Вадим Дудченко
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Hydrogen is the most abundant element. When a neutral hydrogen atom gets blasted with energy, its electron can be boosted to a larger orbit with a higher energy level. Then the electron can jump from one orbit level to another, which produces a photon. When the electron moves to the inner-most orbit from the orbit directly adjacent, it emits a photon with a particular wavelength in the ultraviolet spectrum, called Lyman-alpha emission. Since Lyman-alpha emissions are abundantly produced through the recombination of electrons and protons in Milky Way’s HII regions, it is estimated that they carry a large fraction of the photon energy in our Galaxy.

This artist’s illustration shows NASA’s New Horizons spacecraft in the outer Solar System. In the background lies the Sun and a glowing band representing Zodiacal Light, caused by sunlight reflecting off of dust. Image credit: Joe Olmsted / STScI.

The Lyman-alpha ultraviolet background was first detected in the 1960s, and its existence was later confirmed in 1971.

In most of the Solar System, the background is dominated by Lyman-alpha photons emitted by the Sun and scattered by interstellar hydrogen atoms that are passing through.

In the outer Solar System, however, where New Horizons travels, the scattered sunlight component of the Lyman-alpha signal is far less bright and the fainter components from the nearby regions of the Milky Way become easier to distinguish.

“The Galactic Lyman-alpha background comes from hot regions around massive stars which ionize all the matter near them, which is primarily hydrogen, as that is the most abundant element in the Universe,” said Dr. Randy Gladstone, a researcher in the Department of Physics and Astronomy at the University of Texas at San Antonio and Southwest Research Institute.

“When the electrons and protons eventually get back together, or recombine, they nearly always emit Lyman-alpha photons.”

“Hydrogen atoms between the stars scatter these photons into a roughly uniform glow throughout space,” he added.

“They are detectable, but only at the Lyman-alpha wavelength, which is at a wavelength about four times shorter than can be seen by human eyes.”

“It’s so bright from solar Lyman-alpha that we weren’t certain how much the Milky Way Galaxy contributed to its overall brightness. It’s like standing near a streetlamp on a foggy night. The fog scatters the lamp’s light, making it hard to see anything else.”

Galactic Lyman-alpha background: six-great-circle scan of August 30, 2019 overlaid on a model background; the location of the Sun is marked by an orange dot; the outlines of the four nearest local interstellar medium clouds (LIC, G, Blue, and Aql) are overlaid for comparison. Image credit: Gladstone et al., doi: 10.3847/1538-3881/ac23cd.

With the Alice ultraviolet spectrograph aboard NASA’s New Horizons spacecraft, Dr. Gladstone and colleagues were able to accurately measure the brightness of the Galactic component of the Lyman-alpha background for the first time.

“New Horizons has been flying away from the Sun for more than 15 years now,” Dr. Gladstone said.

“The farther we moved away from the Sun, the less we were blinded by the solar component of the Lyman-alpha background.”

With New Horizons now far beyond Pluto, the researchers were able to measure the brightness of the Lyman-alpha background from the Milky Way for the first time: about 20 times less bright than the Lyman-alpha background is near Earth.

“This has been something that’s been guessed at by astronomers for decades. Now we have a much more precise number,” Dr. Gladstone said.

The results were published in the Astronomical Journal.


G. Randall Gladstone et al. 2021. New Horizons Detection of the Local Galactic Lyman-α Background. AJ 162, 241; doi: 10.3847/1538-3881/ac23cd


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