Three supermassive black holes? In the galactic chaos of NGC 6240, ALMA sees just two

I recently wrote about NGC 6240, a collision between two galaxies where it looks like there were not two but three supermassive black holes seen in the observations. One of the two colliding galaxies had two bright spots in its core, which the astronomers interpreted as being two supermassive black holes.

New observations by another team of astronomers now cast some serious doubt on that. They show evidence for only two black holes there. If this is the case, it's not clear what that third object is, though they speculate it might be a very massive star cluster.

As a refresher, NGC 6240 is clearly a chaotic mess, typical of two massive galaxies in the throes of merging after a catastrophic collision. The process is far enough along that it’s difficult to distinguish one galaxy from the other in visible light images, though the evidence of a collision is obvious to the trained eye. Besides the jumbled up main mass, there are also tidal tails, streamers of stars pulled out by the mutual gravitational attraction of the two. These are a smoking gun for collisions.

NGC 6240 is a galactic collision in progress. ALMA shows the black hole in each ...