Astronomers from the Universities of Lancaster in the UK and Leiden in the Netherlands report the discovery of giant halos around early Milky Way type galaxies which are composed of Lyman-α photons that have struggled to escape them.
In order to understand how our own Milky Way galaxy formed and evolved, astronomers can use observations of 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 time-travelling indicator to pin-point the distance.
However, when we travel more than 11 billion years into the past, there is only one major photometric feature our telescopes can identify: Lyman α.
Jorryt Matthee comments: "Newly born stars in very distant galaxies are hot enough to break apart hydrogen in surrounding clouds of gas, which then shine brightly in Lyman-α light, in theory the strongest of such features observable in a distant galaxy.
Yet, in practice, Lyman-α photons struggle to escape from galaxies as gas and dust block and diverge their travel paths. As a consequence, these photons can escape some galaxies more easily than others, although the details are not well understood."
Astronomers developed a unique experiment using the Isaac Newton Telescope (INT) to look at almost 1000 very distant galaxies. They surveyed the sky using the Wide Field Camera (WFC) and a custom-made filter in order to measure where, and how much, Lyman-α emission is produced, and where it comes out of galaxies.
David Sobral says "We have used dozens of dedicated nights on the INT with our own narrow-band filter in order to understand how many Lyman-α photons escape and from which galaxies. We looked back in time 11 billion years, essentially the limit where we can still use multiple features to identify distant galaxies and study them in detail. Most importantly, we were able to predict accurately how many Lyman-α 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, less than 10% escape. In other words, all galaxies forming stars in the distant Universe are surrounded by an impressively large halo of Lyman-α photons, which we can only detect if we conduct extremely deep observations.
On the other hand, galaxies that are bright in Lyman-α light typically are of much lower mass than the Milky Way and have a higher escape fraction.