Two black holes 400 million light-years away from us are caught in the cosmic collision of their former host galaxies. What remains is a complex merging galaxy known as NGC 6240. Now, an international team of astronomers has zoomed in on the black holes and obtained the most detailed image yet of the gas swirling around them.
The feat was only possible with the Atacama Large Millimeter/submillimeter Array (ALMA). ALMA increased the resolution of the images by a factor of ten. This allowed the team to better understand the motions of the molecular gas around the galaxy, as well as its movements in the sphere of influence of the two supermassive black holes. The findings will be published in The Astrophysical Journal.
“The key to understanding this galaxy system is molecular gas,” lead author Ezequiel Treister, of the Pontificia Universidad Católica in Santiago, said in a statement. “This gas is the fuel that is needed to form stars, but it also feeds the supermassive black holes, which allows them to grow.”
The observations contradict a recent study that suggested there are three supermassive black holes in the galaxy. However, when the team traced the motion of the gas, they couldn't find any evidence for a third one. The detailed tracking of the gas also helped the team refine their estimations of the mass of the supermassive black holes, which, as it turns out, are actually smaller than previously thought.
“Previous models, based on surrounding stars, indicated that the black holes were much more massive than we expected, around a billion times the mass of our Sun,” added Anne Medling of the University of Toledo in Ohio. “But these new ALMA images for the first time showed us how much gas is caught up inside the black holes’ sphere of influence. This mass is significant, and therefore we now estimate the black hole masses to be lower: around a few hundred million times the mass of our Sun.”
Another incredible find is that the gas gets much closer to the black holes than expected. This gas will either fall onto the black hole or escape it at high speeds. The latter scenario might help explain how supermassive black holes eventually lose energy and merge with one another.
“This galaxy is so complex, that we could never know what is going on inside it without these detailed radio images,” explained Loreto Barcos-Muñoz of the National Radio Astronomy Observatory in Charlottesville. “We now have a better idea of the 3D-structure of the galaxy, which gives us the opportunity to understand how galaxies evolve during the latest stages of an ongoing merger. In a few hundred million years, this galaxy will look completely different.”