One of the biggest cosmological mysteries right now is when “cosmic reionization” began. To find out, astronomers have been looking deeper into the cosmos (and farther back in time) to spot the first visible galaxies. Thanks to new research by a team of astronomers from University College London (UCL), a luminous galaxy has been observed that was reionizing the intergalactic medium 13 billion years ago.
The team responsible for this discovery was led by Romain Meyer. He says: “By looking at distant galaxies, we look into the early Universe, as the light has traveled for billions of years before reaching us. This is fantastic as we can look at what galaxies were like billions of years ago, but it comes with several drawbacks.”
For starters, Meyer explained, distant objects are very faint and can only be observed using the most powerful ground-based and space-based telescopes. At this distance, there’s also the tricky issue of redshift, where the expansion of the cosmos causes light from distant galaxies to have its wavelength stretched towards the red end of the spectrum.
In the case of galaxies that several billion years old, the light has been shifted to the point that it is only visible infrared (particularly the UV light Meyer and his colleagues were looking for). In order to get a good look at A370p_z1, a luminous galaxy 13 billion light-years away, the team consulted Using data from the Hubble Frontier Fields program – which astronomers are still analyzing.
The Hubble data suggested that this galaxy was very redshifted, indicating that it was particularly ancient. They then made follow-up observations with the Very Large Telescope (VLT) to get a better sense of this galaxy’s spectra. In particular, they looked for the bright line that’s emitted by ionized hydrogen, known as the Lyman-alpha line. Said Meyer:
“The big surprise was to find that this line, detected at 9480 Angstroms, was a double line. This is extremely rare to find in early galaxies, and this is only the fourth galaxy that we know of to have a double Lyman-alpha line in the first billion years. The nice thing with double Lyman-alpha lines is that you can use them to infer a very important quantity of early galaxies: what fraction of energetic photons they leak into the intergalactic medium.”
Another big surprise was the fact that A370p_z1 appeared to be letting 60% to 100% of its ionized photons into intergalactic space, and was probably responsible for ionizing the bubble IGM around it. Galaxies that are closer to the Milky Way typically have escape fractions of about 5% (50% in some rare cases), but observations of the IGM indicate that early galaxies must have had a 10 to 20% escape fraction on average.
This discovery was extremely important because it could help resolve an ongoing debate in cosmological circles. Until now, the questions of when and how reionization occurred has produced two possible scenarios. In one, it was a population of numerous faint galaxies leaking about 10% of their energetic photons. In the other, it was an “oligarchy” of luminous galaxies with a much larger percentage (50% or more) of escaping photons.