🔭 Hubble sees an object it shouldn't have seen

Astronomers couldn't believe their eyes when they discovered MXDFz4.4: they thought observing such an object, a galaxy from such a distant epoch, was impossible. Yet this distant galaxy, immersed in a fog of neutral hydrogen, lets through ultraviolet light that should have been absorbed.

After the Big Bang, the Universe was filled with neutral hydrogen gas, opaque to ultraviolet radiation. This period, called the Epoch of Reionization, lasted about a billion years. Gradually, energy sources ionized this gas, making it transparent. But what were these sources? Astronomers were torn between supermassive black holes and the first massive stars.


In 2023, the James Webb Space Telescope had already found a galaxy capable of ionizing its environment 900 million years after the Big Bang. Today, Hubble goes further: it detected the ultraviolet light of MXDFz4.4, a galaxy that existed 1.4 billion years after the birth of the Universe. This light can only be visible if the surrounding gas has already been ionized.

MXDFz4.4 is a hundred times smaller than our Milky Way, but it forms stars ten times faster. At the heart of this galaxy, a dense cluster of young, hot, massive stars produces the ionizing radiation. Ilias Goovaerts, from the Space Telescope Science Institute, explains that a large number of young, hot stars in a confined space pierces through the opaque gas more effectively.

The researchers compared Hubble's data with Webb's and discovered that the stars in the cluster formed in successive bursts. Each burst of star formation produced a new wave of ultraviolet radiation. Today, we observe the galaxy about 250 million years after it finished reionizing its neighborhood.


These observations confirm that it was indeed the clusters of massive stars in young galaxies that played a major role in dispersing the cosmic fog. Marc Rafelski, deputy project scientist for the Hubble mission, indicates that finding other galaxies from slightly later epochs will allow refining these measurements and understanding how our view of the Universe cleared up. The results are published in The Astrophysical Journal.

The Epoch of Reionization

After the Big Bang, the Universe was filled with a hot plasma that cooled to form neutral hydrogen, opaque to ultraviolet light. This dark period lasted until the first stars and galaxies began emitting radiation capable of stripping electrons from hydrogen atoms, a process called ionization.

The Epoch of Reionization gradually made the Universe transparent to ultraviolet light, allowing the light from the first galaxies to reach us. Observations of MXDFz4.4 show that clusters of massive stars were the main agents of this transformation, long before supermassive black holes took over.