Published: April 18, 2018 By

NGC 6240 as seen by the Hubble Space Telescope. Credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Researchers at CU Boulder have completed an unprecedented “dissection” of twin galaxies in the final stages of merging.

The new study, led by research associate Francisco Müller-Sánchez, explores a galaxy called NGC 6240. While most galaxies in the universe hold only one supermassive black hole at their center, NGC 6240 contains two—and they’re circling each other in the last steps before crashing together. 

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 NGC 6240’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."

Galaxies like NGC 6240—which play host to two well-fed supermassive black holes of a class called Active Galactic Nuclei (AGNs)—are relatively rare. They have attracted a lot of attention, however, because they provide a snapshot of an important stage in the evolution of galaxies like our own Milky Way. Scientists believe that such galaxies are created from the merger of two parent galaxies.

But NGC 6240 is weird in other ways, Müller-Sánchez said. Unlike the Milky Way, which forms a relatively tidy disk, bubbles and jets of gas shoot off from NGC 6240, extending about 30,000 light years into space and resembling a butterfly in flight. Scientists have suspected that this butterfly may be linked to the galaxy’s twin hearts. 

“Galaxies with a single AGN never show such a phenomenal structure,” he said.

To test the idea, the CU researchers and their colleagues combined observations from three different telescopes: the Hubble Space Telescope, the Very Large Telescope in Chile and Apache Point Observatory in New Mexico—which is owned by a consortium of universities including CU Boulder.

In research published today 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.

“The data from these three telescopes allowed us to determine the location and velocity of different types of gas in the galaxy,” said Rebecca Nevin, a graduate student in APS and a co-author on the paper. “This helped us uncover two winds—one that is driven by dual supermassive black holes, and one that is driven by star formation.”

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

Such an outflow can have big implications for the galaxy itself. When two galaxies merge, he said, 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. The team suspects that they are seeing such “negative feedback” on star formation begin to ramp up in NGC 6240.

“NGC 6240 is in a unique phase of its evolution,” said Julie Comerford, an assistant professor in APS 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 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.