enrich the interstellar medium,” with heavy elements like carbon and silicon.
To chart the structure of the Milky Way’s massive envelope of ionized hydrogen, Reynolds and his collaborators built on the early work at the Physical Sciences Lab and Pine Bluff Observatory, building a large device atop Kitt Peak. Known as WHAM or the Wisconsin H-Alpha Mapper, the National Science Foundation-funded device on Kitt Peak was used over a two-year period to take more than 37,000 30-second exposures of the sky in a detailed survey of the ionized gas in the galaxy.
“We now have a complete picture of hydrogen in the interstellar medium,” says Reynolds of the gas in the Milky Way. Similar structures of ionized hydrogen have subsequently been found in other galaxies as well.
Reynolds work has helped astrophysicists flesh out their understanding of the structure of our galaxy and its components. And his accomplishments were recently recognized by the American Astronomical Society, which awarded him the Beatrice M. Tinsley Prize, given every two years “for research of an exceptionally creative or innovative character.”
Citing Reynolds, the society noted that his discovery has “presented major challenges to both observers and theorists, with major implications for the complicated, dynamic, and sometimes violent processes that comprise the cycle of birth and death of stars in the galaxy.”
Terry Devitt from UW Communications can be contacted at firstname.lastname@example.org.