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5 mins read 15 Oct 2020

Heavy Metal Helps Scientists Understand Star Formation

Revisiting a simple assumption about the amount of heavy metals in galaxies leads astronomers to a better understanding of when stars were formed.

Artist's impression of some galaxies. Credit: ICRAR

Astronomers studying the history of star formation in galaxies have discovered an ingredient missing from their models – metals. Accounting for the missing metals has helped them to reconstruct when most of the stars in the universe formed, and, for the first time, their results match what has been observed in the universe.

The results were made possible by a new algorithm that models the energy and wavelengths of light from galaxies to figure out how old the stars are. The code – known as ProSpect – was applied to almost seven thousand nearby galaxies from the Galaxy and Mass Assembly (GAMA) survey.

The trick was to include the galaxy’s metallicity – that is, the abundance of elements in a galaxy that are heavier than hydrogen and helium. Previous models had assumed that the metallicity of galaxies remained constant, but as these heavy elements are continually being made inside of stars there is actually a gradual build-up of metals over time.

The ability to reconstruct so many nearby galaxies from the GAMA survey and successfully extrapolate over the whole universe confirms theories about when most of the stars in the universe formed.

Island Universes

Hubble image of Abell 2744. Abell 2744 is a galaxy cluster that is 4-million light-years across and contains the mass of about 4-trillion Suns. Credit: NASA/ESA/G. Bacon (STScI)

Just one hundred years ago, it was thought that all the stars in the universe were contained in the Milky Way. That theory was only dispelled after Edwin Hubble calculated that the Andromeda Nebula, as it was known at the time, was about nine hundred thousand light years away. The fuzzy clouds thought to have been embryonic solar systems were revealed to be other galaxies.

Through a combination of the exquisite data sent back from the telescope that bears Hubble’s name, and advanced computer simulations that include dark matter and dark energy in addition to the normal baryonic matter with which we are all familiar, we know today that there are around two trillion galaxies in just the part of the universe that we can see. If they were all the size of the Milky Way, there would be approximately two hundred sextillion stars (that’s a two with twenty-three zeros after it – an incomprehensibly large number).

Astronomers know that the bulk of the stars in the Milky Way formed at least eight billion years ago, and that there was another episode of rapid star formation about one billion years ago. Indeed, most of the stars in the universe were born relatively early in its history – around three or four billion years after the Big Bang.

The results of this research will allow astronomers to piece together cosmic history with even greater precision.

A Better Model

Artist’s impression of the ProSpect code analysing a galaxy. When increasing galaxy metallicity is taken into account, theoretical models match observed galaxies. Credit: ICRAR

For the last decade or so, studying star formation rates has been done by trying to look at galaxies all the way to the edge of the observable universe, a difficult task requiring the most powerful telescopes. But the new, more accurate, model required only a sample of nearby galaxies to produce a cosmic star formation history that matched observations. Lead researcher Dr. Sabine Bellstedt of The International Centre for Radio Astronomy Research (ICRAR) and the University of Western Australia explained.

“By simply improving how we deal with a single assumption – letting metallicities in galaxies build up over time, rather than forcing them to be constant – we were able to reproduce observed star formation rates using only a relatively small sample of galaxies that are quite close to us!”

The analysis also meant that statements about star formation history in different types of galaxies could be made. For example, while half the stars in the most massive galaxies had formed eleven billion years ago, this milestone was reached much more recently in the least massive galaxies, perhaps only four billion years ago.

Annotated graph showing the history of star formation from the Big Bang to now. Half the mass in massive galaxies had been formed 11-billion years ago. Credit: ICRAR

Dr. Bellstedt expects that the techniques her team have used will also improve measurements of other properties of galaxies in the future. Understanding the chemical composition of galaxies may just be the start.

“The fact that we’re now modelling the metallicity histories of galaxies more accurately means that we can start making estimates of the metal content of galaxies – something we've not been able to do before. Typically measurements of galaxy metallicity are made using spectra, but this is often tricky, and requires careful calibration. Providing an alternative method to measure metallicity will hopefully help us to better understand the chemical enrichment of galaxies in the future.”

The next challenge is to expand the sample of galaxies being studied using this technique, in an effort to understand when, where and why galaxies die and stop forming new stars. And Dr. Bellstedt says that more distant galaxies will definitely be in the mix.

“All we need in order to analyse a galaxy using ProSpect is a measurement of how far away it is and a series of measurements of the brightness of the galaxy in different wavelengths. When galaxies are very distant, it's hard to see their structure in detail, but it’s still relatively straightforward to add up all the light we collect using telescopes. Applying ProSpect to more distant galaxies is something we're working on at the moment, using new data collected at the Anglo Australian Telescope.”

And with potentially trillions of galaxies to study, we’ll be following this research with interest.