Shedding new light on star formation

Australian astronomers have developed a new model to study the birth of stars, spanning cosmic time from the beginning of the Universe until today.

The algorithm, created by researchers at the University of Western Australia’s node of the International Centre for Radio Astronomy Research (ICRAR) along with an international team, succeeded in accurately reconstructing star formation in nearly 7000 nearby galaxies, matching telescope observations for the first time.

“It’s absolutely mind-blowing stuff,” says ICRAR’s Aaron Robotham, who designed the code.

Astronomers are constantly trying to improve our understanding of how and when stars form, as this in turn informs our knowledge of how galaxies evolved and how the Universe is assembled.

Most stars burst to life in the first few billion years after the Big Bang. Studying them involves peering across the cosmos to observe far-distant galaxies billions of light-years away – and therefore billions of years back in time. This is tricky, requiring the use of powerful telescopes.

Instead, astronomers can “cheat” by turning their gaze to stellar nurseries in younger, nearby galaxies and using these as a template to create models of earlier star formation events.

However, when such models are checked for accuracy against observations from the distant Universe, the two don’t match up.

This indicates that something is missing, according to lead researcher Sabine Bellstedt. “That missing ingredient, it turns out, is the gradual build-up of heavy metals within galaxies.”

For the sake of simplicity, previous models assumed that the amount of heavy metals in a galaxy – known as its metallicity – remains constant. But this is not the case.

“Stars can be thought of as enormous nuclear-powered processing plants,” Bellstedt says. “They take lighter elements like hydrogen and helium, and, over billions of years, produce the heavier elements of the periodic table that we find scattered throughout the Universe today.”

The new algorithm, called ProSpect, accounts for these increases in heavy metals such as oxygen, silicon and iron.

As described in a paper in Monthly Notices of the Royal Astronomical Society, ProSpect was then able to accurately model the energy and wavelengths of light coming from thousands of close galaxies. This was confirmed by direct observations from the Galaxy and Mass Assembly (GAMA), project drawing on data from ground- and space-based survey telescopes around the world.

“This is the first time we’ve been able to constrain how the heavier elements in galaxies change over time based on our analysis of these 7000 nearby galaxies,” Robotham says.

“With this tool, we can now dissect nearby galaxies to determine the state of the Universe and the rate at which stars form and mass grows at any stage over the past 13 billion years.”

This will provide key insight into the formation and evolution of early galaxies.

The results also confirm the time period in which the majority of stars were formed.

“Most of the stars in the Universe were born in extremely massive galaxies early on in cosmic history – around three to four billion years after the Big Bang,” Bellstedt says.

“Today, the Universe is almost 14 billion years old, and most new stars are being formed in much smaller galaxies.”

Next, Bellstedt, Robotham and colleagues will expand their sample of galaxies in an effort to understand when and why galaxies stop forming new stars and die.