UNIVERSITY PARK, Pa. — The period of rapid galaxy growth in the universe about 10 billion years ago required massive amounts of hydrogen gas, according to astronomers, but they couldn’t find it — until now. Using observations from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), an international team of scientists, including three Penn State researchers, discovered tens of thousands of gigantic hydrogen gas halos that surrounded galaxies in the early universe. The halos, called “Lyman-alpha nebulae,” date to an era known as cosmic noon — about 2 to 3 billion years after the Big Bang — when galaxies were most rapidly increasing in size. To spur this growth, the galaxies would have needed access to vast reservoirs of hydrogen gas, a key building block for stars. Until recently, however, astronomers had only identified a handful of these essential structures, now it appears they have discovered the mother lode.
“This investigation has increased the number of known halos from approximately 3,000 to more than 33,000,” said Caryl Gronwall, research professor of astronomy and astrophysics in the Penn State Eberly College of Science and co-author of the study. “These are not rare curiosities, and we also now have a broader range of sizes — a more representative sample that might help us tease out the origin and evolution of the first galaxies.”
A paper describing the study recently published in The Astrophysical Journal.
“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, the researchers explained. However, if it’s near an object that is emitting enormous quantities of energy — for example, near a galaxy or group of galaxies containing massive stars emitting ultra-violet radiation — that energy can cause the hydrogen to glow.
“Previous astronomical work in this area identified a number of these structures, but with the sensitivity and wide sky coverage of the HETDEX instrumentation we have been able to move beyond the brightest, most extreme examples of this class,” said Robin Ciardullo, professor of astronomy and astrophysics at Penn State and a co-author on the paper. “The blind survey nature of HETDEX also allows detection of the full variety of the halo structures that can be missed in observations that target specific galaxies.”
Using the Hobby-Eberly Telescope at McDonald Observatory in Texas, HETDEX 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.”
HETDEX uses a set of 156 spectrographs — tools that can identify and analyze light — mounted on the HET. Each spectrograph gathers the light from distant galaxies and splits it into its individual wavelengths. These spectra can reveal the chemical composition and temperature of objects in the universe, and how fast they are moving toward or away from us.
“The Hobby-Eberly Telescope is one of the largest in the world,” said 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 structures containing multiple galaxies.
“Those are the fun ones,” Mentuch Cooper said. “They look like giant amoebas with tendrils extending into space.”
To find the structures, the team selected the 70,000 brightest of the over 1.6 million early galaxies that have been identified by HETDEX to date. With the aid of supercomputers at the Texas Advanced Computing Center, they examined each galaxy for evidence of a surrounding halo: a compact central region of hydrogen and a thinner cloud extending beyond it.
Nearly half of the galaxies possessed hydrogen halos, but this fraction is likely an underestimate, the research team explained. They suspect that the faintest systems simply aren’t bright enough to fully reveal their halos.
The team said they hope their discovery will assist our understanding of the early universe, for example, the evolution of its structures, the distribution of matter and the movement of objects.
“There are various models for galaxies in this epoch that largely work and seem to make sense, but there are gaps and holes,” Davis said. “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.”
In addition to Gronwall and Ciadullo, the team at Penn State included Donald Schneider, distinguished professor of astronomy and astrophysics.
This work was based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, Penn State, Ludwig-Maximilians-Universitaet Muenchen and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.
Editor’s note: A version of this story originally appeared on the HETDEX website.