UT researchers propose methods of supermassive black hole formation

Elizabeth Robinson

Almost as old as the universe itself, the supermassive black holes that form the centers of galaxies and their origin have long puzzled astronomers. However, researchers may have found one possible solution.

One hypothesis for the origin of these supermassive black holes is the merger of relatively smaller massive black holes. Earlier this year, an international research team including UT astronomers published their findings from a computer simulation successfully showing that such massive black holes could form quickly from the collapse of early massive stars.

The key proposal of the study, published in Science Magazine, is that these massive stars formed when fast-traveling gas was trapped inside enormous clusters of dark matter, a hypothetical form of matter that can only be detected through its gravitational effects.

“The scenario (is) firstly that dark matter clumps form, and that secondly, gas is trapped inside,” said UT postdoctoral researcher Shingo Hirano, one of the study’s authors. “This is a very common scenario in the world of cosmology.”

Modern stars develop inside of nebulae, but these hypothetical massive stars would have instead formed from when hydrogen and helium gas traveling faster than the speed of sound became trapped inside of clumps of dark matter, Hirano said.

“Via gravity, (dark matter) forms a very dense structure and eventually forms a clump,” Hirano said. “The gravity of these dark matter clumps can trap supersonic gas inside. The formation of the dense gas structure forms the star.”

The hypothetical star would have been about 30,000 times larger than our sun, Hirano said. Stars can only grow that large if they are fueled only by hydrogen and helium, the two most basic elements.

“Right after the formation of the universe, it was basically just hydrogen and helium,” UT astronomy professor Karl Gebhardt said. “The only time you get the heavier elements is when … a star blows up and spills out all the heavy elements (from its core).”

After these first massive stars of hydrogen and helium collapsed and scattered heavier elements throughout the universe, new stars formed with heavier elements already inside of them, keeping them from growing larger, Gebhardt said. According to a Cornell University study published in 2015, heavier elements have a cooling effect on stars, and thus limit their growth.

Clumps of dark matter still exist today in the form of dark matter halos, Hirano said. These dark matter halos affect the way galaxies rotate, but the chances of these halos forming new massive black holes is slim, he added.

“Most of the formation (of black holes) in terms of dark matter clumping happened early in the universe,” Gebhardt said. “As the universe expands out, everything’s getting less and less dense … In the past, things were closer together so it was easier to clump together.”

According to Gebhardt, the jury is still out on whether supermassive black holes formed from the merger of a few large black holes or many smaller ones. However, he said he believes that today’s supermassive black holes may have formed from a combination of pathways.

“In (our) study, (black holes) can form in any part of the universe,” Hirano said. “You can easily find this scenario. In other scenarios, people assume very specific conditions to form massive black holes.”