Hubble telescope spots 'impossible' light from a galaxy that shouldn't have been visible

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. It's the earliest such detection on record, arriving only around 250 million years after the end of a major cosmic transition called the Epoch of Reionization, the researchers explained in a study published June 23 in The Astrophysical Journal.

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 across the universe — a process astronomers are still working to fully understand.

"This was thought to be impossible," Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore and first author of the new study, said to Live Science. "What’s really special about this galaxy is that it’s getting through so much of the intergalactic medium [the ionized plasma between galaxies]. It’s the furthest away so it has the most intergalactic medium to get through."

What makes MXDFz4.4 unusual is its combination of size and star-formation rate. The galaxy is roughly 100 times smaller by area than the Milky Way, yet it forms stars around 10 times faster, packing a large number of massive young stars into a compact space. According to Goovaerts, that crowding effect helps the galaxy punch clear channels through its surrounding gas, letting ionizing light escape both the galaxy and, eventually, the murky space between galaxies. The team estimates that somewhere between half and all of the galaxy's ionizing light is escaping.

The discovery, made in October, 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 there before. 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," Goovaerts said. "We were excited from day one, but then it took months for it to mature and to extract all the properties about the galaxy."

The finding relied on an unusually rich set of observations: an extremely deep Hubble image taken from 40 hours of observations; JWST imaging across many wavelengths, used to characterize the galaxy's stars and star-formation history; 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's Multi-Unit Spectroscopic Explorer instrument. That spectrum confirmed the galaxy's distance through its Lyman-alpha emission line — which serves as a "hydrogen fingerprint," or a glow given off by excited hydrogen gas, that astronomers can use to measure cosmic distance and time.

No other galaxy from this early period had previously shown detectable ionizing light, making MXDFz4.4 one of a kind so far, study co-author Marc Rafelski, deputy mission head for the Hubble Space Telescope at STScI, noted in the statement. .

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.

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