An international team of astronomers has discovered enormous halos of blue, faint light — made of so-called Lyman-alpha photons — around early Milky Way-type galaxies.
This artist’s impression shows a Milky Way-type galaxy, surrounded by a large halo of blue, faint light, made of Lyman-alpha photons. Image credit: L. Calçada / ESO.
In order to understand how our 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, scientists use the technique of spectroscopy to analyze their light, dispersing it into a spectrum.
They 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.
“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,” said team member Dr. Jorryt Matthee, an astronomer at Leiden Observatory in the Netherlands.
“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), operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, Dr. Matthee and co-authors developed a unique experiment to study almost 1,000 distant galaxies.
They surveyed the sky using INT’s 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.
“We have used dozens of dedicated nights on the INT to understand how Lyman-alpha photons escape, and from which galaxies,” said team member Dr. David Sobral, of Leiden Observatory and the University of Lancaster.
“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 centers of galaxies like the Milky Way.
“Even if we account for all the photons at a large distance from the center, fewer than 10% escape,” Dr. Sobral said.
“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,” he added.
“We now need to understand exactly how and why that happens.”
The astronomers reported their results in a pair of papers (paper #1 and paper #2) in the Monthly Notices of the Royal Astronomical Society.
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Jorryt Matthee et al. 2016. The CALYMHA survey: Lyα escape fraction and its dependence on galaxy properties at z = 2.23. MNRAS 458 (1): 449-467; doi: 10.1093/mnras/stw322
David Sobral et al. 2017. The CALYMHA survey: Lyα luminosity function and global escape fraction of Lyα photons at z = 2.23. MNRAS 466 (1): 1242-1258; doi: 10.1093/mnras/stw3090
