Here’s how you might get incredibly large galaxies in the early universe

By | June 1, 2023

One of the most interesting (and puzzling) discoveries made by the James Webb Space Telescope (JWST) is the existence of “incredibly 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” (about 1 billion years after the Big Bang). This period is believed to hold the answers to many cosmological mysteries, not least of which is the appearance of the first galaxies in the Universe. But after Webb took pictures of these early galaxies, astronomers noticed something puzzling.

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

The research team was led by Professor Avishai Dekel of 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 Racah Institute and Tel Aviv University Dr Kartick Sarkar, Professor Yuval Birnboim, Dr Nir Mandelker and Dr Zhaozhou Li. Their results have been presented in a paper entitled “Efficient training of massive galaxies at cosmic dawn by feedback-free starbursts”, recently published by the Monthly Notices of the Royal Astronomical Society.

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This image shows one of the farthest known galaxies, called GN-108036, dating to 750 million years after the Big Bang that created our universe. Credit: NASA/ESA/JPL-Caltech/STScI/University of Tokyo

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 came from the Cosmic Microwave Background (CMB) and from those released by the neutral hydrogen clouds that enveloped the Universe. Once the galaxies started forming, radiation from their hot, massive stars (1000 times more massive than our Sun) started reionizing the neutral hydrogen.

This period is known as the Epoch of Reionization (about 1 billion years after the Big Bang), in which the Universe gradually became transparent and visible to modern instruments. Thanks to Webb’s extreme sensitivity to infrared light, astronomers have pushed the edge 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 hasn’t been enough time since the Big Bang for so many galaxies to form and become so massive. As Professor Dekel shared in a press release from the Hebrew University of Jerusalem:

“As early as the first half-billion years, researchers have identified galaxies that each contain about ten billion stars like our sun. This discovery has surprised researchers who have tried to identify plausible explanations for the conundrum, 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 proposed 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 non-feedback starbursts (FFBs). To break it down, the prevailing theories of galaxy formation indicate that the hydrogen permeating the early Universe collapsed into gigantic spherical clouds of Dark Matter, where it gathered together to form the first population of stars (Population III).

Artist’s illustration of Population III stars, the first stars in the Universe.
Credit: Wikimedia Commons

These theories also state that the stars were almost entirely composed of hydrogen, which was slowly melted within them to create heavier elements (metals). These elements were distributed throughout the early galaxies as Population III stars reached the end of their lives and exploded from their outer layers in supernovae. Consequently, more recent stellar populations (Population II and I) have had higher metal content (aka “metallicity”). To date, astronomers have observed that the star formation efficiency (SFE) of galaxies is low, with only about 10% of the gas falling into the clouds becoming stars.

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

This highly efficient star formation explains the abundance of massive galaxies observed from Webb so right after the Big Bang. As Dekel concluded, the implications of their theory will be the subject of further investigation:

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

Artist’s impression of GNz7q, an early ancestor of modern supermassive black holes (SMBHs). Credit: NASA/ESA/STScI.

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

Further reading: Hebrew University of Jerusalem, MNRAS

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