Super-massive black holes quickly repoint their jets

With masses up to 10¹⁵ times greater than that of the Sun, galaxy clusters are the largest concentrations of matter in the universe. Within these objects, the space between the galaxies is filled with a gravitationally bound hot plasma. Given time, this plasma accretes on the galaxies, cools down and eventually forms stars. However, observations indicate that the rate of star formation is slower than expected, suggesting that processes are at play that prevent the gas from accreting. Violent bursts and jets coming from super-massive black holes in the centre of galaxy clusters are thought to quench star formation. A new study indicates that these jets rapidly change their directions.

Super-massive black holes form the centre of all galaxies, including our own, and can undergo periods of activity during which powerful jets are emitted along their spin axes. In the case of galaxy clusters, these bursts can be spotted in real time by looking at their radio emission, while their histories can be traced using X-ray observations. As the jets are emitted, they crash into the intra-cluster plasma, sweeping up material and leaving behind bubbles, or cavities, in the plasma. As the plasma emits in the X-ray region, these bubbles reveal themselves as voids when viewed with X-ray detectors. After their creation, they continue to move through the plasma and remain visible long after the original jet has disappeared (see image).

Francesco Ubertosi of the University of Bologna and co-workers studied a sample of about 60 clusters observed using the Very Long Baseline Array, which produces highly detailed radio information, and the Chandra X-ray telescope. The team studied the angle between the cavities and the current radio jet and found that most cavities are simply aligned, indicating that the current jet points in the same direction as those responsible for the cavities produced in the past. However, around one third of the studied objects show significant angles, some as large as 90°.

Violent bursts and jets are thought to quench star formation

This study therefore shows that the source of the jet, the super-massive black hole, appears to be able to reorient itself over time. More importantly, by dating the cavities the team showed that this can happen within time scales of just one million years. To get an idea of the rapidity of this change, consider that the solar system takes 225 million years to revolve around the super-massive black hole at the centre of the Milky Way. Analogously, Earth takes 365 days for one revolution around the Sun. Therefore, if the Milky Way’s super-massive black hole altered its spin axis on the timescale of one million years, it would be as if the Sun were to change its spin axis in a matter of a few days.

These observations raise the question of how the re-orientation of jets from super-massive black holes takes place. The authors find that the results are unlikely to be due to projection effects, or perturbations that significantly shift the position of the cavities. Instead, the most plausible explanation is that the spin axes of the super-massive black hole tilt significantly, likely affected by complex accretion flows. The results therefore reveal important information about the accretion dynamics of super-massive black holes. They also offer important insights into how stars form in these clusters, as the reorientation would further suppress star formation.