In our top story, we have news of a missing supermassive black hole. 

It is currently believed that all massive galaxies should have a black hole in their core that is proportional in size to the spheroid of that galaxy and the motions of its stars. While some small galaxies have been found without any obvious central black hole, massive galaxies have consistently had either obvious stellar motions or high energy emissions (or both) that are consistent with a massive object.

This was, at least, true until folks started looking at the central galaxy in the cluster Abell 2261.

Once upon a time, we referred to these systems as cD galaxies, and it was believed they were an outcome of how clusters form. Today they are more often referred to as giant elliptical galaxies and are thought more to be that first massive object that formed in the center of a dark matter halo in the early days of the universe. 

In this revised picture, these galaxies should be just like any other elliptical galaxy, just bigger, and more likely to be actively merging with or consuming other systems. Astronomers, however, have been utterly unable to find high-energy, X-ray light coming from this system, and the motions of star clumps that can be resolved weren’t the definitive data that was needed either. In the radio, there was evidence of past emissions, roughly 50 million years ago but nothing indicating there is something there now.

While it’s entirely possible there is a supermassive black hole in the central galaxy of Abell 2261 that isn’t consuming anything, and thus isn’t giving off any X-ray emission; and while it’s possible this galaxy doesn’t have any close in, fast-moving star clumps we’ll be able to resolve well enough to use orbital motions to detect that supermassive black hole; while it is entirely possible there is a supermassive black hole in there that we just can’t detect… the fact that we can’t detect it has folks looking for an alternative explanation.

It’s thought that this galaxy has undergone a recent merger, and dynamically it is possible that the original supermassive black holes from each galaxy merged in such a way that they ended up rocketing off out of the system as gravitational waves were released in the opposite direction. There are aspects of this galaxy consistent with this story: the center of the galaxy has far more stars than is expected, and the densest part of the galaxy is 2000 ly off-center, which is extraordinary.

If — and the data here is not at all definitive — if it turns out that supermassive black hole mergers can asymmetrically release gravitational waves and send the newly formed supermassive black hole flying, then we should see more supermassive black hole-missing galaxies, and we also have a cool new thing to look for in our gravitational-wave data. Currently, detectors are only sensitive to smaller mergers, but the future has the potential to bring news systems sensitive to different kinds of events.

We don’t know exactly what is going on, and we can’t know with our current technology. The JWST may be able to reveal more details if it launches, but until then, we have a system with a secret. It may or may not have a central supermassive black hole, and either solution is scientifically awesome.

This research is described in a new paper accepted to the American Astronomical Society journals, and available on arXiv, with first author Kayhan Gultekin.

More Information

Chandra X-ray Observatory press release 

“Chandra Observations of Abell 2261 Brightest Cluster Galaxy, a Candidate Host to a Recoiling Black Hole,” Kayhan Gultekin et al., to be published in AAS Journals (preprint on arxiv.org)