Royal Astronomical Society 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


RAS web:

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

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

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.