Observing history in the making

For over two years, astronomers have been intrigued by a perplexing finding from the primordial universe — numerous objects referred to as “small red dots” located so distantly that the light took billions of years to reach the eyes of scientists.   

Initially recognized by the James Webb Space Telescope, these unusually dense remnants from the cosmic origin have ignited fervent discussions: Are they closely packed galaxies, or do they encompass massive black holes?

Now, two astrophysicists from Harvard have introduced a novel theory: These far-off entities are newly formed galaxies evolving within gradually rotating halos of dark matter — and examining them could unveil crucial new knowledge regarding the universe’s formation.

“Telescopes serve as time-travel instruments,” stated Fabio Pacucci, a Clay Fellow at the Harvard-Smithsonian Center for Astrophysics and principal author of the recent study. “When you observe the moon, you perceive it as it was a second ago, and if you view the sun, it’s as it appeared eight minutes ago. Observing these small red dots means you are looking back billions of years.”

Dark matter halos are thought to have significant roles in galaxy formation and the evolution of the cosmos. (Dark matter remains an enigmatic substance that is imperceptible because it neither absorbs, reflects, nor emits light, though it is believed to make up the majority of matter in the universe.)

“When you observe the moon, you perceive it as it was a second ago, and if you view the sun, it’s as it appeared eight minutes ago. Observing these small red dots means you are looking back billions of years.”

The halos have not been directly witnessed, and their existence is inferred from other observational data, including the movements of stars and gas, and the bending of light.

“A dark matter halo serves as a cradle for galaxy formation,” explained Pacucci. “The larger the dark matter halo, the larger the galaxy at its core.”

The mysterious dots represent some of the most astonishing findings from the strongest telescope ever launched into the cosmos. Launched in 2021, the James Webb Space Telescope (JWST) was intended to investigate the “cosmic dawn” — the period when the initial stars and black holes formed after the Big Bang 13.8 billion years ago.

Orbiting approximately 1 million miles away from Earth, the JWST identified hundreds of unusually red and condensed sources dubbed the “small red dots” (LRDs).

Their unique hue is attributed to various factors, including the presence of dust and the phenomenon known as “redshift” (where light shifts towards the red end of the spectrum as it travels great distances).

LRDs emerged around 600 million years post-Big Bang and subsequently faded away.

These dots are remarkably compact and relatively luminous, implying they either contain gigantic black holes (which emit bright light despite their name) or condense unimaginable numbers of stars within galaxies merely one-fiftieth the size of our Milky Way.

“They’re akin to cosmic fireworks,” remarked Pacucci. “They magically materialize and remain highly visible for about 1 billion years. Then they simply vanish.”

In a study recently published in The Astrophysical Journal Letters, Pacucci and Abraham “Avi” Loeb, Frank B. Baird Jr. Professor of Science, propose the theory of galaxies forming within gradually rotating halos of dark matter to clarify the abundance, compactness, and redshift distribution of LRDs.

Their model suggests that the LRDs discovered so far represent merely the slowest-spinning galaxies — the bottom 1 percent of the distribution.

In essence, the LRDs are not a fundamentally different category of galaxies, but rather a minor group that displays extraordinary characteristics.

Some of their seemingly inexplicable traits may stem from observational biases: Current technologies can only detect compact, low-spin halos because they focus light into bright cores; meanwhile, larger, more diffuse galaxies at greater redshifts remain unseen — despite being more prevalent.

Loeb stated, “If you assume the small red dots are typically in the first percentile of the spin distribution of dark matter halos, then you can account for all their observed properties.”

Dale Kocevski, a leading researcher of LRDs and chair of the physics and astronomy department at Colby College, noted that the theory suggested by Pacucci and Loeb “makes considerable sense.”

“This possibly enhances our foundational understanding of these entities,” he remarked. “Moreover, it provides a physical model that we can evaluate moving forward.”

Pacucci is optimistic that the LRDs will ultimately be recognized as the defining discovery of the JWST. (Like many colleagues, he also speculates that the small red dots do harbor supermassive black holes, though that aspect is not part of the new theory.)

He foresees they will engender new understandings regarding the formation of galaxies and black holes during the cosmic dawn.

“We are currently debating the essence of a fundamentally novel type of galaxy that we have never observed before,” declared Pacucci. “This will deeply alter our perspective on the early evolution of the universe.”