Here's How You Could Get Impossibly Large Galaxies in the Early Universe

One of the most interesting (and confounding) discoveries made by the James Webb Space Telescope (JWST) is the existence of “impossibly large galaxies.” As noted in a previous article, these galaxies existed during the “Cosmic Dawn,” the period that coincided with the end of the “Cosmic Dark Age” (roughly 1 billion years after the Big Bang). This period is believed to hold the answers to many cosmological mysteries, not the least of which is what the earliest galaxies in the Universe looked like. But after Webb obtained images of these primordial galaxies, astronomers noticed something perplexing.

The galaxies were much larger than what the most widely accepted cosmological model predicts! Since then, astronomers and astrophysicists have been racking their brains to explain how these galaxies could have formed. Recently, a team of astrophysicists from The Hebrew University of Jerusalem Jerusalem published a theoretical model that addresses the mystery of these massive galaxies. According to their findings, the prevalence of special conditions in these galaxies (at the time) allowed highly-efficient rates of star formation without interference from other stars.

The research team was led by Professor Avishai Dekel from The Racah Institute of Physics at the Hebrew University of Jerusalem and the UC Santa Cruz Institute for Particle Physics (SCIPP). He was joined by colleagues from the Racah Institute and Tel Aviv University, Dr. Kartick Sarkar, Professor Yuval Birnboim, Dr. Nir Mandelker, and Dr. Zhaozhou Li. Their results were presented in a paper titled “Efficient formation of massive galaxies at cosmic dawn by feedback-free starbursts,” recently published by the Monthly Notices of the Royal Astronomical Society.

According to the Lambda-Cold Dark Matter (LCDM) model, which best explains what we have observed of the cosmos, the first stars and galaxies formed during the “Cosmic Dark Age.” The name refers to how the only sources of photons during this period were from the Cosmic Microwave Background (CMB) and those released by the clouds of neutral hydrogen that shrouded the Universe. Once galaxies began to form, the radiation from their hot and massive stars (1000 times more massive than our Sun) began reionizing the neutral hydrogen.

This period is known as the Epoch of Reionization (ca. 1 billion years after the Big Bang), where the Universe gradually became transparent and visible to modern instruments. Thanks to Webb’s extreme sensitivity to infrared light, astronomers have pushed the boundary of what is visible, spotting an abundance of massive galaxies that existed just half a billion years after the Big Bang. According to the LCDM model, there simply wasn’t enough time since the Big Bang for so many galaxies to have formed and become so massive. As Professor Dekel shared in a press release from The Hebrew University of Jerusalem:

“Already in the first half-billion years, researchers identified galaxies that each contain about ten billion stars like our sun. This discovery surprised researchers who tried to identify plausible explanations for the puzzle, ranging from the possibility that the observational estimate of the number of stars in galaxies is exaggerated, to suggesting the need for critical changes in the standard cosmological model of the Big Bang.”

According to the model put forth by Dekel and his colleagues, the prevalence of special conditions in these galaxies would have allowed for high rates of star formation. These include the high density and low abundance of heavy elements and feedback-free starbursts (FFBs). To break it down, prevailing theories of galaxy formation indicate that the hydrogen permeating the early Universe collapsed into giant spherical clouds of Dark Matter, where it collected together to give birth to the first population of stars (Population III).

These theories further state that the stars were almost entirely composed of hydrogen, which was slowly fused in their interiors to create heavier elements (metals). These elements were distributed throughout early galaxies when Population III stars reached the end of their lifespans and blew off their outer layers in supernovae. As a result, more recent stellar populations (Population II and I) have had higher metal content (aka. “metallicity”). To date, astronomers have observed that galaxies’ star-formation efficiency (SFE) is low, with only about 10% of the gas falling into clouds becoming stars.

This low efficiency results from the remaining gas being heated or blown out of galaxies by stellar winds or shock waves generated by supernovae. In contrast, Dekel and his team theorized that low-metallicity massive stars were subject to a process they call “feedback-free starburst” (FFB). In essence, star-forming clouds in the early Universe had a density that allowed gas clouds to collapse rapidly into stars 1 million years before they would have developed winds and supernovae. This created a “window of opportunity” where the absence of feedback allowed the rest of the gas to form stars.

This high-efficiency star formation explains the abundance of massive galaxies observed by Webb so soon after the Big Bang. As Dekel concluded, the implications their theory has will be the subject of follow-up investigations:

“The publication of this research marks an important step forward in our understanding of the formation of primordial massive galaxies in the Universe and will no doubt spark further research and discovery. The predictions of this model will be tested using the accumulating new observations from the Webb Space Telescope, where it seems that some of these predictions are already confirmed.”

Of particular interest to astronomers are the primordial supermassive black holes (SMBHs) one thousand times as massive as our Sun that existed about 1 billion years after the Big Bang. Astronomers were surprised to observe SMBHs this massive at the center of early galaxies since (once again) it was assumed that they didn’t have enough time to form. Future observations will attempt to find the seeds of these black holes using Webb and observatories like the Laser Interferometer Space Antenna (LISA). Dekel and his colleagues hope they find these seeds among clusters of FFBs that went supernova.

Further Reading: Hebrew University of Jerusalem, MNRAS