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Map of the universe's Lyman-alpha emissions collected by New Horizons looking away from the sun. (Credit: SwRI)

This story was adapted from a version published by the Southwest Research Institute.

The spacecraft’s extensive observations of Lyman-alpha emissions have resulted in the first-ever map from the galaxy at this important ultraviolet wavelength, providing a new look at the galactic region surrounding our solar system.

“Understanding the Lyman-alpha background helps shed light on nearby galactic structures and processes,” said Randy Gladstone, a researcher at the Southwest Research Institute (SwRI) in Boulder, Colorado, and lead author of the study. “This research suggests that hot interstellar gas bubbles like the one our solar system is embedded within may actually be regions of enhanced hydrogen gas emissions at a wavelength called Lyman-alpha.”

The team in The Astronomical Journal. Michael Shull, professor emeritus in the Department of Astrophysical and Planetary Sciences at ��ƷSM����ӰƬ, served as a co-author of the study.

New Horizons launched in 2006, and, after passing by Pluto in 2015, the spacecraft traveled outside the dustiest regions of Earth’s solar system—a good vantage point for viewing Lyman-alpha emissions.

Lyman-alpha is a specific wavelength of ultraviolet light emitted and scattered by hydrogen atoms. It is especially useful to astronomers studying distant stars, galaxies and the interstellar medium, as it can help detect the composition, temperature and movement of these distant objects.

After New Horizon’s primary objectives at Pluto were completed, scientists used the Alice instrument to make broader and more frequent surveys of Lyman-alpha emissions as the spacecraft traveled farther from the sun. These surveys included an extensive set of scans in 2023 that mapped roughly 83% of the sky.

The results indicate a roughly uniform background Lyman-alpha sky brightness 10 times stronger than expected from previous estimates. Shull explained that this intense glow is likely produced an “interstellar greenhouse effect.”

“The strong Lyman-alpha emission line was scattered millions of times by the hydrogen gas, bouncing around space outside the solar system like interstellar ping-pong balls,” he said.

The study also found no evidence that a hydrogen wall, thought to surround the sun’s heliosphere, substantially contributes to the observed Lyman-alpha signal. Scientists had theorized that a wall of interstellar hydrogen atoms would accumulate as they encountered the edge of our heliosphere, the vast region of space dominated by the solar wind as it interacts with the interstellar medium. However, the New Horizons data saw nothing to indicate the wall is an important source of Lyman-alpha emission.

“The Lyman-alpha emission map produced by New Horizons represents one of our first glimpses of the interstellar gas clouds that surround the ‘Local Hot Bubble,’” Shull said. “It’s amazing to think that the hot bubble and interstellar structures were shaped by exploding stars just a few millions years ago.”

SwRI’s Alan Stern, a co-author of the new study and principal investigator for New Horizons, added:

“These are really landmark observations, in giving the first clear view of the sky surrounding the solar system at these wavelengths, both revealing new characteristics of that sky and refuting older ideas that the Alice New Horizons data just doesn’t support. … This Lyman-alpha map also provides a solid foundation for future investigations to learn even more.”