Weighing the most distant black holes

A new technique that can measure the mass of supermassive black holes may revolutionise our understanding of their formation and how they help to shape galaxies.

The method involves detecting the tell-tale tracer of carbon monoxide within a cloud of mostly hydrogen gas circling a supermassive black hole at the centre of a distant galaxy. The researchers detect the velocity of the spinning gas, and from this they are able to determine the black hole’s mass.

The team used the new method on the supermassive black hole at the centre of a galaxy, NGC 4526, in the constellation of Virgo.

Tim Davis from the European Southern Observatory who is lead author of the paper published in Nature said: “We observed carbon monoxide molecules in the galaxy we were monitoring using the Combined Array for Research in Millimetre-wave Astronomy (CARMA) telescope. With it super-sharp images we were able to zoom right into the centre of the galaxy and observe the gas whizzing around the black hole. This gas moves at a speed which is determined by the black hole’s mass, and the distance from it. By measuring the velocity of the gas at each position, we can measure the mass of the black hole.”

Little detailed information exists on supermassive black holes; it has taken 15 years to measure the mass of just 60 of them. This is because other supermassive black holes are just too far away to examine properly, even with the Hubble Space Telescope.

This novel method that uses new telescopes such as CARMA and ALMA (Attacama Large Millimetre/submillimetre Array) promises to extend the ‘weighing-in’ of black holes to thousands of distant galaxies. The techniques will also enable the study of black holes in spiral galaxies.

Michele Cappellari from Oxford University’s Department of Physics and an author of the paper said: “Because of the limitations of existing telescopes and techniques we had run out of galaxies with supermassive black holes to observe. Now with this new technique and telescopes like ALMA we will be able to examine the relationship between thousands of more distant galaxies and their black holes giving us an insight into how galaxies and black holes co-evolve.”

Reference: 'A black-hole mass measurement from molecular gas kinematics in NGC4526'