WORLD NEWS
Star formation is at 40% of its level 4.5 billion years ago, while atomic hydrogen is at 74% — not a simple fuel shortage
In a study combining FAST radio telescope and DESI observations, it was determined that over the past 4.5 billion years, the cosmic star-formation rate fell to about 41% of its former level, while raw measurements put neutral atomic hydrogen at about 74%. This means that it is difficult to explain the phenomenon of fewer stars being formed simply by depletion of atomic hydrogen fuel.
It is not a direct image of how much gas is left in individual galaxies. The very weak 21 cm signals from about 2.5 million galaxies were statistically overlaid to measure the average, and the hydrogen reduction can be made smaller by systematic error correction.
What was measured and how?
The researchers combined the 21 cm radio spectrum of neutral atomic hydrogen from FAST with optical spectroscopic data from DESI. Using about 2.5 million galaxies covering about 12,000 square degrees of sky, we estimated the average hydrogen density in the universe by superimposing signals that were too faint to be detected individually. It ranges from redshift 0 to 0.41, or about the last 4.5 billion years.
How big is the difference between the two decline rates?
4.5 billion years ago, the star formation rate density was about 2.46 times that of today. The density of neutral atomic hydrogen during the same period was 1.35 ± 0.10 times the current value in raw data, and was estimated to be 1.12 ± 0.10 times after conservative systematic error correction. The reduction in star formation is much greater than the reduction in atomic hydrogen.
Why isn’t atomic hydrogen the fuel for stars?
Stars are born primarily in cold, dense clouds of molecular hydrogen. Neutral atomic hydrogen is an important reservoir, but it must first be converted to molecular hydrogen and the clouds will cool sufficiently and clump together. Therefore, even if the total amount of atomic hydrogen does not decrease significantly, star formation may decrease if molecular hydrogen conversion or star formation efficiency decreases.
What is still unclear?
Methods of overlapping signals produce precise averages for a very large number of galaxies, but can hide the diversity of individual galaxies. Systematic errors such as sample selection, signals from neighboring galaxies in the beam, and hydrogen mass corrections also remain. The researchers applied several corrections, but the exact size of the decrease will need to be narrowed down with follow-up observations.
What to check next
Molecular hydrogen and atomic hydrogen must be measured together in the same galaxy group, and differences depending on galaxy mass, environment, and shape must be compared. Observations that directly measure the 21 cm signal further back in time are also needed. Rather than confirming a single cause for the decline in star formation, these results set a new standard that rules out simple fuel depletion explanations.