New observations show that ancient galaxies were able to recycle the gas from previous stars into new generations of stars. This recycled gas could have been enough to supply all of the material needed for continued star formation. This discovery sheds new light on the evolution of galaxies and stars.
Figure1 : Conceptual image of an ancient massive galaxy recycling the gas to sustain its growth. The background is an image of the Subaru and Keck telescopes. (Credit: Tsinghua University/NAOJ)
Gas provides the material for galaxies to form new stars. When stars die in supernova explosions, they can expel gas back out of the galaxy. So continuing star formation requires a supply of gas falling into the galaxy. But it has been unclear if star formation is driven by a supply of pristine new gas or if galaxies are able to recycle the gas from previous generations of supernovae.
To answer this question an international team of researchers led by Tsinghua University, observed a galaxy which existed 11 billion years ago. Pristine gas will consist mostly of hydrogen, with a little helium. On the other hand, recycled gas will contain heavier elements produced by nuclear fusion in stars. In data from the Keck II telescope and Subaru Telescope, the team detected signs of hydrogen, helium, and carbon extending out to 300,000 light-years around the galaxy. The ratios of the elements are similar to what is seen in the Sun, which is surprising for such an ancient galaxy which would be expected to be closer to the pristine state of new gas.
The team was also able to map the motion of the gas. Comparison with models shows that the heavy element enriched gas is flowing back into the galaxy, delivering about 700 times the mass of the Sun in recycled gas each year. This is much more than is needed to fuel the star formation rate seen in the galaxy, about 81 times the mass of the Sun each year, indicating that gas recycling alone is enough to fuel star formation in the galaxy.
The leader of the research team, Zheng Cai, Associate Professor at Tsinghua University, comments on the role of the Subaru Telescope in this project, "The Subaru Telescope had a very important role in getting the emissions from hydrogen to measure the ratio between hydrogen and the heavier elements."
These results appeared as Shiwu Zhang et al. "Inspiraling streams of enriched gas observed around a massive galaxy 11 billion years ago" in Science on May 4, 2023.
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.
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