Stars bursting into life may be why the earliest galaxies are so bright

Computer simulations suggest that bright bursts by newly forming stars are the solution to the so-called “impossible early galaxy problem.”

Early galaxies are an issue

Among the James Webb Space Telescope’s (JWST) many scientific observations are the images of the earliest known galaxies in the universe. JWST has repeatedly broken its own records since coming online in July 2022, giving glimpses of highly evolved galaxies only 325 million years after the Big Bang.

Galaxy brightness is normally determined by mass. The more massive a galaxy, the brighter it appears through our telescopes. And more massive galaxies must hold more stars, suggesting a highly advanced stage of galactic evolution where billions of stars have had time to form.

But how could such mature galaxies form in such a short amount of time in the early days of the cosmos? Astronomers liken it to a child growing into an adult within just a couple of years.

So puzzling is this early galaxy problem that it has led some to suggest a radical rethink – that the universe is two times older than the 13.8-billion-year estimate accepted by most cosmologists.

That theory has been hotly contested, and most astrophysicists and astronomers are looking for other means to understand the early galaxy problem.

A new star theory

A team led by astrophysicists at Northwestern University in the US has used computer simulations to come up with a different answer: that these early galaxies aren’t so massive after all.

Their findings, published in the Astrophysical Journal Letters, point to bright, irregular bursts of star formation leading to less massive galaxies being able to glow just as bright.

“The discovery of these galaxies was a big surprise because they were substantially brighter than anticipated,” says senior author Claude-André Faucher-Giguère. “Typically, a galaxy is bright because it’s big. But because these galaxies formed at cosmic dawn, not enough time has passed since the Big Bang. How could these massive galaxies assemble so quickly? Our simulations show that galaxies have no problem forming this brightness by cosmic dawn.”

“The key is to reproduce a sufficient amount of light in a system within a short amount of time,” study lead Guochao Sun adds. “That can happen either because the system is really massive or because it has the ability to produce a lot of light quickly. In the latter case, a system doesn’t need to be that massive. If star formation happens in bursts, it will emit flashes of light. That is why we see several very bright galaxies.”

The power of JWST

Astronomers knew very little about the “cosmic dawn” – the period between 100 million and 1 billion years after the Big Bang – before the JWST launched into space.

“Prior to JWST, most of our knowledge about the early universe was speculation based on data from very few sources,” Sun says. “With the huge increase in observing power, we can see physical details about the galaxies and use that solid observational evidence to study the physics to understand what’s happening.”

Simulating starburst

The astrophysicists simulated the formation of early galaxies as part of Northwestern’s FIRE (Feedback In Realistic Environments) project. These computer simulations combine astrophysical theory with advanced algorithms.

They were able to model galaxy formation, growth and development and found that stars form in “bursts” – a process given the creative name “bursty star formation.”

In large galaxies like our Milky Way, stars form at a constant rate.

“Bursty star formation is especially common in low-mass galaxies,” Faucher-Giguère explains. “The details of why this happens are still the subject of ongoing research. But what we think happens is that a burst of stars form, then a few million years later, those stars explode as supernovae. The gas gets kicked out and then falls back in to form new stars, driving the cycle of star formation. But when galaxies get massive enough, they have much stronger gravity. When supernovae explode, they are not strong enough to eject gas from the system. The gravity holds the galaxy together and brings it into a steady state.”

“Most of the light in a galaxy comes from the most massive stars. Because more massive stars burn at a higher speed, they are shorter lived. They rapidly use up their fuel in nuclear reactions. So, the brightness of a galaxy is more directly related to how many stars it has formed in the last few million years than the mass of the galaxy as a whole.”

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