Astronomers have discovered a unique way to weigh supermassive black holes at the centres of galaxies.


The method involves measuring the distances between the galaxies that contain them, and indicates supermassive black holes may not be as massive as previously thought.

Scientists believe that mysterious, massive dark objects that lurk at the centre of nearly all galaxies are black holes that can be observed with the right equipment.

They can have masses that can exceed a billion suns, according to the study published in Nature Astronomy.

Such supermassive black holes may power quasars, and may halt the formation of stars by releasing copious amounts of energy which heats up and fragments the gas in their host galaxies.

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Researchers say they are tiny compared with their host galaxies – the size of a grape compared with the Earth.

But despite this, most observations suggest the bigger the galaxy, the bigger the supermassive black hole in it.

According to the astronomers, you would not expect the size of the black hole to know about the size of the galaxy it resides in, but there must be an intimate link between supermassive black hole growth and galaxy evolution.

Supermassive black holes may not be as massive as previously thought © Chris Marsden, University of Southampton/PA
"These findings have significant implications for our understanding of the evolution and growth of supermassive black holes" said Dr Francesco Shankar © Chris Marsden, University of Southampton/PA

This has not been proven, and in the study, the international team led by Dr Francesco Shankar of the University of Southampton, with Dr Viola Allevato at the Normale di Pisa and other partners in the US, Germany, Italy and Chile, set out to explain this link.

The masses of supermassive black holes are usually calculated by measuring the velocity of the surrounding stars or gas.

This is usually done with extremely sensitive telescopes and complex observations.

Dr Shankar said: “These findings have significant implications for our understanding of the evolution and growth of supermassive black holes.

“What we have discovered suggests a greater ability to release energy, and less strength in powering gravitational waves as supermassive black holes merge.”

Galaxies and their supermassive black holes are believed to reside in halos made of dark matter.

Numerical simulations show that more massive dark matter halos deviate more from a random spatial distribution – more strongly clustered.

Therefore, say the scientists, their clustering strength can be used to weigh the halos.

Reader Q&A: Are black holes hot or cold?

Asked by: Martin Hearne, Chorley

Black holes are freezing cold on the inside, but incredibly hot just outside. The internal temperature of a black hole with the mass of our Sun is around one-millionth of a degree above absolute zero.

Just outside the hole, however, the material being pulled into the hole’s gravity well is accelerated to near the speed of light. The molecules of the material collide with such vigour that it is heated up to a temperature of hundreds of millions of degrees. When astronomers study black holes, this is the material that they see.

The radiation from the material masks the tiny amount of radiation escaping from the hole itself, and so what the astronomers observe is the very hot outside environment, rather than the freezing cold environment inside.

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They expect more massive black holes to be hosted by more massive halos, so the clustering of the black holes can be used to estimate the masses of their hosts.

In turn this can be used to constrain the masses of the black holes themselves.


By comparing simulations with recent data on the spatial distribution of galaxies, the group found evidence that supermassive black holes are, on average, not as massive as previously thought.


Alexander McNamaraOnline Editor, BBC Science Focus

Alexander is the former Online Editor at BBC Science Focus.