Those Impossibly Massive Early Galaxies Might Just Be Surprisingly Bright

On July 12th, 2022, in an event live-streamed from the NASA Goddard Spaceflight Center, the James Webb Space Telescope’s (JWST) first images were released! Among them was the most detailed image of SMACS 0723, showing galaxy clusters and the gravitational lenses they produced. These lenses allowed astronomers to see deeper into the cosmos and spot galaxies as they appeared less than one billion years after the Big Bang (ca. 13 billion years ago). Upon further examination, however, they noticed something rather surprising about these early galaxies: they were much larger than expected!

According to the standard model of cosmology, the earliest galaxies in the Universe did not have enough time to become as bright, massive, and mature as they appeared. This raised many questions about our cosmological models and whether or not the Universe was older than previously thought. According to new simulations by a Northwestern University-led team of astrophysicists, these galaxies may not be so massive after all. According to their findings, they appear larger due to irregular and very bright bursts of star formation.

The research was led by Guochao (Jason) Sun, a Postdoctoral Fellow with the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the Department of Physics and Astronomy at Northwestern University. He was joined by researchers from the Flatiron Institute’s Center for Computational Astrophysics, the Theoretical AstroPhysics Including Relativity and Cosmology (TAPIR) Center at Caltech, the Kavli Institute for Astrophysics and Space Research at MIT, and the University of California, Davis. The paper that describes their research appeared on Tuesday, October 3rd, in The Astrophysical Journal Letters.

Claude André Faucher-Giguère, an associate professor of physics and astronomy at Northwestern, is also the leader of the Faucher-Giguère galaxy formation group and one of the study’s lead authors. As he explained in a Northwestern Now press release, the appearance of these galaxies so soon after the Big Bang was a big shock to astronomers:

“The discovery of these galaxies was a big surprise because they were substantially brighter than anticipated. 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 galaxies identified in the SMACS 0723 deep fields existed during Cosmic Dawn. This period began roughly 100 million to 1 billion years after the Big Bang when the first stars and galaxies formed. These galaxies caused the vast clouds of neutral hydrogen that permeated the Universe to become ionized (aka. the Epoch of Reionization). This led to the Universe becoming transparent to light and visible to modern-day instruments, effectively ending the so-called “Cosmic Dark Ages,” a period previously inaccessible to astronomers.

By viewing these galaxies with the JWST’s advanced Mid-Infrared Instrument (MIRI), astronomers hope to see precisely how the largest structures in the Universe evolved over time. This is expected to provide invaluable insight into the physics governing the cosmos and resolving the biggest cosmological mysteries (i.e., the Hubble Tension, Dark Matter, Dark Energy, etc.). Very little was known about Cosmic Dawn before these observations, and what astronomers saw defied expectation and led to some attempts to provide possible explanations.

In their new study, Sun and his team performed advanced computer simulations to model how the earliest galaxies formed right after the Big Bang. The simulations were part of the Feedback of Relativistic Environments (FIRE) project co-founded by Prof. Faucher and astrophysicists from the California Institute of Technology (Caltech), Princeton University, and the University of California at San Diego. The FIRE simulations combine astrophysical theory and advanced algorithms to model galaxy formation. The resulting models allow researchers to probe how galaxies form, grow, and change shape by accounting for energy, mass, momentum, and chemical elements returned from stars. As Sun indicated:

“The key is to reproduce a sufficient amount of light in a system within a short amount of time. 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.”

When the team ran their simulations, they discovered that stars formed in bursts (aka. “bursty star formation”), which occurs when stars form in an alternating pattern. This phenomenon is rare in massive galaxies like the Milky Way today, where stars form at a steady rate and gradually increase over time. In contrast, the simulations showed that in early galaxies, many stars formed at once, followed by millions of years where very few new stars were born before another burst would occur. With this pattern accounted for, the simulations produced the same abundance of bright galaxies observed by the JWST. Said Faucher-Giguère:

“Bursty star formation is especially common in low-mass galaxies. 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.”

These findings highlight a major difference between galaxies as they existed in the early Universe versus today. Modern galaxies are largely filled by Population II stars, which are older, relatively sparse in terms of elements heavier than helium (aka. “metal-poor”), and are commonly found closer to the center of galaxies. Then there are the metal-rich Population I stars that formed more recently and are commonly found in the spiral arms of galaxies. In contrast, the earliest galaxies were populated by Population III stars, which had no heavy elements and were massive and bright.

Because massive stars burn faster, they are shorter-lived, lasting for hundreds of millions of years instead of billions (or trillions, in the case of some red dwarfs) before exhausting their hydrogen fuel and collapsing. Since most of the light coming from a galaxy is emitted by its most massive stars, the brightness of these early galaxies is more directly related to how many stars formed in recent eons than the mass of the galaxy as a whole.

“The JWST brought us a lot of knowledge about cosmic dawn,” added Sun. “Prior to JWST, most of our knowledge about the early Universe was speculation based on data from very few sources. 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.”

Research conducted by other astrophysicists has also explored the possibility that star formation may be responsible for the perceived size of the galaxies observed at Cosmic Dawn. However, the Northwestern-led team is the first to use detailed computer simulations to demonstrate it. Furthermore, their models explained Webb’s observations without the need for additional considerations or conditions that go beyond the standard model of cosmology.

Further Reading: Northwestern University