Anna Xu, Contributing Illustrator

New research reveals that the supermassive black holes at the center of galaxies are spinning nearly at their theoretical limit.

Tonima Ananna GRD ’20, who is currently a postdoctoral research associate at Dartmouth, is the lead author on a paper recently accepted in the Astrophysical Journal on the subject of supermassive black holes. In the past, the scientific understanding of black hole spin was limited by a lack of data on black holes obscured by dust.

But newly developed techniques allowed Ananna to obtain this information, and she successfully incorporated it into her new, more comprehensive model for all black holes. She found that black holes are spinning extremely fast. This implies that black holes grow mostly by devouring objects near them, as opposed to growing by merging with other black holes. Her results also begin to explain how the current universe was formed after the Big Bang.

“There’s this theoretical limit to the efficiency, to the spin of a black hole. And Tonima’s solution was quite near that,” said Professor Meg Urry, Ananna’s thesis advisor. “It was quite high and she kept saying, there must be something wrong.”

But their numbers were correct.

Black holes have three properties: charge, spin and mass. This study contains the first comprehensive estimation of black hole spin, according to Urry.

Black holes grow in two ways: by merging with other black holes and by accretion, which is the reeling in of other nearby objects. Ananna calculated how efficiently rapid growth supermassive black holes converted the matter they were eating into radiation. A high efficiency value implies that the black hole is spinning quickly.

She explained that if their efficiency values were extremely high, that meant the black holes grow mainly by accretion. If they were very large but inefficient, they grew by two or more black holes merging with each other. What Ananna found is that the black holes were incredibly efficient, so they, therefore, must be spinning very fast.

Her study also begins to explain why certain parts of the universe’s early history are hidden from us. As light travels at a speed limit, the farther away we peer into the universe, the farther back we are seeing in time, Ananna explained. However, astronomers cannot see into the era right after the Big Bang, during which the universe was very hot. Light bounced around off of free electrons, and the universe was “ionized,” according to Priyamvada Natarajan, professor of astronomy and physics at Yale. Therefore, very little light escaped, so it is not visible to telescopes today.

As the universe cooled down, however, electrons and protons combined to form neutral hydrogen atoms, so the universe was no longer ionized. Although more light escaped than before, it was still a small enough amount that this era is referred to as the Dark Ages.

It was not until between 150 million and 1 billion years after the Big Bang that the stars and galaxies seen in the universe today started forming. These new hot objects reionized the hydrogen by separating the protons and electrons, and light waves were no longer trapped. Astronomers can therefore look back and observe objects after this time period.

The big question is what contributed more to this reionization process: galaxy formation or a type of supermassive black holes called quasars.

“Every quasar is individually brighter than a galaxy because quasars outshine their galaxy,” Natarajan said. “But they’re rarer, so the question is always … to make this tradeoff and see which of these two populations of potential objects, which produce reionizing radiation get implicated in causing reionization.”

Ananna’s work contributed to answering this question because it included data from a population of obscured black holes.

Natarajan explained that with this additional information, one can confirm that galaxies are “significantly responsible for reionization.”

Ananna’s model explains so much about black hole growth because it fits many types of supermassive black holes. She explained that the new data on dust-obscured black holes did not fit the old models, and her initial attempts to create a new model were failing.

“We didn’t have the mathematical machinery to do it,” Ananna said. “We were stumped for a month and a half. I was learning machine learning, from this course … so I decided to use that new machinery which wasn’t really used all that much in astronomy in 2017, 2018.”

The model she designed now fits all of the new data that has come out since she created it. She has continued to use it for her future research, including in other surveys of black holes.

“Tonima has the world’s best model now,” Urry said. “It’s state of the art, surpasses anything anyone else has ever done.”

Supermassive black holes are defined as black holes with a mass of more than one million times our Sun.

Annika Salmi | annika.salmi@yale.edu