Astronomers recently used the Hobby-Eberly Telescope Dark Energy Survey (HETDEX) to draw the most detailed three-dimensional hydrogen radiation distribution map to date, and for the first time outlined a "hidden cosmic web" of galaxies and gas intertwined in the universe about 10 billion years ago. This map tracks the Lyman alpha line light emitted by hydrogen atoms after absorbing energy from nearby stars. Its time span covers the evolution of the universe from about 9 billion to 11 billion years ago.
Lyman alpha radiation is usually particularly strong in regions with active star formation and has long been an important clue to finding early bright galaxies. However, the locations of a large number of faint galaxies and diffuse gas clouds that emit the same signal have been difficult to locate. Maya Lujan Niemeyer of the Max Planck Institute for Astrophysics in Germany, a member of the Heterdex team responsible for this mapping work, said that observing the early universe helps to understand how galaxies evolved into what they are today and the role of intergalactic gas in this process. However, these targets are extremely distant and weakly luminous, which poses a huge challenge to observational capabilities.
In order to detect these previously "invisible" sources, researchers used a new method of "spectral line intensity mapping". Instead of identifying individual galaxies one by one, they measured the overall radiation intensity of a certain element in large-scale space, thereby capturing the cumulative glow of countless faint targets at once. Julian Muñoz, co-author of the paper and astronomer at the University of Texas at Austin, vividly said that traditional sky surveys are like marking only the brightest city lights in the night sky on an airplane, while intensity mapping is like looking down at the earth through a "smeared window." Although the image is blurry, it records all light sources, not just the brightest part.
This work is the first to use such a large data set to perform intensity mapping of Lyman alpha radiation with high accuracy. The HETDEX project relies on the Hobby-Eberly Telescope of McDonald Observatory. It was originally designed to study dark energy by measuring the positions of more than one million bright galaxies. In the process, it accumulated extremely large spectral data - more than 600 million spectra, covering an area of the sky equivalent to the total area of more than 2,000 full moons in the sky. Carl Gebhardt, the project leader and chair of the Department of Astronomy at the University of Texas at Austin, pointed out that only about 5% of the data is actually used in daily scientific analysis, and most of the rest has been sleeping on the hard drive, which contains huge potential.
Niemeyer emphasized that HETDEX almost "shuts in light" in a sky area, but the galaxies that are really bright enough to be included in the original scientific target sample are just the tip of the iceberg, and more light is hidden in the seemingly empty background. In order to "fish out" the structure of the cosmic web from the massive amount of original observations, the team developed special software and used the supercomputer at the Texas Advanced Computing Center to centrally process about half a petabyte of data.
The research team then used the existing catalog of bright galaxies as "signposts" to infer the possible distribution locations of fainter galaxies and luminous gas clouds around them. Because gravity pulls matter together, bright galaxies often mark areas where matter is highly concentrated. Eiichiro Komatsu, scientific director of the Max Planck Institute for Astrophysics who participated in the study, said that these known galaxies can be regarded as stakes, and they can be used to estimate the distance and distribution of faint sources. The resulting map not only shows clearer details around bright galaxies, but also reveals for the first time the rich and complex structure in the originally "blank" areas between galaxies.
The astronomical community has previously conducted a large number of computer simulations on the evolution of the universe during this period, but after all, they were only theoretical deductions. The measured cosmic network obtained this time provides a solid benchmark for testing related astrophysical processes. The researchers next plan to cross-reference this hydrogen Lyman alpha intensity map with survey results of other elements. For example, the cold and dense gas surrounding star birth regions is often revealed by carbon monoxide radiation. The corresponding intensity map is expected to reveal more details about the environment of young stars that produce Lyman alpha radiation.
Muñoz said that this work is first of all a "first detection", which is of great significance in itself. It also opens up a new way to systematically outline the universe using intensity mapping methods. He believes that the Hobby-Eberly Telescope plays a pioneering role in this field. As a variety of complementary new instruments are put into use one after another, mankind may usher in a "golden age" for mapping the large-scale structure of the universe.