How to ‘weigh’ a Black Hole

Whilst popping a black hole on a scale may be impractical, to say the least, researchers have developed a new method to ‘weigh’ the central engines of active galaxies.

Supermassive black holes sit at the centre of almost every galaxy, meaning that determining their characteristics — mass, spin and electric charge — are important to our understanding of how galaxies evolve.

Astrophysicists from the Moscow Institute of Physics and Technology (MIPT) have devised an ingenious new way to measure the mass of a black hole. They tested their method on the active galaxy Messier 87 — located 54 million light-years from Earth.

Virgo A galaxy, also known as M87, and its jet. Credit: NASA/JPL-Caltech/IPAC

The key to the team’s new method of mass measurement hinges on observations of the relativistic jets — long, thin beams of matter ejected at speeds comparable to that of light — blasted outwards from the centres of active galaxies.

As these jets are too powerful to have been emitted by stars, the current scientific consensus regarding their origins posits that they are emitted from the galactic engine lurking at the centre of active galaxies — more commonly known as galactic nuclei. And at the heart of this engine, a supermassive black hole powering the entire galaxy.

The key factor in determining mass hinges on the fact that these jets often display ‘breaks’ in shape. The theoretical model used to predict this change in shape can determine how massive a black hole has to be to generate the observed break.

“The new independent method for estimation of black hole mass and spin is the key result of our work,” says Elenka Nokhrina, lead author of the paper published in the Monthly Notices of the Royal Astronomical Society and deputy head of MIPT.

“Even though its accuracy is comparable to that of the existing methods, it has an advantage in that it brings us closer to the end goal. Namely, refining the parameters of the core ‘motor’ to deeper understand its nature.”

The reason that Messier 87 was chosen to pilot this new method is that it has been meticulously studied by astronomers since its discovery in 1781. Despite initially suspecting that it was a nebula — astronomers had identified Messier 87 as a galaxy by 1918, thus making it the first-ever galaxy identified outside our own.

This familiarity means that the plasma jet it emits is also well-studied, as are associated conditions such as its velocity, temperature and surrounding particle number density. With the jet studied so thoroughly, researchers have been able to identify the fine detail of its boundary, discovering in-turn, a curious ‘break’ in its shape — a change in form from a parabola to a more conical shape.

A radio interferometry image of the M87 galaxy at a 2-centimeter wavelength with sub-parsec resolution. Credit: Yuri Kovalev/MIPT

This break is not unique to Messier 87’s jet, however, having been identified in the relativistic jets of a least a dozen other galaxies. Despite its commonality, the break phenomenon remains most clearly identified in the jet of Messier 87.

The huge amounts of observations of M87’s jet break shape allowed the researchers to examine its connection to the central black hole’s gravitational influence.

Examination of the jet boundary — comprised of segments of two distinct curves — and the distance between the core and the break of the jet, coupled with the jet’s width allows astrophysicists to measure both the mass and spin of the black hole. In order to perform such an examination, the team developed their method — employing a theoretical model, computer calculations and observations from telescopes.

The researchers' method requires them to describe the jet as a flow of magnetised fluid. This means that the shape of the jet would be determined by the electromagnetic field within it. This field is dependant on characteristics of the jet, such as its speed, the charge of the particles within the jet and current within the jet. Another factor to be considered is the rate at which the black hole accretes surrounding matter to its surface.

These various elements meet in a complex interplay with physical phenomena to cause the break in the plasma jet.

Whilst very early days for the model, it offers an interesting method to measure the qualities of the universe’s most mysterious objects.