Black holes: Twin Galaxies in the final stages of merging

two merged black holes

Researchers at the University of Colorado, Boulder completed an unprecedented “dissection” of the Twin Galaxies at the final stage of the merger.

The new study led by CU Boulder research associate researcher Francisco Müller-Sánchez explores a galaxy named NGC 6240.

Although most of the galaxies in the universe have only one supermassive black hole in the center, NGC 6240 contains two – they hover in the last step before the collision.

The research reveals how gases ejected by those spiraling black holes, in combination with gases ejected by stars in the galaxy, may have begun to power down NGC 6240’s production of new stars.

Müller-Sánchez’s team also shows how these “winds” have helped to create the galaxy’s most tell-tale feature: a massive cloud of gas in the shape of a butterfly.

“We dissected the butterfly,” said Müller-Sánchez of CU Boulder’s Department of Astrophysical and Planetary Sciences (APS).

“This is the first galaxy in which we can see both the wind from the two supermassive black holes and the outflow of low ionization gas from star formation at the same time.”

The team zeroed in on NGC 6240, in part, because galaxies with two supermassive black holes at their centers are relatively rare.

Some experts also suspect that those twin hearts have given rise to the galaxy’s unusual appearance. Unlike the Milky Way, which forms a relatively tidy disk, bubbles and jets of gas shoot off from NGC 6240, extending more than 30,000 light years into space and resembling a butterfly in flight.

“Galaxies with a single supermassive black hole never show such a phenomenal structure,” Müller-Sánchez said.

In research published April 18 in Nature, the team discovered that two different forces have given rise to the nebula. The butterfly’s northwest corner, for example, is the product of stellar winds, or gases that stars emit through various processes.

The northeast corner, on the other hand, is dominated by a single cone of gas that was ejected by the pair of black holes—the result of those black holes gobbling up large amounts of galactic dust and gas during their merger.

Those two winds combined evict about 100 times the mass of Earth’s sun in gases from the galaxy every year. That’s a “very large number, comparable to the rate at which the galaxy is creating stars in the nuclear region,” Müller-Sánchez said.

Such an outflow can have big implications for the galaxy itself. He explained that when two galaxies merge, they begin a feverish burst of new star formation.

Black hole and stellar winds, however, can slow down that process by clearing away the gases that make up fresh stars—much like how a gust of wind can blow away the pile of leaves you just raked.

“NGC 6240 is in a unique phase of its evolution,” said Julie Comerford, an assistant professor in APS at CU Boulder and a co-author of the new study.

“It is forming stars intensely now, so it needs the extra strong kick of two winds to slow down that star formation and evolve into a less active galaxy.”

Other co-authors on the study include Rebecca Niven, a graduate student in APS at CU Boulder, Richard Davies at the Max Planck Institute for Extraterrestrial Physics in Germany, Ezequiel Treister at the Pontifical Catholic University of Chile and George Privon at the University of Florida.

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