Astronomers with the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) have detected enormous hydrogen halos, called Lyman-alpha nebulae, around more than 30,000 galaxies 10 billion to 12 billion years ago, suggesting the raw material for galaxy growth was far more abundant than once thought.
An enormous halo of hydrogen gas found in HETDEX data and superimposed over its location as seen in deep imaging from the NASA/ESA/CSA James Webb Space Telescope; present 11.3 billion years ago, this system glows from the combined light of many galaxies within it, with the brightest region represented in red. Image credit: Erin Mentuch Cooper, HETDEX / NASA / ESA / CSA / STScI.
Hydrogen gas is notoriously hard to detect because it doesn’t generate its own light.
However, if it’s near an object that’s throwing off a lot of energy — say, a galaxy or group of galaxies full of UV-emitting stars — that energy can cause the hydrogen to glow.
To detect this, astronomers need to dedicate a lot of time on precise instruments, which are often in high demand.
While previous astronomical surveys have found some of these halos, their instruments were only able to pick up on the brightest, most extreme examples.
And targeted observations of early galaxies are usually so zoomed in that they cut off all but the smallest halos.
As a result, everything in between the little guys and the big honkers has remained elusive.
Observations from HETDEX are starting to fill in this gap. Using the Hobby-Eberly Telescope at McDonald Observatory, the survey is charting the position of over one million galaxies in its quest to understand dark energy.
“We’ve captured nearly half a petabyte of data on not only these galaxies but the regions in between,” said HETDEX principal investigator Dr. Karl Gebhardt, chair of the University of Texas at Austin’s astronomy department.
“Our observations cover a region of the sky measuring over 2,000 full Moons. The scope is enormous and unprecedented.”
“The Hobby-Eberly Telescope is one of the largest in the world,” added HETDEX scientist Dr. Dustin Davis, a postdoctoral researcher at the University of Texas at Austin.
“And the instrument HETDEX uses produces 100,000 spectra in each observation. So, we have huge amounts of data and there are all kinds of neat, fun, weird things waiting for us to find.”
To find hydrogen halos, the astronomers selected the 70,000 brightest of the over 1.6 million early galaxies that have been identified by HETDEX so far.
With the help of supercomputers at the Texas Advanced Computing Center, they looked to see how many of these showed evidence of a surrounding halo.
According to the team, these halos measure from tens of thousands to hundreds of thousands of light-years across.
Some are as simple as a football-shaped cloud surrounding a single galaxy; others are sprawling, irregular blobs containing multiple galaxies.
“Those are the fun ones,” said HETDEX data manager Erin Mentuch Cooper, a researcher at the University of Texas at Austin.
“They look like giant amoebas with tendrils extending into space.”
A paper on the findings was published March 11, 2026 in the Astrophysical Journal.
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Erin Mentuch Cooper et al. 2026. Lyα Nebulae in HETDEX: The Largest Statistical Census Bridging Lyα Halos and Blobs across Cosmic Noon. ApJ 1000, 38; doi: 10.3847/1538-4357/ae44f3
