ISSN 2330-717X

Giant Halos Discovered Around Early Milky-Way Galaxies

Artist impression of early Milky way type galaxy. Credit: ESO/L.CalçadaArtist impression of early Milky way type galaxy. Credit: ESO/L.Calçada

Astronomers have looked 11 billion years into the past to discover giant halos around early Milky-Way galaxies.

The discovery was made by astronomers led by David Sobral from Lancaster University in the UK and Jorryt Matthee from Leiden University in the Netherlands. The team reports its findings in the journal Monthly Notices of the Royal Astronomical Society.

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 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 feature which telescopes can identify: Lyman-alpha photons.

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

Astronomers developed a unique experiment using the Isaac Newton Telescope (INT) in La Palma to look at almost 1000 very distant galaxies in a stereoscopic way. They surveyed the sky using the Wide Field Camera (WFC) and custom-made filters in order to measure where Lyα is produced, how much, and where it comes out of galaxies.

Dr David Sobral said, “We have used dozens of dedicated nights on the INT with our own narrow-band filter in order to understand how Lyα photons escape from distant galaxies. We have 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. We were able to predict how many Lyα photons were effectively produced in each distant galaxy and where this happened. Then we compared them with the ones that actually reach the INT.”

The results show that while these photons are so used to study the very early Universe, 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 center, less than 10% escape.

Dr Sobral said: “In other words, all galaxies forming stars in the distant Universe seem to be surrounded by an impressively large, faint halo of Lyα 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.”

Astronomers expect that the James Webb Space Telescope will be able to extend these studies to even higher look-back times, opening up a new window into the study of galaxy formation and evolution. Studying how the escape fraction evolves with redshift can tell us about the kind of stars producing Lyα photons, and the properties of interstellar and intergalactic gas.


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