Astronomers using NASA’s Hubble Space Telescope have found something they never expected — ultraviolet light from a galaxy that existed just 1.4 billion years after the big bang. This discovery helps explain how the universe emerged from an era of opaque darkness.
The End of the Neutral Fog
The galaxy, cataloged as MXDFz4.4, existed at the end of the Era of Reionization, a transformative period in our universe. During roughly the first billion years of the cosmos, the gas between stars and galaxies was completely opaque to energetic ultraviolet light. As time wore on, this neutral hydrogen fog became transparent (ionized).
“Observing a galaxy like this was thought to be impossible,” said lead author Ilias Goovaerts. “Researchers expected the ‘fog’ that filled the early universe would be too thick and obscure our view of its ionizing light. Hubble not only spotted that light, but it also revealed incredible details about the galaxy.”
Thousands of galaxies appear across the black background of space. A white square highlights the tiny blue pinpoint of MXDFz4.4. Credit: NASA, ESA, CSA, STScI
The Great Light "Escape"
Young, massive stars emit ultraviolet light capable of ionizing hydrogen atoms. As this light traveled for over 12 billion years to reach Hubble, space expanded, and the light stretched into visible light. MXDFz4.4 is exceptionally unique:
- Size and Speed: The galaxy is about 100 times smaller by area than our Milky Way but is forming stars 10 times faster.
- Radiation Leak: A massive amount of young, hot stars are crammed together. Researchers estimate that 50 to 100% of their energetic ionizing light escapes the surrounding gas.
- Supernovae Blasts: Massive stars live short lives. Many explode as supernovae, blowing colossal holes in the gas that allow even more light to escape into intergalactic space.
A Tale of Three Telescopes
Hubble could not do this alone. These conclusions are supported by data taken by NASA’s James Webb Space Telescope (JWST) and the European Southern Observatory’s Very Large Telescope (VLT).
Illustration portraying galaxy MXDFz4.4 when it existed 1.4 billion years after the Big Bang. Credit: NASA, ESA, Leah Hustak (STScI)
Data from the VLT pinpointed exactly when MXDFz4.4 existed: 1.4 billion years after the Big Bang. Meanwhile, Webb’s data was used to determine the galaxy’s mass and proved that recent star formation happened in intense bursts.
Expanding What We Know. Before this discovery, researchers had only identified a few galaxies emitting ionized light from later cosmic times. MXDFz4.4 brings researchers significantly closer to drawing firm conclusions about how the Era of Reionization unfolded and cleared our view of the cosmos.
