The Rules Of Star Formation Have Changed Over The Course Of The Universe

Galaxy 434625, marked with a square, as seen by Hubble how it looked 11 billion years ago. Star-formation was apparently different then. HST/NASA /ESA

The rate at which new stars are formed is one of the most important things astronomers can find out about distant galaxies, and it is well correlated with many other properties, like their mass. So, discovering that one of these correlations doesn’t work in the early universe is a big deal.

An international team led by scientists at ETH Zurich in Switzerland looked at galaxies 2 billion years after the Big Bang and discovered that their metallic content – elements other than hydrogen and helium – was about 20 percent of the average metallicity of galaxies nowadays. But surprisingly, there was no difference between strongly star-forming galaxies and objects with a lower rate of star-formation.

In galaxies today, the metallicity depends on the star-formation rate; galaxies that are strongly star-forming have fewer metals than galaxies with lower star-formation.

"There is no connection between the rate at which stars were born in an earlier galaxy and the amount of heavy elements it contains," said Professor Marcella Carollo, senior author of the study, in a statement.

"The findings on metallicity provide us with important clues into the physical balance of galaxies over epochs and the formation of stars and planets.”

The discrepancy highlighted by the team suggests that the conditions that governed star formation in the early universe must be different from the condition galaxies experience now. An important factor that could have played a role then is the accretion rate of gas from the intergalactic medium, which might have a more important role of star formation in the mass assembly of young galaxies.

A study like this, which is published in the Astrophysical Journal, tells us that the empirical relationship that we observe in the local universe can’t always extend to the early epochs of the cosmos. But understanding the true link between metallicity, star formation rates, and the mass of galaxies will get us closer to a complete picture of galaxy evolution.

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