Astronomers have spotted an ancient galaxy shining through the cosmic fog of the early universe, revealing a detailed view that was thought to be impossible.
Using NASA's Hubble Space Telescope, along with data from the James Webb Space Telescope (JWST) and the European Southern Observatory's Very Large Telescope (VLT), researchers detected ionizing ultraviolet photons — energetic light capable of stripping electrons from hydrogen atoms — coming from the galaxy called MXDFz4.4.
The galaxy existed just 1.4 billion years after the Big Bang, and its ultraviolet light traveled across the universe for over 12 billion years to reach us.
'This was thought to be impossible'
'This was thought to be impossible,' said Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute and first author of the study published June 23 in The Astrophysical Journal. 'What's really special about this galaxy is that it's getting through so much of the intergalactic medium. It's the furthest away so it has the most intergalactic medium to get through.'
For hundreds of millions of years after the Big Bang, the space between galaxies was filled with a fog of neutral hydrogen gas that blocked this kind of light. Over time, radiation from the first stars and galaxies ionized that gas, clearing the fog and letting light travel freely — a process called the Epoch of Reionization.
A compact star factory
What makes MXDFz4.4 unusual is its extreme star formation. The galaxy is roughly 100 times smaller by area than the Milky Way, yet it forms stars around 10 times faster, packing massive young stars into an incredibly compact space. This crowding effect helps the galaxy punch clear channels through surrounding gas, letting ionizing light escape both the galaxy and the murky space between galaxies.
The team estimates that somewhere between half and all of the galaxy's ionizing light is escaping — an extraordinary proportion.
Serendipitous discovery
The discovery came about somewhat by chance. While preparing an unrelated funding proposal just days before a major deadline, Goovaerts examined an existing deep Hubble image to check whether anyone had looked for this kind of signal. Within a couple of hours, he had a promising signal. 'It was very, very quick from us having the idea to me going, okay, there's something here and this is exciting.'
The finding relied on an unusually rich set of observations: an extremely deep Hubble image from 40 hours of observations; JWST imaging across many wavelengths; and one of the deepest spectra ever taken of a single patch of sky, gathered over roughly six days of observing time with the VLT.
Researchers say bursts of vigorous star formation like the one seen in MXDFz4.4 may have played an important role in clearing the early universe's hydrogen fog — and that more galaxies like it are likely still waiting to be found.
