Photons Couldn’t Easily Escape Ancient Galaxies

An artist's impression of a spiral galaxy surrounded by a large halo of Lyman-alpha photons. ESO/L. Calçada

Galaxies have not always been bright beacons of light shining in a dark, unforgiving universe. In the very early universe, according to new research, they were struggling to shine through the large gas clouds that surrounded them, with less than 10 percent of their photons escaping.

Astronomers from the University of Lancaster and the University of Leiden, led respectively by David Sobral and Jorryt Matthee, have discovered giant halos of light surrounding primordial galaxies that look like what the Milky Way might have been like more than 10 billion years ago. The discovery is reported in the Monthly Notices of the Royal Astronomical Society.

The observations, conducted with the Isaac Newton Telescope (INT) on La Palma in the Canary Islands, focus on a particular type of photon (particle of light) known as Lyman-alpha photons, which are associated with the light emitted by hydrogen – the main component of gas clouds.

“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," Sobral said in a statement. "We now need to understand exactly how and why that happens."

The photons are a bit like a crazy pinball, bouncing around and looking for a way to escape the galaxies. Lyman-alpha light is in the ultraviolet side of the spectrum and it's produced in great abundance by young, hot stars.

Ancient galaxies are full of them, and looking for the Lyman-alpha signature should be an easy way to discover far away galaxies. But it turns out that those Lyman-alpha photons are not so good at leaving their galaxies behind.

“We have used dozens of dedicated nights on the INT to understand how Lyman-alpha photons escape, and from which galaxies,” continued Sobral. “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.”

Only 1 to 2 percent of the Lyman-alpha photons were able to escape the core of the galaxies. Even when including larger distances, the total fraction escaping the galaxies remains lower than 10 percent.

New instruments like the James Webb Space Telescope will be able to look further back into the universe and, hopefully, answer the questions posed by this research.

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