One of the surprising discoveries of the Webb telescope is an early population of compact red galaxies at redshifts above 7, a time when the universe was 20 times younger than it is today. The galaxies are redder than expected based on their cosmological redshift, suggesting additional reddening by a layer of dust.
Some of these galaxies contain as much mass in evolved stars as our own Milky Way. Nevertheless, they are a hundred times smaller in radius, on the order of a few hundred light years. These compact galaxies show an increase of a factor of a million in the number of stars per unit volume compared to the Milky Way.
If we lived in such a galaxy, the solar system’s Oort cloud would have been reduced to a percentage of its current size by the gravity of passing stars. These little red rubies in the sky are often called “little red dots.”
The stellar mass required to light up these red galaxies, in the context of the expected abundance of galaxies in the standard cosmological model, requires that they rapidly convert almost all of their gas into stars over the hundreds of millions of years they have after the Big Bang. Such a complete conversion is unlikely, suggesting that a significant fraction of their light is contributed by a central supermassive black hole.
The existence of a black hole in the small red dots is supported by spectroscopic detection of broad emission lines. These represent streams of gas traveling at up to one percent of the speed of light, thousands of kilometers per second, as would be expected if the stream of gas originated in the immediate vicinity of a black hole.
However, no X-rays have been detected from these galaxies so far, as is typically observed for quasars. The required mass of the black hole is above expectations based on the correlation between the masses of stars and black holes in the present-day universe. What could be the possible origin of these small red galaxies, which could be pregnant with supermassive black holes in their bellies?
On the day I arrived at Harvard University thirty years ago, a brilliant young physics student named Daniel Eisenstein came to my office and asked if I would take him on as a doctoral student. I immediately agreed and drafted a research project that we could investigate together. It was about a new idea I had about the origin of quasars, the most massive black holes at the centers of galaxies. At that time, in 1993, the earliest quasars had been observed from the “young adult” universe at one-third its current age.
My idea to seed quasars in the early universe came from the realization that the size of galaxies is determined by their spin. The smaller the spin, the more compact their eventual disk, where cold gas is held against gravity by rotation. The amount of rotation is derived from the tide that galaxies experience as their matter reverses from cosmic expansion and begins to collapse into a gravitationally bound system. Because different galaxies are born in different environments, their spin levels would differ, reflecting variations in the external tide.
These variations result in a probability distribution of galactic spins that Daniel and I calculated in a 1995 paper. We showed that this spin distribution is nearly independent of the galaxy mass or its formation time. In a follow-up paper, we argued that a galaxy with a low spin at the tail of the spin distribution would naturally host a compact disk of gas with less angular momentum than a typical galaxy. The gas from this compact disk would flow away more efficiently to the sink of a central black hole, forming a quasar.
Furthermore, the gas disk in a low-spin galaxy, due to its small size, would form stars more rapidly. Therefore, we have suggested that low-spin galaxies could be the progenitors of quasar black holes.
When my brilliant postdoc, Fabio Pacucci, alerted me to the mysterious properties of the little red dots discovered by the Webb telescope, I immediately thought of my papers with Daniel. Fabio and I plan to investigate the connection between the little red dots and low-spin galaxies in more depth in a future paper.
The scientific literature is vast, and I don’t expect my colleagues to remember a paper written thirty years ago. Those of us with long scientific memories have to keep connecting the dots, literally speaking in this context of the ‘little red dots.’
In the grand scheme of academia, most papers are forgotten. But the most rewarding aspect of pursuing scientific knowledge is that data from nature eventually leads us to the truth, even if the underlying ideas were proposed decades ago and are now forgotten.
Avi Loeb is the director of the Galileo Project, founder of Harvard University’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and former chair of the astronomy department at Harvard University (2011-2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chairman of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of Extraterrestrial: The First Sign of Intelligent Life Beyond Earth and co-author of the textbook Life in the Cosmos, both published in 2021. His new book, Interstellar, will be published in August 2023.