How do you weigh a supermassive black hole?

Supermassive black hole
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Astronomers have discovered a novel way to ‘weigh’ supermassive black holes at the centres of galaxies. They were able to do this by measuring the distance between the galaxies that contain them.

It is common for mysterious, massive dark objects to lurk at the centre of galaxies. Many astronomers believe these to be black holes with masses than can exceed a billion Suns.

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

Despite being incredibly small compared to their host galaxies, most observations suggest that the bigger the galaxy, the bigger the supermassive black hole it hosts. This suggests that there must be an intimate link between supermassive blackhole growth and galaxy evolution.

In a new study published in Nature Astronomy, an international team led by Dr Francesco Shankar of the University of Southampton, UK, researcher set out to explain the link between the size of supermassive black holes and the size of their hosts.

Measuring the mass of a supermassive black hole is usually achieved by measuring the velocity of the surrounding stars or gas, which has been proved to be challenging. However, galaxies and their supermassive black holes are believed to reside in haloes made of dark matter.

Numerical simulations show that more massive black haloes deviate more from a random spatial distribution, more strongly clustered. Thus, their clustering strength can be used to weigh halos. It is assumed that 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. Additionally, astronomers can use this to constrain the masses of the black holes themselves.

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.”

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