Astronomers from the University of Wisconsin have released the most comprehensive data yet from the Wisconsin H-Alpha Mapper (WHAM) observatory, which has been looking at the ionized hydrogen that occupies interstellar space above and below the orbital plane of the Milky Way.
Ionized hydrogen shines at a particular deep-red wavelength, known as H-alpha, so astronomers can easily find it across the cosmos. WHAM has shown that our galaxy has an incredible amount of ionized hydrogen. It’s found in a structure 75,000 light-years across and 6,000 light-years thick, rotating with the galactic plane.
“It’s kind of like a galactic atmosphere,” Matt Haffner, who has been WHAM director since 2005, said in a statement. “We’re tracing the same kind of emission in the visible part of the spectrum that gives rise to bright nebulae. But over much of the galaxy, it’s just very, very faint.”
The team has called the structure the Reynolds layer, after Ron Reynolds, former WHAM director who retired 12 years ago. Reynolds discovered the vast reservoir of ionized hydrogen in the late 1970s by constructing a makeshift instrument connecting a hole he sawed in the ceiling of an office to a spectrometer to create a novel telescope.
“No one expected to see ionized hydrogen out in the middle of nowhere,” he said in a 2004 interview. “It’s all over the sky, but it is brightest in the plane of the galaxy.”
Hydrogen becomes ionized when its electrons are ripped away from its protons by powerful light. These electrons and protons then recombine emitting the characteristic H-alpha light. Young stars are the usual suspects in the ionization of interstellar gas, and that’s why astronomers are unsure how this hydrogen is ionized thousands of light-years away from the galactic plane.
“For us to see this emission everywhere, the gas has to be actively ionized,” Heffner added. “What are the sources of energy that keep it going?”
One possibility is that young and incredibly hot stars could be better at ionizing than we thought, exciting hydrogen atoms over huge galactic distances. Cracking this puzzle might teach us something new about the life of middle age galaxies like our own.