The twin Voyager probes are so far from the sun that they can see a kind of light from the Milky Way that we on Earth cannot. The observations could act as a Rosetta stone for understanding star formation in more distant and ancient galaxies.
The veteran Voyagers, which were launched in 1977 and are slowly approaching the outer limit of the solar system, have detected a particular wavelength of light called Lyman-alpha emission coming from our home galaxy for the first time.
The light is useful because it is a trace of star formation in other galaxies. Hot young stars blast their surroundings with high-energy photons, stripping electrons from hydrogen atoms. Those stripped atoms eventually find another electron and absorb it to become whole again. When they do, they can emit two kinds of photons: H-alpha and Lyman-alpha.
A tale of two photons
H-alpha photons, which are in the visible part of the electromagnetic spectrum, tend to sail through interstellar gas unimpeded. But Lyman-alpha photons, which are in the shorter-wavelength ultraviolet range, tend to be absorbed by this gas and later re-emitted.
The complicated histories and trajectories of Lyman-alpha photons make H-alpha photons more straightforward telltales of star formation – for stars that are closer than about 8 billion light years. If H-alpha photons make a longer journey than that, the expansion of the universe stretches them too much for our telescopes to detect. This means we cannot learn about star formation in the early universe by studying H-alpha photons reaching us now.
Lyman-alpha light has a shorter wavelength to begin with, and even stretching it out over vast distances leaves it in the easily detectable visible region of the spectrum. "The only way to observe primordial galaxies and check the first stars that formed in the early universe is to look at Lyman-alpha," says Rosine Lallement of the Paris Observatory in France.
But even though we can see this visible-light Lyman-alpha coming from galaxies across the universe, we are blind to the unstretched version in our own. That's because the sun pumps out vast quantities of ultraviolet Lyman-alpha light, and this is then scattered by gas within the solar system. The light from any other Lyman-alpha sources in the Milky Way is still ultraviolet when it reaches us, so gets lost in the glare of the sun's output, like candle flames next to a searchlight.
Now Lallement and her colleagues have confirmed that the Voyager spacecraft have seen past this haze into star-forming regions in the wider galaxy. "You still see this glow [from the sun], and it is the dominant radiation you measure," she says. "But you begin to see, superimposed, the small galactic sources which are much farther away, which are coming from the spiral arms and the star-forming regions in the Milky Way disc."
The emission showed up when astronomers began scanning the sky using ultraviolet spectrographs on the probes in 1993, when Voyager 1 was about 40 astronomical units away from the sun (1 AU is the distance from the Earth to the sun).
Lallement and colleagues analysed 10 years of scans taken by Voyager 1 and five years of scans from Voyager 2. They found that spikes in Lyman-alpha emissions matched previously observed H-alpha emissions from star-forming regions in the Milky Way.
Because only Lyman-alpha emission is visible in very distant galaxies, and only H-alpha emission had previously been detected in the Milky Way, seeing them both in our galaxy will allow astronomers to better interpret star formation in other galaxies, Lallement says.
"In the Milky Way, you know exactly where the new stars are, the motion, the kinematics, you have everything you need to exactly test your propagation models," she says. "Because we know everything in detail, it is now possible to have a test case for those models."
Jeffrey Linsky of the University of Colorado at Boulder says the find is a bonus. "The primary mission of the Voyagers was to study the outer planets and their environments," he writes in a commentary about the study. "Who would have anticipated the new accomplishments of these spacecraft more than 20 years later?"
No more scans
To conserve power, the Voyager systems that scanned the skies for Lyman-alpha signals have been turned off for years. "It is ironic that the electrical power on the Voyagers is decreasing with time as the Lyman-alpha foreground keeps improving," says Linsky.
But Lallement says she doesn't mind. At 119 and 97 AU from the sun, Voyagers 1 and 2 are near the edge of the region of space dominated by the sun and its magnetic field. Turning off the ultraviolet spectrographs will allow the spacecraft to keep sending data back to Earth until at least 2020.
"For the first time, we will really get information as if we were travelling in interstellar space," Lallement says. "I don't complain about that."
Journal references: Science, DOI: 10.1126/science.1197340 and DOI: 10.1126/science.1200166
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