Astronomers at the Universities of Nottingham and Birmingham have uncovered the first direct evidence that the extremely massive black holes lurking at the centres of galaxies have gradually put on weight by consuming a steady diet of gas and stars. This discovery is to be presented at the OXCAM2 conference in Oxford on March 27, 2000, where astronomers will be discussing recent developments in the study of supermassive black holes. A paper on the subject will be published in the Monthly Notices of the Royal Astronomical Society on April 1.

An artist's impression of a supermassive black hole. The swirling disc of gas contains material in the final stages of falling into the central black hole. Such accretion explains the observed progressive increase in black holes' masses with age. Photo: CXC/A. Hobart.
 

It has been known for a number of years that the centres of almost all galaxies contain small, very massive, dark objects. Such an object can weigh in excess of a billion times the mass of the Sun, yet may occupy a region not much larger than the solar system. The only explanation that astronomers have been able to come up with for such extreme properties is that these objects are supermassive black holes, but very little is known about how these exotic objects came to be at the centres of so many galaxies. Were the black holes there before the galaxies formed around them, or have they grown over time by sucking in some of the stars and gas that make up their host galaxies? What makes this a difficult question to answer is that the galaxies we see today have typically been in existence for many billions of years, so the rate at which a black hole would have to acquire mass to build up to its current size is far too low to be detectable.

In order to get around this problem, Professor Michael Merrifield of the University of Nottingham and Drs Duncan Forbes and Alejandro Terlevich of the University of Birmingham have adopted a different approach. As Prof Merrifield explains, "If you didn't know how people grow as they get older, you wouldn't have to watch one individual over a complete lifetime to find out; just by looking at a snapshot of a large family that spans a range of ages from toddler to great-grandparent, you could infer that children grow quite rapidly for the first decade or so of their lives, but that older people don't continue to develop at anywhere near the same rate. We have used the same reasoning to discover how black holes grow with age."

To determine the ages of galaxies, the astronomers have compared the detailed properties of the starlight they emit to what would be expected for galaxies of differing ages. Using this technique, they have been able to determine the ages of 23 nearby galaxies, including such familiar objects as the Andromeda Galaxy, which are known to contain black holes at their centres. The analysis revealed a wide range in the ages of these galaxies, from a youthful four billion years to a venerable twelve billion years. Comparing the ages to the masses of the central black holes, the researchers discovered that the masses of black holes in young galaxies tend to be relatively modest, while older galaxies contain progressively more massive black holes.

It thus appears that these black holes have built up to their current stature by acquiring mass over the entire lifetime of the galaxies that they live in, with no signs that this growth has come to an end. "One of the basic properties of a black hole is that material can fall into it, but can't get out again," said Merrifield. "What we seem to be seeing is the consequence of this one-way traffic, with gas and stars from the surrounding galaxy dragged in by gravity, making each black hole more and more obese as it gets older."