Recoiling Supermassive Black Holes and Stellar Flares

Astrophysicists love to simulate huge collisions, and they don’t get much bigger than this. From the discoverers of the first ever observed black hole collision back in April, new observational characteristics have been researched and Max Planck astrophysicists believe that after two supermassive black holes (SMBHs) have collided, they recoil and drag flaring stars with them. By looking out for anomalous X-ray flares in intergalactic space, or off-galactic nuclei locations, repelled black holes may be spotted powering their way into deep space at velocities of up to 4000 kms^-1…

Back in April, I followed the discovery of the first ever SMBH collision observed (from the Universe Today article “Supermassive Black Hole Kicked Out of Galaxy: First Ever Observation“). This event occurred during a galactic merger as the central SMBHs attempted to merge. If the conditions are right (and if the black holes are spinning rapidly), on contact, the colliding singularities may recoil. This recoil (similar to the jolt that is felt after firing a rifle) is generated by the huge disruption in space-time, generating powerful gravitational waves, shunting the smaller black hole in the opposite direction. The process can be envisioned as two spinning tops slowly approaching each other; when they touch, the huge angular momentum blasts the tops away from each other.

Now, the Max Planck team who made the groundbreaking discovery of SMBH recoil (and made the first detailed observation of a black hole torus), headed by Stefanie Komossa have just submitted a paper to Astrophysical Journal Letters with more details on how to identify a black hole collision and recoil. To set the scene, should the conditions be right and black hole coalescence result in a massive recoil, it is possible that the gravitational waves generated by such a collision could produce a kick from 200 kms^-1 (for two non-spinning black holes) to 4000 kms^-1 for two maximally spinning, equal mass black holes. Should two galaxies merge, and the resulting SMBH binaries eventually spiral into one another, it isn’t so hard to imagine the smaller SMBH could be ejected from its parent galaxy.

But how do you spot a recoiling black hole? There are indirect methods (such as gravitational lensing), but ideally astrophysicists need a direct indication of the presence of a speeding SMBH post-collision. As mentioned by Komossa in her research published in April, the ejected black hole may have an associated accretion disk that will remain gravitationally bound to the massive object. In fact, the first ever observed recoiling black hole was observed by measuring the broad spectral emission lines of the gas surrounding the ejected black hole. From this measurement, the velocity of the black hole could be deduced. In this case, the black hole was travelling an impressive 2650 kms^-1. Although this is useful, are there any other indicators for the presence of a speeding SMBH?

Primarily, at the point of collision inside the merged galaxy, residual X-ray emission could be observed by Komossa. But with the publication of this new research, there’s a possibility that stars from the galactic nucleus may be gravitationally dragged with the recoiling SMBH as it travels out of the galaxy and through intergalactic space.

1. Powerful off-nuclear X-ray flares and feedback trails
In this situation, quasar-like X-ray flares may be observed within the galactic disk. Since there is no known mechanism that causes such luminous, short-lived (months-year) stellar emissions, the only possibility could be an ejected SMBH, tidally disrupting gravitationally bound stars, passing from the galactic nucleus and into the galactic disk.

2. Intergalactic flares and other signatures
Like #1, anomalous X-ray flaring stars will be observed. Only this time, the emissions will be located in intergalactic space, well away from the host galaxy. After the massive gravitational disruption of collision and recoil, the gravitationally bound stars will still be tidally perturbed by the ejected SMBH, triggering X-ray flares for a long while. Eventually, the stars surrounding the SMBH will continue to evolve, creating planetary nebulae and turning into old white dwarfs after several billion years. In some cases, fossil SMBHs may be revealed as ancient white dwarfs stray too close to the SMBH, triggering a supernova event.

Whether or not stellar signatures of ejected SMBHs will be observed, the Max Planck group are churning up some incredible SMBH discoveries, so keep an eye on these guys…

Source: arXiv:0807.0223v1 [astro-ph]