An international group of researchers has reported the first ever detection of a particular atomic hydrogen line emission from the Milky Way, using instruments on board the Voyager spacecraft. This emission, has already been seen from much more distant objects and is used to find star-forming galaxies. It is also used to probe the epoch of reionization – the formation of the first stars post the "dark ages" of the universe – and so is of considerable significance to astronomers.

Astronomers routinely look at astronomical objects and phenomena that are many millions of light-years away – currently, the furthest object observed is at a distance of 13.14 billion light-years. But surprisingly, there are phenomena that occur within the bounds of our parent galaxy – the Milky Way – that have not been seen or studied. An example of this is the "Lyman-alpha (Lyα) emission" (121.6 nm) that is generated when there is an electron transition between the first and second energy levels of hydrogen. The galactic Lyα emission is an essential marker of the young-star-formation rate in the Milky Way, the ionization environment in which the atmospheres of young planets evolve, and the amount of shocked gas in the interstellar medium.

Star birth marker

Although Doppler-shifted Lyα lines emission has been seen from other galaxies, it has been undetectable for the Milky Way as a result of very bright local sources that drown out the galactic Lyα radiation, in a similar manner to which the glare of bright lights from a city blocks the light from all but the brightest of stars. This local brightness is mainly attributed to the "H glow" – solar Lyα photons scattered by neutral hydrogen gas in the solar system. Like getting away from a big city to see a clear sky, astronomers are using data from the two Voyager spacecraft – both of which have now reached the heliosheath at the very edge of the solar system and are beyond the worse of the H glow.

Using the recently acquired data from the Voyager spacecraft, launched by NASA in 1977, Rosine Lallement from the Observatoire de Paris run by the French research council (CNRS), and colleagues in the US and Russia are the first to study this galactic emission and confirm that most of it originates in the regions where hot young stars are being formed. As both spacecraft are moving out of the solar system, they can perceive the much fainter radiation that comes from the galaxy. "For us, it is like beginning to see small candles within a brightly lit room," explains Lallement. The team has been busy "disentangling the two [local and distant] signals" that have two different consequences.

Glowing Milky Way

In the distant case, the astronomers study the amount of ultraviolet Lyα radiation emitted by a galaxy because it corresponds to the rate at which stars are being born within that galaxy – that is, the star formation rate (SFR) of the galaxy. Lallement explains that one of the major goals for astronomers is to pinpoint when stars first appeared in the young universe, and so detecting the Lyα emissions from the most distant galaxies and correctly interpreting the signal is essential. "However, the correspondence between Lyα and the SFR is not an easily derived due to the complex manner in which the radiation propagates through the distant galaxy. A single Lyα photon can be absorbed and scattered by the numerous neutral hydrogen clouds present within galaxies, and hence its history is essentially lost due to its complex "random walk" from its origin to its escape from the ionized regions of a distant galaxy."

She goes on to say that the star, gas and dust distributions are not known in those galaxies that are extremely faint and so accurately observing and deciphering the Lyα galactic signal to test and calibrate Lyα photon propagation models for distant galaxies was impossible. "In the case of the Milky Way we have for the first time the Lyα signal and all the necessary information, thus models can be tested," says Lallement. The Lyα emission can trace the SFR in much more distant galaxies, with redshifts from z = 2 to z = 6.

Closer to home

The study of the other signal – the local signal – is important to understand the heliosphere – the bubble formed due to the solar wind that contains our entire solar system and marks the extent of the Sun's environment, including the boundary between the Sun and the ambient galactic interstellar medium. Both Voyager spacecraft are currently crossing the boundary region, moving into the galactic gas. "The H glow due to penetrating neutral hydrogen atoms from interstellar space is part of the whole structure. Understanding how this local H glow evolves with the distance to the Sun and confirming the best models of the glow brings information that is complementary to in situ data", says Lallement, explaining that although the data from the Voyager spacecraft shows a preliminary distribution of the emissions; precise maps will have to wait a dedicated mission. NASA's New Horizons spacecraft, on its way to Pluto, has an ultraviolet-imaging spectrometer that could observe galactic Lyα emission in a more systematic way.

Unfortunately, power on board the Voyager spacecraft decreases as they move further and further away from the Sun. Indeed, no data will be received beyond 2020–2025. To save power, the UV instruments on board are no longer capable of pointing towards a certain source; data are still recorded but in a fixed direction. Hopefully, they will still generate new and useful information about the galactic Lyα emissions as well as the interstellar gas until then.

The research is published in Science 10.1126/science.1197340.