One of the surprising discoveries of the Webb telescope involves an early population of compact red galaxies at redshifts above 7, when the Universe was 20 times younger than it is today. The galaxies are redder than expected from their cosmological redshift, indicating additional reddening by a veil of dust. Some of them contain as much mass in evolved stars as our own Milky Way galaxy. Nevertheless, they are a hundred times smaller in radius, of order a few hundred light years. These compact galaxies manifest an increase by a factor of a million in the number of stars per unit volume relative to the Milky Way. If we were to reside in such a galaxy, the Oort cloud of the solar system would have been stripped to a percent of its current size by the gravitational tide of passing stars. These tiny red rubies in the sky are commonly dubbed `little red dots.’
Medieval Europeans wore ruby gemstones to symbolize health, loyalty, prosperity, wisdom and success in love. If it was feasible, I would have threaded these red rubies in the sky on three necklaces and given them to my wife and two daughters to wear.
The mass in stars needed to light up these red galaxies requires, in the context of the expected abundance of galaxies in the standard cosmological model, that they convert nearly all their gas into stars rapidly, over the hundreds of millions of years available to them after the Big Bang. Such full 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 little red dots is supported by spectroscopic detection of broad emission lines, representing outflows of gas with a speed of up to a percent of the speed of light, thousands of kilometers per second, as expected if the outflow originated from the immediate vicinity of a black hole. Nevertheless, no X-rays were detected as of yet from these galaxies, as typically observed from quasars. The required mass of the black hole is above expectations based on the correlation between the mass in stars and black holes in the present-day universe. What could be the possible origin of these little red galaxies, which might be pregnant with overmassive black holes in their bellies?
On the day that I arrived at Harvard University, thirty years ago, a brilliant young student from the Physics department named Daniel Eisenstein, showed up in my office and asked whether I would accept him as a graduate student. I promptly agreed and defined a research project for the two of us to explore. It involved a new idea that I had about the origin of quasars, the most massive black holes at the centers of galaxies. Back then in 1993, the earliest quasars were observed from the `young adult’ Universe at a third of its current age.
My idea for seeding quasars in the infant Universe stemmed from the realization that the size of galaxies is dictated by their spin. The smaller the spin is, the more compact is their final disk, where cold gas is held against gravity by rotation. The amount of rotation is derived from the tide the galaxies experience as their matter turns around from cosmic expansion and starts to collapse into a gravitationally bound system. Since different galaxies are born in different environments, their level of spin would be different, 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 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 disk of gas with less angular momentum than a typical galaxy. The gas from this compact disk would drain more efficiently towards 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 rapidly. We therefore suggested that low-spin galaxies could be the progenitors of quasar black holes.
When my bright postdoc, Fabio Pacucci, alerted me to the puzzling properties of the little red dots discovered by the Webb telescope, I was immediately reminded of my papers with Daniel. Fabio and I plan to explore the connection between the little red dots and low spin galaxies more thoroughly in a future paper.
The scientific literature is vast and I do not expect my colleagues to remember a paper that was written thirty years ago. It remains the duty of those of us with a long scientific memory to connect 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 forgotten by now.
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ABOUT THE AUTHOR
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 the 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 chair 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 a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
