Press Release

Diet of Gas and Dust Makes Black Holes Put on Weight

By SpaceRef Editor
March 20, 2000
Filed under

RAS Press Notice

Date: 20 March 2000
For immediate release

Ref. PN 00/03


Dr Jacqueline Mitton
RAS Press Officer
Office & home phone: Cambridge ((0)1223) 564914
FAX: Cambridge ((0)1223) 572892

This press release and accompanying figures can also be found at

Contacts for this release:

Professor Michael Merrifield
phone: (0) 115 951 5186
mobile: (0) 411 382612

Dr Duncan Forbes
phone: (0) 121 414 6474

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 27 March 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
1st April.

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.”



Caption: An artist’s impression of a supermassive black hole. The swirling disk 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.

This image may be reproduced in connection with this story and should be credited to CXC/A. Hobart.

A low resolution JPEG version is attached, and can be downloaded from the WWW at:

A higher resolution JPEG version is available at

These images can also be downloaded from


‘Bulges and Massive Black Holes’
Monday March 27 2000, 11am-5pm
NAPL Lecture Theatre,
Nuclear and Astrophysics Laboratory, Keble Road, Oxford
Organisers: Ofer Lahav (Cambridge) and Joe Silk (Oxford)

We now know that most bulges contain massive nuclear black holes, and that these black holes power quasars and most AGN. But did
bulges or black holes form first, or is the formation of a bulge generally accompanied by the formation of a massive black hole? Have
bulges been significantly modified by their embedded black holes, either through the influence of outflows and jets from the holes, or as a
consequence of the black holes scattering stars and particles of dark matter? What makes some black holes active and others dormant?
What roles do black holes play when their bulges merge? Are there naked massive black holes or bulges without central black holes?

The purpose of this informal one-day meeting is to discuss the observational evidence for, and the theoretical understanding and
implications of, the correlation between massive black holes and bulges.

This is the second in a new series of one-day seminars on topics in physical cosmology. They are being organized alternately at the
Universities of Oxford and Cambridge, at roughly two-month intervals.

Contact for information on the meeting in Oxford on 27th March:
Prof. Joe Silk
Nuclear and Astrophysics Laboratory
Keble Road
Oxford OX1 3RH, UK

Tel: +44 (0)1865 273300
Fax: +44 (0)1865 273390

Vol. 313, Number 2, published 1st April 2000. Page L29.

SpaceRef staff editor.