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How to measure a supermassive black hole

An international team of scientists have measured a supermassive black hole with unprecedented accuracy, using observations made by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

The black hole in question lies at the heart of a galaxy named NGC 1332, 73 million light years from Earth. It's 660 times more massive than our sun, and is circled by a cloud of gas moving at around 1.8 million kilometres per hour.

Published in The Astrophysical Journal Letters, the paper Measurement of the black hole mass in NGC 1332 from ALMA observations at 0.044 arcsecond resolution describes how the team used ALMA's high-resolution observations of carbon monoxide emissions from a giant disk of cold gas orbiting the black hole and measurements of the gas's speed to accurately create a model that reproduced the disk's kinematic properties – the characteristics of its motion.

The findings, which put the hole's mass at MBH = (6.64+0.65 −0.63) × 108 M⊙, show that high resolution observations, such as those produced by ALMA can be used to establish the size of holes, based on the dimensions of the gaseous disks orbiting them, assuming, as the paper notes, that the disks "exhibit suitably clean rotation".

Study co-author, professor Andrew J. Baker of Rutgers University in the USA says that "this has been a very active area of research for the last 20 years, trying to characterize the masses of black holes at the centers of galaxies. This is a case where new instrumentation has allowed us to make an important new advance in terms of what we can say scientifically."

Part of the importance of being able to make such measurements is that they'll help to further our understanding of the relationship between the growth of galaxies and that of their black holes.

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