Astronomers using NASA’s Hubble Space Telescope have made an unexpected discovery that sheds new light on one of the universe’s greatest transformations.
They detected ultraviolet light from a distant galaxy that existed just 1.4 billion years after the Big Bang, providing important clues about how the early universe became transparent.
The galaxy, known as MXDFz4.4, lived near the end of a period called the Era of Reionization. During the universe’s first billion years, space was filled with vast amounts of neutral hydrogen gas.
This gas acted like a thick fog, absorbing energetic ultraviolet light and making the cosmos largely opaque.
Over time, however, the fog disappeared. The hydrogen gas became ionized, meaning its atoms lost electrons and no longer blocked ultraviolet light.
This gradual process transformed the universe into the transparent cosmos we see today. Scientists have long debated exactly how this change happened.
MXDFz4.4 may provide part of the answer.
Researchers were surprised to detect ultraviolet light escaping from this galaxy because they believed the remaining hydrogen fog in the early universe would have hidden such light from view.
The galaxy contains a densely packed cluster of young, massive stars. These stars produce enormous amounts of energetic ultraviolet radiation capable of ionizing hydrogen gas.
The researchers believe this radiation is clearing the gas both inside and around the galaxy, allowing the light to escape into space.
The light from MXDFz4.4 traveled for more than 12 billion years before reaching Earth. During its journey, the universe continued expanding, stretching the light into longer wavelengths that Hubble could detect.
Scientists have found many galaxies from this period in cosmic history, but none had previously shown this kind of escaping ionizing light. That makes MXDFz4.4 particularly important.
The observations revealed that the galaxy’s stars formed in powerful bursts during the last few million years of the galaxy’s existence. Even more remarkable is the galaxy’s compact size. MXDFz4.4 covers an area roughly 100 times smaller than our Milky Way, yet it is forming stars at about ten times the Milky Way’s rate.
This intense concentration of young stars appears to be crucial. Large numbers of hot, massive stars packed into a small region can produce enough radiation to punch holes through surrounding gas clouds.
The researchers estimate that between 50% and 100% of the galaxy’s ionizing light escapes into space.
Massive stars also live short lives. Many explode as supernovas after only a few million years, releasing enormous amounts of energy that can blast open even larger gaps in the surrounding gas, allowing additional light to escape.
The discovery relied on the combined power of several observatories. Hubble provided the ultraviolet observations, while NASA’s James Webb Space Telescope supplied information about the galaxy’s mass, older stars, and star formation history. Data from the European Southern Observatory’s Very Large Telescope helped determine exactly when the galaxy existed.
Astronomers hope to find more galaxies like MXDFz4.4. Each new discovery will help explain how countless young galaxies gradually cleared the hydrogen fog that once filled the universe, transforming it from a dark and opaque place into the vast, transparent cosmos we can observe today.
