Astronomers at the Universities of Nottingham and Birmingham in the UK have obtained the first direct evidence that the massive black holes at the centres of galaxies put on weight as they get older by eating gas and stars.

It has been known for a number of years that the centres of almost all galaxies contain small, very massive, black holes.

Little is known about how they came to be at the centres of all galaxies. It may be that they were there before the galaxies formed around them, or have they grown over time by sucking in some of the stars and gas from their host galaxies?

Family snapshot

To answer this question astronomers have gathered data on galaxies of all ages.

According to Professor Michael Merrifield of the University of Nottingham, "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."

They determined the ages of 23 nearby galaxies, including such well known 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 four billion to 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 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."

The research will be published in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society