During the early universe, the formation of galaxies and stars was the pinnacle of all time.
Only a few hundred million years after the Big Bang, almost half of the stars that ever existed have already formed.
The rate of star formation has declined tenfold since then, but there is a missing piece in history about what fuels the rapid outbreak of stellar births early in the universe and why growth has slowed since then.
The mystery focuses on atomic hydrogen, a key ingredient in star formation and fuel for this early surge of productivity.
To try to solve this, a team of scientists measured atomic hydrogen in more than 7,000 galaxies during a period of the universe in which the galactic formation was at its peak. Their findings are described in detail in a study published Wednesday in the journal Nature. The data reveals new clues about the earliest years of space and how it continues to evolve today.
During the early universe, atomic hydrogen made up most of the gas in younger galaxies.
Stars form when interstellar gas enters galaxies, creating hydrogen atoms that then turn into molecular hydrogen. This directly feeds the birth of the stars.
Hydrogen atoms have been found in nearby galaxies, but atomic gas is more difficult to detect in more distant galaxies. As a result, the exact details of the gas that feeds early star formation have remained a mystery.
To monitor the process in these distant galaxies, the scientists behind the new study used the giant Metrewave radio telescope near Pune, India, to measure neutral atomic hydrogen emissions in a total of 7,653 galaxies.
The galaxies were over 0.4 redshift, which is a unit of measure for the distance of the object from Earth. The redshift depends on how light shifts to shorter or longer wavelengths when objects in space approach or move away from us. The farther away the object is, the more light shifts toward the red end of the spectrum.
The galaxies studied here are located in the early universe, in a region of space and time when galaxy formation was at its peak – about four to seven billion years after the Big Bang.
At that time, galaxies contained about 2.5 times more of this gas than their stellar masses than galaxies today. The results suggest that the huge amount of this gas in these early galaxies may explain the higher rate of star formation during this time in space history.
The study suggests that atomic hydrogen is consumed relatively quickly in the process of star formation. This means that the accumulation of gas would have to be somewhat continuous to cope with the birth rate of these young stars, the study found.
In this sense, it may not be surprising that star formation has slowed, given that hydrogen gas production has not remained high enough to meet the high yield of baby stars.
The study does not conclude whether the gas was most often found in larger galaxies or was distributed among all galaxies equally. So there are still some missing details that scientists will have to fill in with future observations of the distant universe.
The scientists behind this study hope to use the upcoming square-kilometer array, a radio telescope designed specifically to detect hydrogen emissions from distant galaxies, to try to answer these basic questions about the formation of our universe.
Summary: Baryonic processes in the evolution of galaxies involve the fall of gas on galaxies to form neutral atomic hydrogen, which is then converted to a molecular state (H2) and, finally, the transformation of H2 to the stars. Therefore, understanding the evolution of galaxies requires understanding the evolution of stars and neutral atomic and molecular hydrogen. It is known that stars have a cosmic density of stellar formation peaks at redshifts from 1 to 3 and decreases by an order of magnitude over the next 10 billion years.1; the reasons for this decline are unknown. As for the gas, the weakness of the hyperfine transition of HI at 21 cm wavelengths – the main tracker of the HI content in galaxies – means that so far it has not been possible to measure the atomic gas mass of galaxies at redshifts, more high of about 0.4; this is a critical gap in our understanding of galaxy evolution. Here we report a measurement of the mean HI mass of star-forming galaxies at a redshift of about one obtained by stacking2 their individual HI 21-centimeter emission signals. We obtain an average HI mass similar to the average stellar mass of the sample. We also estimate the average star formation rate of the same galaxies on the 1.4 GHz radio continuum and find that HI mass can feed the observed star formation rates for only 1 to 2 billion years in the absence of fresh gas. This suggests that the accumulation of gas on galaxies at redshifts of less than one may have been insufficient to maintain high levels of star formation in star-forming galaxies. This is probably the reason for the decrease in the density of cosmic star formation in redshifts below one.