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How to weigh a black hole without getting off the couch


There’s a simple relationship between the angle of the arms of a spiral galaxy and the mass of the black hole at the galaxy’s heart


An artist’s impression of a black hole at the centre of a galaxy.
An artist’s impression of a black hole at the centre of a galaxy.
James Josephides

For years, scientists have relied on a complicated series of technical observations and calculations to determine the mass of black holes, but new research has shown that in some cases this arduous task can be replaced with a method so simple that even school-children can do it.

In the traditional method, since black holes do not emit any discernible light, scientists calculate their mass from the orbits of the stars and gas around them, which requires extremely complex observations that could only be carried out by astronomers with the right equipment and training.

A team of researchers led by Benjamin Davis of the Swinburne University of Technology in Victoria, however, has shown that it is possible to accurately estimate the same hard-won measurements for some black holes just by looking at the spiral arms of the hole’s host galaxy.

This discovery, which is documented in Monthly Notices of the Royal Astronomical Society, builds on work by Sir James Jeans and Edwin Hubble conducted nearly a century ago, which noted the correlation between the bulge at the centre of a spiral galaxy and how its arms were distributed.

Specifically, Jeans and Hubble found that while spiral galaxies with large central bulges had tightly wound spiral arms, those with smaller bulges displayed more widely dispersed arms.

Almost a decade ago, one of the coauthors of the new report, Marc Seigar from the University of Minnesota Duluth in the US, added to this understanding when he uncovered a relationship between central black hole mass and spiral arm geometry.

By carefully analysing a larger sample of galaxies using images collected by an array of space telescopes, Davis and his team have now revised this connection, which is capable of predicting lower-mass black holes in galaxies with spiral arms.

Interestingly, the research also indicates that black holes and the discs of their galaxies – the planes in which the spirals develop – must co-evolve.

Davis and his colleagues claim that this relationship can help astronomers hunt down a population of suspected, but missing black holes between 100 and 100 000 solar masses, which would consequently help explain the processes behind gravitational wave production.

The equation, for anyone playing at home, is log (MBH/M) = (7.01 ± 0.07) − (0.171 ± 0.017)[|ϕ| − 15°], where MBH/M is the mass of the black hole measured in solar masses and ϕ is the pitch angle of the galaxy’s spiral arms.

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Angus Bezzina is a writer from Sydney, Australia.
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