An international team of astronomers using the data from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) to peer back 10 to 12 million years in time to the universe’s ‘Cosmic Noon’ has discovered tens of thousands of giant hydrogen halos surrounding several ancient galaxies.
While previous studies suggested that these phenomena, called Lyman-alpha nebulae, were rare, the new data indicate that these massive hydrogen halos were common during this epoch, when the universe was still growing. The discoveries also expand the range of known Lyman-alpha nebulae sizes, adding critical data about the evolution of the universe’s first galaxies.
Giant Hydrogen Halos Glow When Blasted with Enough UV Light
Astronomers have known about massive hydrogen halos surrounding early galaxies, but only a handful had been discovered, leading to the general assumption that they were rare. Erin Mentuch Cooper, a HETDEX data manager and the lead author on the study detailing the discovery, said that this small data set has limited the conclusions scientists could draw about this seemingly rare ‘curiosity.’
“We’ve been analyzing the same handful of objects for the past 20 or so years,” the researcher explained.
When scientists try to model Lyman-alpha nebulae, they must account for the vast reservoirs of hydrogen these structures would need to access to form. However, the researchers note that hydrogen does not generate its own light, making it notoriously difficult to detect.
Although the lack of self-generated light from hydrogen makes its detection challenging, the researchers noted that if the gas is near an object such as a galaxy or a group of galaxies full of UV-emitting stars “throwing off a lot of energy,” the hydrogen will glow. Detecting this glow requires significant observation time using precision instruments that are always in high demand, but it is possible.
The research team notes that these challenges have led to a handful of detections of the brightest, most “extreme” hydrogen halos in larger astronomical surveys, as well as smaller detections in targeted, small-scale surveys.
“As a result, everything in between the little guys and the big honkers has remained elusive,” they explained.
‘Enormous and Unprecedented’ Survey Finds Thousands of Lyman-Alpha Nebulae
To fill this gap, the team used observations from the Hobby-Eberly Telescope at McDonald Observatory. Study co-author Dustin Davis, a postdoctoral fellow at UT Austin and a HETDEX scientist, noted that the telescope, one of the largest in the world and which produces 100,000 spectra per observation, is currently charting the positions of over one million galaxies to help scientists understand dark energy.
“So, we have huge amounts of data, and there are all kinds of neat, fun, weird things waiting for us to find,” Davis explained.
Study co-author Karl Gebhardt, a HETDEX principal investigator and chair of The University of Texas at Austin’s astronomy department, said that the team captured nearly half a petabyte of data. This included 70,000 of the 1.6 million brightest early galaxies previously spotted by HETDEX. The department chair noted that this extensive data capture also included data on the galaxies and “the regions in between.”
“Our observations cover a region of the sky measuring over 2,000 full Moons,” Gebhardt explained, adding that the scope is “enormous and unprecedented.”
After running the data through supercomputers at the Texas Advanced Computing Center, the team made a startling discovery. Instead of finding giant hydrogen halos surrounding a handful of ancient galaxies, the supercomputers found that nearly half of the targeted galaxies had them.
“The study increased the known number of hydrogen gas halos by a factor of ten: from roughly 3,000 to over 33,000,” they explained.
A closer analysis of the data revealed several similarities and differences between the thousands of newly discovered Lyman-alpha nebulae. For example, the smallest measured tens of thousands of light-years across, whereas the largest spanned hundreds of thousands of light-years.
The researchers also identified hydrogen halos of different shapes. Some were described as a simple, football-shaped cloud surrounding a single galaxy, while others were described as “irregular blobs” encompassing multiple galaxies. Mentuch Cooper said the irregular halos are the ‘fun’ ones since “they look like giant amoebas with tendrils extending into space.”
“HETDEX is letting us find many more of these halos and measure their shapes and sizes,” the researcher added. “It has really allowed us to create an amazing statistical catalogue.”
“We Can Fix or Throw Out the Models and Try Again”
Although the discovery has dramatically increased the number of known Lyman-alpha nebulae, the researchers said they think that even this larger number is likely an “underestimate.”
“We suspect the faintest systems simply aren’t bright enough to fully reveal how large they are.” Mentuch Cooper explained.
When discussing the impacts of their discoveries, the HETDEX team said they hope it will help inform other studies about the early universe, including its evolution, matter distribution, movement of objects “and more.”
“With 33,000 halos to study, the problem will no longer be where to find them, but which one to choose,” they explained.
Davis said their work could also help fill gaps in models designed to replicate this historical epoch using actual data.
“Now we can focus in on individual halos and see at a greater detail the physics and mechanics of what’s going on,” the researcher explained. “And then we can fix or throw out the models and try again.”
The study “Lyα Nebulae in HETDEX: The Largest Statistical Census Bridging Lyα Halos and Blobs across Cosmic Noon” was published in the Astrophysical Journal.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.