Tiny black hole lurks in neighbouring galaxy
Jul 19, 2001
The first galaxy without a super-massive black hole at its centre has been found by astronomers. If there is any sort black hole in the nearby galaxy M33 - which is about 3 million light years away - it must be thousands of times less massive than the black holes in other galaxies, according to the study by David Merritt and colleagues at Rutgers University. The discovery is likely to shape new theories of the evolution of black holes and their host galaxies (D Merritt et al 2001 Science to appear).
Merritt and colleagues studied images of the spiral galaxy M33 that were taken by the Hubble Space Telescope, and had a resolution ten times better than previous Earth-based observations. The team measured the velocities of stars circulating in the nucleus of M33. In other galaxies, the gravity of the central black hole makes the stars closest to it move very rapidly. But Merritt's team found that stars close to the centre of M33 were moving slowly.
Merritt and colleague Laura Ferrarese recently discovered how the mass of the central black hole in a galaxy relates to the velocities of the stars in orbit around it. This allowed them to calculate that the black hole in M33 must be less than 3000 times the mass of the Sun, even accounting for the orbital eccentricities and gravitational interactions of the stars. This means the black hole at the centre of M33 is of 'intermediate' mass at most.
Astronomers previously thought that all galaxies were powered by super-massive black holes, and believed that large and small black holes evolved differently. But the search for the black hole at the centre of M33 suggests that these ideas may be flawed.
"The upper limit on the mass is still consistent with the relation that we discovered a year ago, and implies that small black holes - if they exist - formed in much the same way as very massive ones", says Ferrarese.
About the author
Katie Pennicott is Editor of PhysicsWeb