Astronomers using data from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) have discovered tens of thousands of gigantic hydrogen gas halos, called “Lyman-alpha nebulae,” surrounding galaxies 10 billion to 12 billion years ago. Known as Cosmic Noon, this is an epoch in the early universe when galaxies were growing their fastest. To spur this growth, they would have needed access to vast reservoirs of hydrogen gas, a key building block for stars. However, until recently, astronomers had only found a handful of these essential structures.
A new study published in The Astrophysical Journal has now increased the known number of hydrogen gas halos by a factor of ten: from roughly 3,000 to over 33,000. This confirms suspicions that they are not rare curiosities. The study also increases the range of known sizes, providing a more representative sample for astronomers to study as they continue to tease out the origin and evolution of the first galaxies.
“We’ve been analyzing the same handful of objects for the past 20 or so years,” said Erin Mentuch Cooper, HETDEX data manager and lead author on the study. “HETDEX is letting us find many more of these halos and measure their shapes and sizes. It has really allowed us to create an amazing statistical catalogue.”
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, you 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, it 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 Karl Gebhardt, HETDEX principal investigator, chair of The University of Texas at Austin’s astronomy department, and co-author on the paper. “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 Dustin Davis, a postdoctoral fellow at UT Austin, a HETDEX scientist, and co-author on the study. “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.”
The newly revealed 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 Mentuch Cooper. “They look like giant amoebas with tendrils extending into space.”
To find them, the team 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: a compact central region of hydrogen and a thinner cloud extending beyond it.
Nearly half did. What’s more, this fraction is likely an underestimate, explained Mentuch Cooper. “We suspect the faintest systems simply aren’t bright enough to fully reveal how large they are.”
The team hopes their discovery will help others study the early universe: how its structures evolved, the distribution of matter, the 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.
“There are various models for galaxies in this epoch that largely work and seem to make sense, but there are gaps and holes,” explained Davis. “Now we can focus in on individual halos and see at a greater detail the physics and mechanics of what's going on. And then we can fix or throw out the models and try again.”
