Struggle to escape distant galaxies creates giant halos of scattered photons

In order to understand how our own Milky Way galaxy formed and evolved, astronomers rely on observing distant galaxies. As their light takes billions of years to reach us, telescopes can be used as time machines, as long as we have a clear indicator to pinpoint the distance to the objects being observed. As with closer galaxies, stars and planets, astronomers use the technique of spectroscopy to analyse their light, dispersing it into a spectrum.

Scientists then look for characteristic features (spectral lines) that tell them about properties including the composition, temperature and movement of the object. With the most distant galaxies, only one spectral feature typically stands out, the so-called Lyman-alpha line associated with hydrogen gas.

Jorryt Matthee comments: "Newly born stars in very distant galaxies are hot enough to break apart hydrogen in surrounding clouds of gas, which then shines brightly in Lyman-alpha light, in theory the strongest such features observable in a distant galaxy. Yet in practice, Lyman-alpha photons struggle to escape galaxies as gas and dust block and diverge their travel paths, making it a complex process to understand."

Using the Isaac Newton Telescope (INT) on La Palma in the Canary Islands, astronomers developed a unique experiment to study almost 1000 distant galaxies. They surveyed the sky using the Wide Field Camera and custom-made filters, in order to measure where the Lyman-alpha is produced, how much of it there is, and where it comes out of galaxies.

David Sobral says: "We have used dozens of dedicated nights on the INT to understand how Lyman-alpha photons escape, and from which galaxies. We looked back in time 11 billion years, essentially the limit of where we can identify distant galaxies and study them in detail. Most importantly, we were able to predict accurately how many Lyman-alpha photons were effectively produced in each galaxy and where this happened. Then we compared them with the ones that actually reach the INT."

The results show that only 1-2% of those photons escape from the centres of galaxies like the Milky Way. Even if we account for all the photons at a large distance from the centre, fewer than 10% escape.

"Galaxies forming stars in the distant Universe seem to be surrounded by an impressively large, faint halo of Lyman-alpha photons that had to travel for hundreds of thousands of light years in an almost endless series of absorption and re-emission events, until they were finally free. We now need to understand exactly how and why that happens," adds Sobral.

When the James Webb Space Telescope begins operation in 2018, astronomers expect to be able to look even further back in time, opening up a new window on the first galaxies and stars. Studying how the escape fraction evolves over time can tell us about the kind of stars producing these photons, and the properties of interstellar and intergalactic gas.