BEIJING, May 10 (Xinhua) - What was the universe like 4.2 billion years ago? Based on the latest ultra-deep sky survey data from the 500-meter Aperture Spherical Radio Telescope (FAST) known as "China's Eye of Heaven," Chinese and foreign astronomers have collaborated to discover the most distant neutral hydrogen (HI) galaxy in the universe to date. This finding provides observational evidence for the existence of more massive neutral hydrogen galaxies in the universe 4.2 billion years ago.
The latest observations from "China's Eye of Heaven" have led to a significant astronomical research paper on a sample of the most distant neutral hydrogen galaxies. The study was led by Chinese astronomers in collaboration with peers from Australia, the United States, Russia, and others. The paper was published online on May 10 in the international academic journal "Astrophysical Journal Letters."
The research presented here shows: the black solid line represents the neutral hydrogen spectral line observed by FAST, the red solid line is the fitted line, the blue solid line is the optical spectrum, and the red dashed line represents the corresponding optical velocity in the galaxy. Image credit: National Astronomical Observatories, Chinese Academy of Sciences.
Dr. Xi Hongwei, the first author of the paper and a researcher at the National Astronomical Observatories, Chinese Academy of Sciences, explained that hydrogen is the earliest element formed in the universe, typically existing in the form of neutral hydrogen. Neutral hydrogen is also a crucial component of gas circulation in galaxies, providing fuel for star formation and serving as an important probe for studying the dynamical structure of galaxies. The rotation curves of neutral hydrogen provide observational evidence for the existence of dark matter, laying a solid foundation for the establishment of the standard cosmological model.
The FAST Ultra-Deep Survey (FUDS) project, led by Dr. Peng Bo, a co-corresponding author of the paper and a researcher at the National Astronomical Observatories, Chinese Academy of Sciences, fully utilizes the high sensitivity of FAST and the wide field advantage of the 19-beam receiver to conduct deep "blind searches" for distant and faint neutral hydrogen galaxies.
Dr. Peng Bo highlighted that in this study, the FAST Ultra-Deep Survey discovered six neutral hydrogen galaxies at a distance of approximately 5 billion light-years, making it the farthest sample of neutral hydrogen galaxies directly detected by the 21-centimeter emission line to date. One of these galaxies possesses the largest known neutral hydrogen mass. By estimating the density of massive neutral hydrogen galaxies in the sample, the FAST Ultra-Deep Survey team found that in the universe 4.2 billion years ago, there were more massive neutral hydrogen galaxies.
The research presents: overlaying the neutral hydrogen column density detected by FAST on the tricolor image from the DESI Legacy Survey, with red circles indicating the size of the FAST beam at that redshift; the bottom right corner shows an enlarged optical counterpart. Credit: National Astronomical Observatories, Chinese Academy of Sciences.
Subsequently, the collaborative team further identified the optical counterparts of these six distant neutral hydrogen galaxies through spectroscopic data from the Dark Energy Spectroscopic Instrument (DESI) and multi-band observations using the 5-meter Hale, 10-meter Keck, and 6-meter BTA telescopes. This indicates that FAST can offer a new way to detect distant neutral hydrogen galaxies in distant galaxy surveys.
The research presented here illustrates the relationship between redshift, lookback time, and distance. Different data points show the lookback time and comoving distance of the highest redshift galaxies discovered by various sky surveys. Image credit: National Astronomical Observatory, Chinese Academy of Sciences.
Previous studies have shown that the Parkes 64-meter radio telescope in Australia searched for galaxies within 600 million light-years in the southern sky, detecting about 5000 neutral hydrogen galaxies; while the Arecibo 305-meter radio telescope in the United States searched for galaxies within 800 million light-years in the northern sky, detecting around 30,000 galaxies. These galaxy samples from the nearby universe provide the first insights into the mass distribution of neutral hydrogen galaxies, enabling astronomers to systematically study the relationship between neutral hydrogen gas, stellar mass, and star formation rates.
