Astronomers have demonstrated how one galaxy that existed when the cosmos was only 1.4 billion years old transformed the gas in and around itself: Light from its young, massive, closely clustered stars blasted through opaque, electrically neutral gas, causing it to ionize and clear. This galaxy, cataloged MXDFz4.4, lived at a time when a universal event known as the Era of Reionization was wrapping up.

MXDFz4.4 is the earliest of its kind. It is the only galaxy at this distance to date that appears in a deep Hubble Space Telescope survey in a particular visible-light filter that uniquely captures the energetic light escaping from its young stars. “Hubble returned the only view that shows the galaxy’s ionizing photons — light capable of clearing the ‘fog’ in and around the galaxy,” explained Ilias Goovaerts, the first author of a new paper in the Astrophysical Journal and a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

Hubble’s observations are supported by both the James Webb Space Telescope and the European Southern Observatory’s ground-based Very Large Telescope (VLT). Webb helped the team estimate the properties of the galaxy and analyze its older stellar population, which is not responsible for converting the gas. The VLT dated exactly when this galaxy existed.

This rare example helps astronomers pin down the sources of high-energy light that caused the gas in the entire universe to gradually transition — permanently clearing our view.