Observations of Dramatic Galactic Collision Unlock Clues to the Reionization of the Universe

An international team led by astrophysicists from Stockholm University has revealed unprecedented insights into the Epoch of Reionization, a crucial period that occurred within a billion year after the Big Bang and during which the Universe became transparent to radiation. The study sheds light on the processes through which galaxies participated to change the nature of the Universe. This study, made possible by the remarkable sensitivity of the MeerKAT radio telescope, shows how galaxy mergers could have played an important role during the Epoch of Reionization.

Image of the galaxy Haro 11. The stellar light of the galaxy, shown in white, is surrounded by a halo of ionized gas in red. The newly imaged neutral Hydrogen gas, shown in blue, has been displaced during an interaction between two galaxies that resulted in the creation of Haro 11.

The Epoch of Reionization occurred within the first billion year after the Big Bang and represents a swift transformation of the matter in the Universe. During this period, the neutral Hydrogen gas that filled the Universe became fully ionized. Ionization happens when neutral Hydrogen encounters strong ultraviolet radiation. Ultraviolet radiation carries a lot of energy, which we experience here on Earth when our skin is exposed to UV rays from the Sun and gets burnt. Fortunately for us, the atmosphere absorbs the strongest ultraviolet radiation. In space however, these strong ultraviolet rays are free to travel, and some have sufficient energy to pull apart the proton and electron in the Hydrogen atom: this is what is called ionization. This very energetic radiation was produced in enormous amounts after the creation of the first stars and galaxies that they ionized almost all the Hydrogen in the Universe within a few hundred million years.

While it is widely accepted that the first galaxies were the main source of this ultraviolet radiation, it is not fully known how the radiation managed to pierce through the dense gas within these galaxies to ionize the Universe as a whole. Galaxies themselves contain a large amount of neutral Hydrogen: similarly, to the way the atmosphere around the Earth absorbs the energetic ultraviolet radiation from the Sun and prevents it from reaching us, the Hydrogen gas within galaxies prevents the strong ultraviolet rays from stars to escape galaxies. Understanding how this escape happened is therefore crucial to test our models for Reionization and for the evolution of the Universe. The issue, however, was that no one had ever mapped the neutral Hydrogen gas in a galaxy emitting strong ultraviolet radiation directly. “We knew several of those galaxies had neutral Hydrogen gas in large amounts. So, for the ultraviolet radiation from those galaxies to escape in our direction, something must have happened to the Hydrogen gas around the stars that emit the radiation. However, until recently we did not have telescopes that were sufficiently powerful to observe the structure of the gas itself,” says Alexandra Le Reste, who led the study as part of her doctoral thesis at Stockholm University and the Oskar Klein Centre.

One of the methods to understand how the early Universe became ionized is to study nearby galaxies whose properties are similar to those during the Epoch of Reionization. The galaxy Haro 11, which lies about 300 million light years away from Earth, is the closest and one of only three galaxies in the nearby Universe that is known to emit strong ultraviolet radiation capable of ionizing Hydrogen. For this reason, it has been a focal point for researchers seeking to understand the Epoch of Reionization, providing a unique opportunity for astronomers to study the details of the processes enabling the escape of ultraviolet radiation. However, despite multiple attempts in the past decade, mapping the Hydrogen gas around this galaxy had proven elusive. The signal from Hydrogen itself is very weak, and while Haro 11 is close in comparison to other similar types of galaxy, it is at a distance such that previously available telescopes could not observe the structure of the gas.

The international team of astronomers behind this study used the MeerKAT radio telescope to observe the neutral Hydrogen gas in a galaxy emitting strong ultraviolet radiation for the first time. MeerKAT is a new-generation radio telescope located in South Africa consisting of 64 antennas working together to map the sky. This new telescope allowed the scientists to observe the Hydrogen gas surrounding Haro 11, revealing a highly unusual distribution. In galaxies forming stars, the neutral Hydrogen gas is typically located all around the galaxy and its stars in a relatively symmetric manner. In Haro 11 however, the MeerKAT observations revealed that almost all the gas sat on one side of the galaxy, far away from its stars. Astronomers already knew that Haro 11 had been formed in a collision between two galaxies, but they did not expect that the collision had had such a dramatic effect on the gas. When the two original galaxies merged to form Haro 11 a few hundred million years ago, the gravitational forces pulled the gas away and exposed the regions that produce ultraviolet radiation in Haro 11. “I have worked on the puzzle of ionizing radiation emerging from galaxies for 20 years, and I have never seen anything like this. We have speculated frequently about what mechanisms are at play behind ultraviolet radiation escape from galaxies, but this observation really clinches the gas removal scenario for Haro 11,” says Professor Matthew Hayes, who was involved in the study.

This discovery unveils a previously unknown factor: the effect of galaxy mergers on pulling the gas far away from galaxies, which could help the ultraviolet radiation to escape more easily from stars in these galaxies. It is an important addition to our understanding of how galaxies participated in the Reionization of the Universe. “Galaxies formed in a hierarchical manner, with smaller galaxies merging into bigger ones under the effect of gravitation as the Universe evolved. For this reason, it is expected that galaxy mergers, or collisions, were much more common in the past than they are nowadays. It is not unreasonable to think that the mechanism we identified, with the gas being ejected far from the center of galaxies during galaxy mergers, could play an important role during the Epoch of Reionization” added Le Reste.

MeerKAT is a precursor to the Square Kilometer Array (SKA), that will start operating in 2027 and is set to provide unprecedented insights into the Epoch of Reionization. MeerKAT has only a third of the number of antennas that SKA will have in South Africa and has already reached unparalleled sensitivity. In the future, the SKA will not only allow many sources with faint gas to be observed in great details but will also image the gas in the intergalactic medium during the Epoch of Reionization itself.

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