The number of massive stars in starburst galaxies baffles astronomers

Galaxies in the early universe as well as near the star formation regions challenge current ideas about the evolution of galaxies.

Starburst galaxies in the early universe and star formation regions in nearby galaxies have more in common than previously thought; an international team of astronomers has now found that both contain a much higher proportion of massive stars than calmer galaxies.

At first glance, this insight may seem trivial – yet it challenges current ideas about the evolution of galaxies. This new research is changing our understanding of the cosmic star formation history and the formation of chemical elements.

A team of astronomers headed by Zhi-Yu Zhang from the University of Edinburgh investigated the proportion of massive stars in four distant gas-rich starburst galaxies using the Atacama Large Millimeter / Submillimeter Array (ALMA). We see these galaxies today as they were at a time when the universe was significantly younger. It is therefore unlikely that the young galaxies have already undergone several previous episodes of star formation that could affect the results.

Zhang and his team developed a new technique — analogous to the radiocarbon method (also known as C-14 dating) — to measure the abundance of different types of carbon monoxide in the four very distant, dust-enveloped starburst galaxies. They observed the ratio of two types of carbon monoxide with different isotopes.

“Carbon and oxygen isotopes have a different origin,” Zhang explains. “18O is increasingly produced in massive stars and 13C more in stars with low to medium mass.” Thanks to the new technology, the team was able to see through the dust in these galaxies and for the first time judge the masses of their stars.

The mass of a star is the most important factor in its development. Massive stars shine bright and have a short life, while less massive stars like the sun shine billions of years, but less brightly. The knowledge of the number ratios of stars of different masses, which are formed in galaxies, thus underpins the astronomers’ understanding of the formation and evolution of galaxies in the history of the universe.

Consequently, it gives us crucial insights into the chemical elements available to form new stars and planets and, ultimately, the number of black holes that could combine into the supermassive black holes we see in the centers of many galaxies.

Co-author Donatella Romano of the INAF-Osservatorio di Astrofisica and Scienza dello Spazio di Bologna explains what the team has found: “The ratio of 18O to 13C was about 10 times higher in these starburst galaxies in the early Universe than in galaxies like the Milky Way, which means that there is a much higher proportion of massive stars in these starburst galaxies. ”

The ALMA find is confirmed by another discovery in the local universe: a team led by Fabian Schneider of the University of Oxford in the UK used ESO’s Very Large Telescope to take 800 spectroscopic measurements in the huge star-forming region 30 Doradus in the Large Magellanic Cloud to study the overall distribution of star ages and initial masses.

“We found around 30 percent more stars with more than 30 times the mass of the sun and about 70 percent more than 60 solar masses expected, and our results challenge and even lower the predicted limit of 150 solar masses for the maximum birth mass of stars. Stars may have birth masses of up to 300 solar masses,” explains Schneider.

Rob Ivison, co-author of the new ALMA article, concludes, “Our findings lead us to question our understanding of cosmic history, and astronomers who develop models of the universe must now return to the basic concept and add even more detail.

Shakes Gilles

Editor of The Talking Democrat. He enjoys bike riding, kayaking and playing soccer. On a slow weekend, you'll find him with a book by the lake.