China News Service, Beijing, September 8 (Reporter Sun Zifa) The National Astronomical Observatory of the Chinese Academy of Sciences announced on September 8 that the "China Sky Eye" (500-meter Aperture Spherical Radio Telescope, FAST) has recently made another major breakthrough in the field of neutral hydrogen sky surveys - building the world's largest neutral hydrogen galaxy sample library and drawing the largest cosmic gas map.
critical leap
This important achievement of "China Sky Eye" is the FAST Neutral Hydrogen Sky Survey (FASHI) project led by the team of researcher Jiang Peng of the National Astronomical Observatory of the Chinese Academy of Sciences, in conjunction with the team of researcher Guo Hong of the Shanghai Observatory of the Chinese Academy of Sciences, the Guizhou Radio Astronomy Observatory, and the South American Astronomy Institute of the Chinese Academy of Sciences. It was jointly completed by the National Science Center, Peking University and other collaborators. The relevant paper was published again as a cover article in the academic journal "Chinese Science: Physics, Mechanics, Astronomy" (English version) on September 8. The second phase of the FASHI project data (DR2) was released to the world at the same time.
Cover of the journal in which this research was published. Photo courtesy of the National Astronomical Observatory of the Chinese Academy of Sciences
Data show that within two years of observation, the number of extragalactic neutral hydrogen sources detected by FASHI jumped from 40,000 to 156,000, and the sky area covered soared from 7,600 square degrees to 19,500 square degrees, occupying almost half of the sky. In two years, the "China Sky Eye" has twice refreshed the world record for radio telescope sky surveys. The "China Sky Eye" has also completed a key leap from "world record holder" to "core reference sample".
According to the research team, 13.8 billion years ago, after the Big Bang, the temperature of the universe gradually dropped, and protons and electrons were finally able to combine stably together, forming the first batch of neutral hydrogen atoms. Neutral hydrogen is the oldest and most basic substance in the universe - the "raw material" of stars, the "building material" of galaxies, and the "historical archive" that records the universe's transition from darkness to light.
The 21 centimeter radiation emitted by neutral hydrogen is extremely penetrating and can pass through layers of interstellar dust to transmit the most primitive gas information in the depths of the universe to the earth. The "China Sky Eye" is the "most powerful tool" to capture this "beacon" of the universe.
double breakthrough
In January 2024, the first phase of FASHI data (DR1) reached the top of the world's largest neutral hydrogen galaxy sample at that time with 41,741 detection sources.
Two years later, the FASHI team published the second issue of the data paper on the journal cover, with a warm sunset as the background. It represented the 21-centimeter signal received by the "Chinese Sky Eye" from a distant galaxy as a cosmic messenger traveling hundreds of millions of light years, which meant that the "Chinese Sky Eye" was capturing the oldest beacon in the universe.
The second phase of FASHI data covers an area of about 19,500 square degrees, accounting for nearly half of the entire sky area, and the number of detection sources is as high as 156,411, which is nearly four times that of the first phase of data. International reviewers rated it as "a long-awaited landmark data set in the astronomical community" and said that it "will undoubtedly become a standard reference in the industry", which shows that any similar research in the future will use FASHI as a benchmark.
The research team pointed out that the leap-forward growth of FASHI Phase II data is due to the fact that the FAST telescope has always maintained excellent sensitivity and stability during the five-year observation period from August 2020 to July 2025. The cooperation of its 19-beam receiver and efficient drift scanning strategy has achieved double breakthroughs in the depth and breadth of the survey. The scanning, accumulation and optimization day after day, every square degree of coverage and the detection of every source are the solid footprints of the "Chinese Sky Eye" towards the depths of the universe.
Schematic diagram related to the results of this research. Photo courtesy of the National Astronomical Observatory of the Chinese Academy of Sciences
Relying on the FASHI Phase II sky survey data, the research team has robustly expanded the reliable measurement range of the neutral hydrogen mass function to about 1 million times the mass of the sun for the first time, and locked the statistical accuracy of the neutral hydrogen density in the nearby universe at 0.6%. This is the most accurate measurement of this physical quantity by humans to date, and has set a new accuracy benchmark for the study of gas content in the nearby universe.
Widely used
The research team stated that the gas "ID cards" of these 156,000 galaxies newly released by "China Sky Eye" are expected to play a role in multiple frontier fields of astronomy.
In the field of galaxy evolution, it provides direct observational constraints for studying gas accretion, star formation feedback and galaxy evolution. Humans can completely track the entire process of a galaxy's "breathing".
In the field of cosmology, the accurate neutral hydrogen mass function provides key data for understanding the distribution of baryonic matter in dark matter haloes and testing models of galaxy formation and evolution - gradually revealing the "invisible skeleton" of the universe.
In the field of large-scale structures, the neutral hydrogen cosmic network described by FASHI will complement optical surveys and reveal the distribution patterns of gases in fiber-like structures and cavities. Even in the emptiest places in the universe, the secrets of matter are also hidden.
In the field of gravitational wave and transient source research, the FASHI source table helps to accurately locate the host galaxy of gravitational wave events. Every time a gravitational wave reverberates, its home can be found.
In terms of forward-looking research on the Square Kilometer Array Radio Telescope (SKA) of the "World's Giant Eye", FASHI provides important sample design and system comparison benchmarks for the next generation of radio interference array surveys. The global release of the second phase of FASHI data also lays a key data foundation for the study of galaxy evolution in the SKA era.
