Over the past decade or so, astronomers have amassed an impressive body of evidence that enormous black holes with masses between a million and a billion times the mass of the Sun lurk at the centre of many nearby galaxies, including our own Milky Way. Now astronomers are beginning to study the relationship between these gravitational beasts and the parent galaxies in which they are embedded.

Super-massive black holes are found at the heart of nearly all spiral galaxies. The right image was taken with the STIS aboard the Hubble Telescope showing the highly Doppler shifted starlight (blue is receding, red is approaching) from the black hole at the centre of M84, shown on the left. Photo: Space Telescope Science Institute

For over ten years, astronomers have been monitoring the motion of stars and ionised gas trailing round the centres of nearby galaxies. Gas spiralling round a highly concentrated gravitational field generated by a black hole results in huge orbital velocities -- up to several hundred kilometres per second -- which register as large red and blue shifts as the gas recedes and approaches us, respectively. Once these velocities are known, it is then a simple matter to estimate their masses using Newton's law of universal gravitation.

One long standing question in black hole research is whether they form after their parent galaxies, or whether they originate from the merging of smaller black holes from less massive galaxies. Now a group of U.S. astronomers have taken another step closer to answering this question.

At a conference held in Rochester New York in early June, the results of a survey of large galaxies located within 120 million light years of the Milky Way conducted by a team of 15 US astronomers was announced. Using the Hubble Space Telescope Imaging Spectrograph (STIS), the team found 8 new candidates, bringing the total number of super-massive black holes known to 33.

The team was surprised to observe a near-perfect correlation between the orbital speeds of the stars inhabiting the nuclear bulge and the absolute mass of the black holes at their centres. The more massive the black hole, the more swiftly the bulge stars moved, even though the majority of their constituents are much too far away from the black holes to "feel" their gravity directly.

What's more, they showed that each black hole is roughly 1/500th the mass of its attendant galactic bulge. Together, these data provide strong evidence that the nuclear bulge and the black holes at their centres are dynamically linked. "If black holes are unusually massive whenever galaxies are unusually collapsed, then the black hole masses must be determined by the collapse process," says John Kormendy of the University of Texas, Austin.

Although the surveys performed to date can only be considered as preliminary and incomplete, they nonetheless lend support to the idea that black holes and their parent galaxies grew up together. They reveal that the bulge and the super-massive black holes at their centres are a single dynamical unit and hence may have formed and evolved in tandem.

But much work still remains to be done. "We have a long way to go before we understand the symbiosis between black holes and galactic bulges," says Andrew Wilson of the University of Maryland. Future work must involve surveys of greater numbers of galaxies, located at greater distances from the Milky Way. In addition, computer simulations have not addressed the relationship between the masses of the black holes and their galactic bulges. Only after these studies have been completed can astronomers hope to gain a deeper insight into the genealogy of the galactic components.

Neil English is a regular contributor to Astronomy Now. His book, Cosmic Puberty is published by the Lutterworth Press.