One of the Webb Telescope’s surprising discoveries involves an early population of compact red galaxies with a redshift above 7, a time when the universe was 20 times younger than it is today. The galaxies are redder than expected from their cosmological redshift, suggesting additional reddening by the dust layer.
Some of these galaxies contain as much mass in evolved stars as our own Milky Way galaxy. Nevertheless, they have a hundred times smaller radius, on the order of several hundred light years. These compact galaxies show an increase of millions of stars per unit volume relative to the Milky Way.
If we resided in such a galaxy, the solar system’s Oort cloud would shrink to a percentage of its current size due to the gravitational influx of passing stars. These tiny red rubies in the sky are commonly called “Little Red Dots”.
The stellar mass required to light these red galaxies requires, in the context of the expected abundance of galaxies in the Standard Cosmological Model, that they rapidly converted almost all of their gas into stars during the hundreds of millions of years available to them after the Big Bang. Such a complete conversion is unlikely, suggesting that a significant fraction of their light is contributed by the central supermassive black hole.
The existence of a black hole in small red dots is supported by the spectroscopic detection of broad emission lines, which represent gas outflows at up to a percent of the speed of light, thousands of kilometers per second, as expected if the outflow comes from the immediate vicinity of a black hole.
However, no X-ray emission, as is typically observed from quasars, has yet been detected from these galaxies. The required black hole mass is more than expected based on the correlation between the masses of stars and black holes in the current universe. What could be the possible origin of these small red galaxies that could be pregnant with supermassive black holes in their bellies?
The day I arrived at Harvard University thirty years ago, a brilliant young student from the physics department named Daniel Eisenstein appeared in my office and asked if I would accept him as a graduate student. I immediately agreed and defined a research project for the two of us to explore. It included a new idea I had about the origin of quasars, the most massive black holes at the centers of galaxies. Back then, in 1993, the oldest quasars from the “young adult” universe, a third of its current age, were observed.
My idea of ​​deploying quasars in the newborn universe came from the realization that the size of galaxies is determined by their rotation. The smaller the spin, the more compact their final disc, where cold gas is held against gravity by the spin. The magnitude of the spin is derived from the inflow that galaxies experience as their mass spins out of the cosmic expansion and begins to collapse into a gravitationally bound system. Since different galaxies are born in different environments, their level of rotation would be different, reflecting variations in the outward tide.
These variations lead to the probability distribution of galactic spins that Daniel and I calculated in a 1995 paper. We showed that this spin distribution is almost independent of galaxy mass or formation time. In a follow-up paper, we argued that a low-spin galaxy from the tail of the spin distribution would naturally host a compact disc of gas with a smaller angular momentum than a typical galaxy. Gas from this compact disc could flow more efficiently toward the sinkhole of the central black hole, seeding the quasar.
Additionally, because of its small size, a gaseous disk in a low-spin galaxy would form stars more quickly. Therefore, we proposed that the progenitors of quasar black holes could be low-spin galaxies.
When my bright postdoc Fabio Pacucci pointed out to me the mysterious properties of the tiny red dots discovered by the Webb Telescope, I immediately remembered my papers with Daniel. Fabio and I plan to explore the connection between small red dots and low-spin galaxies more thoroughly 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 must continue to connect the dots, literally talking about “little red dots” in this context.
In the grand scheme of academia, most works are forgotten. But the most rewarding aspect of advancing scientific knowledge is that data from nature eventually leads us to the truth, even if the basic ideas were proposed decades ago and are now forgotten.
Avi Loeb is the head of the Galileo project, founding director 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 Department of Astronomy 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 Council on Physics and Astronomy of the National Academies. He is the best-selling author of the book “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and co-author of the textbook “Life in the Cosmos”, both released in 2021. His new book, “Interstellar”, is released in 2021. August 2023.