spaceSpace and Physics

Luminous Galaxy Caught Mid-Reionization Provides Evidence In Cosmic Mystery


The distant galaxy A370p_z1 (marked by the orange box and arrows) suggests luminous galaxies with high photon leakage are responsible for the re-ionization of the early universe. NASA, ESA, Z. Levay (STSci)

The early universe is full of mysteries, many of which are unsolved. However, observations of a luminous galaxy (A370p_z1) 800 million years after the Big Bang, could lead astronomers closer to solving one of them.

At the end of the universe’s formative stages came a period of cosmic reionization, where atoms in the neutral intergalactic gas (primarily comprised of hydrogen) were stripped of their electrons and became charged (or ionized). Astronomers have already gathered strong evidence to suggest this period ended roughly 1 billion years after the Big Bang, but what exactly caused the re-ionization is still up for debate.


There are two main suspects in the case; a population of multiple faint galaxies which emit roughly 10 percent of their energetic photons (that then proceed to ionize the intergalactic medium), or, a group of luminous galaxies that leak a much greater percentage (more than 50 percent) of their ionizing photons.

To determine which type of galaxy is responsible, astronomers have to measure the “escape fraction” of photons from the earliest galaxies. This can be inferred by the light emitted from excited hydrogen atoms (the Lyman-alpha line). As this signature can be absorbed by yet to be ionized gas around galaxies, it is rarely observed.

However, if the hydrogen emission is detected then that signals a large ionized bubble around a galaxy in the process of reionization. How large the bubble is and a measure of the galaxy’s luminosity helps astronomers to determine whether it acted alone or had an accomplice.

At the virtual annual meeting of the European Astronomical Society (EAS), Romain Meyer, a PhD Student at University College London (UCL), UK, presented the team’s discovery of a luminous galaxy which leaked between 60 to 100 percent of its photons and consequently re-ionized its local bubble likely on its own.


“It is the first time we can point to an object responsible for creating an ionized bubble, without the need for a contribution from unseen galaxies,” Meyer said in a statement. “Additional observations with the upcoming James Webb Space Telescope will enable us to study further what is likely one of the best suspects for the unsolved case of cosmic reionization.”

Not quite a case solved, but suspect “luminous” appears to have the strongest evidence in the case so far.


spaceSpace and Physics