Cosmic seeds of the largest black holes

A surprising fact of cosmic history is that the most massive black holes have always had ten billion times the mass of the Sun, starting when the Universe was 5% of its current age, just 700 million years after the Big Bang . During 95% of cosmic history since then, the ceiling of black hole masses has remained the same. This raises two questions. First, how did these giant babies emerge in the delivery room of the early Universe? And second, why didn’t these giant babies grow in mass as the Universe aged by a factor of 20? In other words, in the cosmic population of black holes, the largest children weighed as much as the largest adults.

As any gardener knows, growth started from seeds. What were the seeds that allowed supermassive black holes to grow so early in cosmic history?

Supermassive black holes gain mass primarily by accreting gas or by merging with each other. As gas settles into a disk around a black hole, it spirals inward like water flowing into a sink. The gravitational energy gained by falling into the mouth of the black hole, located in the innermost stable circular orbit (ISCO), according to Einstein’s general relativity, is radiated. Black holes are the most efficient engines for converting rest mass into radiation, with typical efficiency on the order of 6% for a thin accretion disk around a non-rotating black hole.

Once the luminosity of the radiation exceeds the so-called Eddington limit, first identified by astronomer Arthur Eddington for massive stars, the outward push of the radiation overcomes the inward gravitational pull and power it stops. The Eddington limit is proportional to the central mass and is equal to 350 trillion solar luminosities for a ten billion solar mass black hole. Surprisingly, the luminosities of the brightest black holes in the centers of galaxies, the so-called quasars, respect the Eddington limit in the real universe.

Quasars could be hundreds of times more luminous than the sum of the stars in their host galaxies because they consume gas more vigorously and convert mass to radiation faster and more efficiently. As a result, their host galaxies appear as a faint fuzz around them, like fog around a bright streetlight.

The quasar’s phase is episodic, resembling a brief explosion in the cosmic timeline of billions of years. A quasar’s immense power output expels gas from its vicinity and disables its power. Another way of putting it is that quasars are suicidal; they shine so brightly that they push the food out of the bowl. Since the Eddington limit is proportional to their mass, the final state of quasars naturally leads to a correlation between the final mass of the black hole and the depth of the feeder bowl associated with the star spheroid which maintains the food supply. When I suggested looking for this correlation in observational data in 2000, the idea was initially dismissed; six months later, my idea was confirmed by two independent groups of observers (see section 6.2 on page 47 in the 2013 review paper on the subject).

The Eddington limit implies a 30 million year doubling time for the mass of the black hole. In a 2001 paper I wrote with my former student, Zoltan Haiman, now a professor at Columbia University, we showed that if quasars were seeded by stellar-mass black holes, then they would barely have time to reach the observed mass the first quasars though they grew steadily at the Eddington boundary. But it’s more likely that seeds have only grown for a fraction of cosmic history. If so, how did they grow so big so early?

There are two possible solutions. One is that the first quasars were powered up so much that the radiation was trapped by the falling gas blanket and carried with it into the black hole. After all, we know of similar circumstances in the core of a massive star. After consuming its nuclear fuel and losing pressure support, the core rapidly collapses into a black hole at a feed rate that is a quadrillion times the Eddington limit. The infalling gas is so opaque that it traps the radiation and carries it into the black hole without allowing it to flow outward and resist gravity. As I showed in a 2012 article with my former postdoc, Stuart Wyithe, currently a tenured professor at the University of Melbourne in Australia, the same conditions can be realized within the gas-rich galaxies that give birth to the first quasars in the early universe.

Two of the farthest galaxies seen by the Webb Space Telescope. The galaxy labeled (1) existed only 450 million years after the big bang. The galaxy labeled (2) existed 350 million years after the big bang (Credit: NASA, ESA, CSA).

Alternatively, the first seeds of quasars may have been supermassive stars with up to a million solar masses. As I showed in a 2002 article with my former postdoc, Volker Broom, currently chair of the astronomy department at the University of Texas at Austin, such stars could have formed from the direct collapse of huge gas clouds that did not fragment. in normal stars because cooling by heavy elements or molecular hydrogen has been suppressed in the primordial gas. This formation channel, called “Direct Collapse Black Holes”, is often looked for in high redshift galaxies. The possibility of supermassive stars such as quasar seeds was envisioned in a 1994 paper I wrote with Professor Fred Rasio, editor of The letters from the astrophysicist diary.

Once the gas in galaxies has been consumed by star formation, less is left available to fuel black holes. In a recent article I wrote with my postdoc, Fabio Pacucci, we showed that while supermassive black holes grew mainly through accretion of gas in the early universe, mergers became more important to their growth in the universe. local.

The final puzzle concerns the constancy of the mass ceiling for black holes. The first quasars formed in rare environments where the density of matter is high. These environments behave as if they are part of a denser universe where the cosmic clock is ticking faster. As a result, children born in these regions saturate their growth long before their analogues do so later in cosmic history.

little bells

The most massive black holes reside in clusters of galaxies, surrounded by hot gas, which is more susceptible to their feedback. This could potentially impose a universal limit on their growth regardless of cosmic time.

It is possible that some of the light sources in the deepest images obtained by the Webb telescope and celebrated at the White House on July 11, 2022 contain evidence of the first supermassive stars or even the first quasars. If so, future data may indicate that they emit copious amounts of ultraviolet radiation.

As I showed in a 2018 article with my former postdoc, John Forbes, the bright emission of quasars could sterilize habitable planets throughout the core of their host galaxies. We are fortunate to reside far enough from the center of the Milky Way so that even if its central supermassive black hole, Sagittarius A*, were to shine at its Eddington limit, its radiation flux would be fainter than that of the Sun at all times. wavelengths.

We thank this cosmic coincidence as we count our blessings this Thanksgiving holiday!

Avi Loeb leads Project Galileo, 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 chairs the Breakthrough Starshot project advisory board and is a former member of the President’s Council of Advisors on Science and Technology and past chair of the National Academies Board on Physics and Astronomy. He is the best-selling 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, titled ‘Interstellar’, is expected in publication in June 2023.

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