"Understanding the Lyman-alpha background helps shed light on nearby galactic structures and processes," said SwRI's Dr. Randy Gladstone, the study's lead investigator and first author of the publication. "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.
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.
During its initial journey to Pluto, New Horizons collected baseline data about Lyman-alpha emissions using the Alice instrument, an SwRI-developed ultraviolet spectrograph. A spectrograph is a tool astronomers use to split light into its various colors. Alice specializes in the far-ultraviolet wavelength band.
After the spacecraft's primary objectives at Pluto were completed, scientists used Alice to make broader and more frequent surveys of Lyman-alpha emissions as New Horizons traveled farther from the Sun. These surveys included an extensive set of scans in 2023 that mapped roughly 83% of the sky.
To isolate emissions from the galaxy, the New Horizons team modeled scattered solar Lyman-alpha emissions and subtracted them from the spectrograph's data. The results indicate a roughly uniform background Lyman alpha sky brightness 10 times stronger than expected from previous estimates.
"These results point to the emission and scattering of Lyman-alpha photons by hydrogen atoms in the shell of a hot bubble, known to surround our solar system and nearby stars, that was formed by nearby supernova events a few million years ago," Gladstone 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 emissions.
"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," said co-author and New Horizons Principal Investigator Dr. Alan Stern. "This Lyman-alpha map also provides a solid foundation for future investigations to learn even more."
The black dots represent approximately 90,000 known UV-bright stars in our galaxy. The north and south galactic poles are indicated (NGP and SGP, respectively), along with the flow direction of the interstellar medium through the solar system (both upstream and downstream).
Research Report:The Lya Sky as Observed by New Horizons at 57 au
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