One of the surprising discoveries of the Webb telescope involves an early population of compact red galaxies with redshifts greater than 7, a time when the Universe was 20 times younger than it is today. The galaxies are redder than expected from their cosmological redshift, indicating additional reddening from a dust layer.
Some of these galaxies contain as much mass in evolved stars as our own Milky Way galaxy. However, they are a hundred times smaller in radius, on the order of several hundred light years. These compact galaxies exhibit an increase by a factor of one million in the number of stars per unit volume compared to the Milky Way.
If we inhabited such a galaxy, the Oort cloud of the solar system would have been stripped to a percentage of its present size by the gravitational wave of passing stars. These small red rubies in the sky are commonly called ‘little red dots’.
The stellar mass required to illuminate these red galaxies requires, in the context of the expected abundances of galaxies in the standard cosmological model, that they convert almost all of their gas into stars rapidly during the hundreds of millions of years available to them after Big Bang. Such 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 the spectroscopic detection of broad emission lines, which represent outflows of gas at a speed of up to one percent of the speed of light, thousands of kilometers per second, as expected if the flow of originates from the vicinity of a black hole.
However, no X-rays have yet been detected from these galaxies, as is commonly observed from quasars. The required black hole mass is above expectations based on the correlation between the mass of stars and black holes in today’s universe. What could be the possible origin of these small red galaxies, which may 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. At the time, in 1993, the earliest quasars were observed from the ‘young’ universe at a third of its current age.
My idea for seeding quasars in the baby universe stemmed from the realization that the size of galaxies is dictated by their rotation. The smaller the spin, the more compact their final disc, where the cold gas is held against gravity by the spin. The amount of rotation derives from the tides that galaxies experience as their matter returns from the cosmic expansion and begins to collapse into a gravitationally bound system. Since different galaxies are born in different environments, their spin rate would be different, reflecting changes in the external tide.
These changes result in a probability distribution of galactic rotations that Daniel and I calculated in a paper from 1995. We showed that this rotation 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 less angular momentum than a typical galaxy. Gas from this compact disc would flow more efficiently toward the sink of a central black hole, seeding a quasar.
In addition, because of its small size, the gaseous disk in a low-spin galaxy would form stars more quickly. Therefore, we suggested that low-spin galaxies may be progenitors of quasar black holes.
When my clever postdoc, Fabio Pacucci, alerted me to the strange properties of the tiny red dots discovered by the Webb telescope, I was immediately reminded of my letters with Daniel. Fabio and I plan to explore the connection between small red dots and low-spin galaxies 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 a long scientific memory must continue to connect the dots, literally speaking in this context of ‘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 have been forgotten until now.
Avi Loeb is 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 astronomy department at Harvard University (2011 – 2020). He is a former member of the President’s Council of Advisors on Science and Technology and former chairman of the Board of Physics and Astronomy of the National Academies. He is the best-selling author of Extraterrestrial: The First Sign of Intelligent Life Beyond Earth and a co-author of the textbook Life in the Cosmos, both due out in 2021. His new book, titled Interstellar , was published in August 2023.