Observational Signatures of Black-hole Mergers

Conference Report

Jeremy Schnittman, schnittm@pha.jhu.edu

What happens when the supermassive black holes (BHs) at the centers of merging galaxies spiral together and collide? Will we be able to see an explosion? Will we be able to “hear” the burst of gravitational waves (GWs)? Will the surrounding gas and stars be disrupted, or even ejected from the galaxy? This past spring, the Institute hosted a conference of an unusually diverse group of astronomers and astrophysicists with the goal of answering these tough questions. Motivated in large part by recent advances in the numerical relativity community, the past couple of years have seen a remarkable number of new papers published on the potential observational signatures of colliding BHs.

According to Einstein’s general theory of relativity, which describes the behavior of gravity in the universe, even the enormous amount of energy released in GWs during a BH merger will not produce any significant electromagnetic signal (i.e., photons). In fact, this is precisely why it is so difficult to directly detect GWs, because they interact so weakly with ordinary matter. At present, much of the best observational evidence for BH mergers is circumstantial: most, if not all, galaxies appear to host supermassive BHs at their centers; galaxy mergers are observed to be relatively common, with most galaxies undergoing at least one major merger during their lifetimes; and almost no galaxies show evidence for the presence of two supermassive BHs, suggesting that when galaxies merge, their central black holes must also merge on a relatively short timescale.

The conference began with a session dedicated to the topic of galaxy mergers in the context of large-scale cosmological simulations, including the relations expected (and observed!) between the galactic nuclei and their central BHs. Moving to smaller time and distance scales, the “merger mechanisms” session addressed one of the oldest questions in the field: how exactly do two supermassive BHs come together to merge following the galaxy merger? Also known as “the final parsec problem,” the difficulty is bridging the gap between classical gravitational interactions with the stars in the galaxy, which cause the two BHs to sink to the central region of the galaxy within about a parsec of each other, to the point at which relativistic gravitational wave losses grow large enough to take over—around a hundredth of a parsec. One of the most promising mechanisms involves massive disks of gas and dust, which provide enormous drag forces on the orbiting BHs, accelerating their orbital decay.

Over the millions of years it takes just to get the two BHs close enough to merge, they will interact strongly (albeit non-relativistically) with the surrounding gas and stars near the center of the galaxy. Furthermore, detailed calculations of the merger itself suggest that during the final plunging orbit when the two black holes become one, the asymmetric emission of GWs can impart a recoil, or “kick,” to the final BH. This would sending it flying through the host galaxy at thousands of kilometers per second, dragging with it the nearby material and plowing through anything in its path. In both cases, it seems quite likely that a strong electromagnetic signal would be generated.

One major question addressed at the conference was how to characterize and then detect such a signal. The centers of galaxies are already known for their dynamic and energetic activity, even without the presence of two black holes. How might we distinguish between binary BHs and “regular” BHs? Will the high orbital velocities of the binary expel the surrounding gas, or alternatively, might they actually enhance accretion and the production of energetic particles and radiation? After the merger and resulting kick, will the final BH resemble an active galactic nucleus (AGN)? Again, how might we distinguish between post-merger AGNs and normal AGNs?

In addition to the numerous talks on direct signatures of BH mergers, there was an entire session dedicated to indirect measurements of BH mergers and evolution. In particular, a number of speakers discussed how one might use observations of BH spin (which, along with mass, completely describes the physical properties of a black hole) to infer its merger history. Finally, the conference concluded with an exciting panel discussion highlighting the future capabilities of a number of major missions planned for the coming decade.

As might be accurately concluded from this brief summary, the subject of BH mergers has many more questions than answers right now. The meeting at the Institute was an important first step in formulating the questions and focusing the direction of the field. Even more important, it was a fantastic catalyst to bring together many astronomers and astrophysicists from a wide range of specialties to share their ideas and stimulate even more questions for the future.

Figure 1. Hubble image of the merging galaxies NGC 4038 and NGC 4039, also known as the “Antennae Galaxies.” Such mergers trigger massive bursts of star formation and likely lead to strong active galactic nuclear activity. Over a period of hundreds of millions of years, the supermassive black holes originally at the centers of each galaxy will spiral together and eventually merge. During the final hour of this merger, an enormous flux of gravitational waves will be emitted. A NASA supercomputer simulation of these waves is shown in the inset. (Images courtesy of STScI and NASA/GSFC)

Figure 1. Hubble image of the merging galaxies NGC 4038 and NGC 4039, also known as the “Antennae Galaxies.” Such mergers trigger massive bursts of star formation and likely lead to strong active galactic nuclear activity. Over a period of hundreds of millions of years, the supermassive black holes originally at the centers of each galaxy will spiral together and eventually merge. During the final hour of this merger, an enormous flux of gravitational waves will be emitted. A NASA supercomputer simulation of these waves is shown in the inset. (Images courtesy of STScI and NASA/GSFC)