A new method for weighing super-massive black holes

Paris, 29 November 2018 Scientists have revealed, for the first time outside our Galaxy, the properties of rapidly moving gas clouds in the immediate vicinity of a super-massive black hole, enabling the mass of the black hole to be measured with unprecedented accuracy. The measurement was carried out using the GRAVITY instrument on the Very Large Telescope (VLT, European Southern Observatory) by an international team led by the Max Planck Institute for Extraterrestrial Physics and including researchers from the CNRS, Observatoire de Paris - PSL, Université Grenoble-Alpes and Observatoire de la Côte d'Azur. Researchers usually estimate the mass of super-massive black holes located in the heart of galaxies by observing the motion of stars or gas rotating around them: broadly speaking, the faster they rotate, the more massive the black hole is. For distant galaxies, direct measurement of the motion of gas very close to the black hole has until now been impossible, as these regions of gas are too small to be observable. To estimate the mass of the central black hole, astrophysicists therefore measure the time delay between the emission of light from the immediate environment of the black hole and its reverberation from the gas clouds, and use this to infer the size of the gas structure and, hence, the mass of the black hole. This method is known as 'reverberation mapping'. In this new study, astrophysicists used the VLT's GRAVITY instrument to investigate 3C 273, the first quasar ever identified, located in the centre of a galaxy about 2.5 billion light years away.