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ING web news release
12 January, 2017

Lyman-α Giant Halos Around Early Milky Way Type Galaxies

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.

The figure shows some of the observations conducted with the INT and with the UKIRT in Hawaii of one of the (almost 1000) young Milky Way type galaxies in the very early Universe. The results allowed astronomers to measure where, and how many, Lyman-α photons were produced (indicated by the red contour lines), and then compare with those that have actually escaped (blue contour lines) these distant galaxies. The results reveal large haloes of Lyman-alpha photons that struggled to escape, while the vast majority of these photons never make it out at all. Credit: J. Matthee/D. Sobral. Large format: JPG.

Astronomers expect that using the James Webb Space Telescope they 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 escape fraction evolves with redshift can tell us about the kind of stars producing these photons, and the properties of interstellar and intergalactic gas.

More information:

Jorryt Matthee, David Sobral, Iván Oteo, Philip Best, Ian Smail, Huub Röttgering and Ana Paulino-Afonso, 2016, "The CALYMHA survey: Lyα escape fraction and its dependence on galaxy properties at z = 2.23", MNRAS, 458, 449 [ ADS ].

David Sobral, Jorryt Matthee, Philip Best, Andra Stroe, Huub Röttgering, Iván Oteo, Ian Smail, Leah Morabito, Ana Paulino-Afonso, 2017, "The CALYMHA survey: Lyα luminosity function and global escape fraction of Lyα photons at z=2.23", MNRAS, 466, 1242 [ Astro-ph ].

"Photons struggle to escape distant galaxies", RAS press release PR 17/1, 10 January 2017.

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Last modified: 12 January 2017