For decades, scientists believed the early universe was filled with enormous amounts of hydrogen gas—the basic ingredient needed to form stars.
But proving this idea has been difficult, because hydrogen is almost invisible in space.
Now, a major survey has finally revealed just how widespread these hidden gas clouds really are, offering new insight into how galaxies grew in the universe’s early years.
The discovery comes from the Eberly Telescope Dark Energy Experiment (HETDEX), which has identified more than 33,000 massive hydrogen gas halos, known as Lyman-alpha nebulae.
These structures surround galaxies that existed around 10 to 12 billion years ago, during a period often called “Cosmic Noon,” when galaxies were forming stars at their fastest rate.
Before this study, astronomers had only found about 3,000 of these halos, so the new results represent a tenfold increase.
This finding shows that hydrogen halos are not rare oddities but are actually common features of early galaxies.
According to lead researcher Erin Mentuch Cooper, scientists had been studying the same small group of objects for nearly 20 years. With HETDEX, they can now examine a much larger and more representative sample, helping them better understand how galaxies formed and evolved.
One reason these halos have been so hard to detect is that hydrogen gas does not emit light on its own.
It only becomes visible when it is energized by nearby sources, such as galaxies filled with hot, young stars that produce strong ultraviolet radiation. This radiation causes the hydrogen to glow faintly, but detecting that glow requires highly sensitive instruments and long observation times.
Earlier surveys could only detect the brightest and most extreme examples of these halos. In addition, many observations focused closely on individual galaxies, missing the larger surrounding structures. As a result, many of the more typical halos remained hidden until now.
HETDEX has changed that by collecting an enormous amount of data. Using the Hobby-Eberly Telescope in Texas, the project has mapped over a million galaxies and gathered nearly half a petabyte of information.
Its observations cover a huge area of the sky, equivalent to more than 2,000 full moons, allowing scientists to study not just galaxies but also the space around them.
The newly discovered halos vary widely in size and shape. Some are relatively simple, forming smooth, oval clouds around a single galaxy. Others are much larger and more complex, stretching across hundreds of thousands of light-years and containing multiple galaxies. Some even resemble giant, irregular blobs with long extensions, like cosmic amoebas drifting through space.
To find these halos, researchers analyzed tens of thousands of bright early galaxies and looked for signs of surrounding hydrogen clouds. They found that nearly half of these galaxies had halos, and the true number may be even higher, since faint halos are harder to detect.
With this much larger sample, scientists now have a powerful new tool for studying the early universe. By examining these halos in detail, they hope to better understand how matter was distributed, how galaxies grew, and how cosmic structures evolved over time.
Source: University of Texas at Austin.
