A fundamental connection between supermassive black holes — the most
massive singular objects in our Universe — and the galaxies at whose
centers these objects reside has been discovered by a team of
astronomers working at the Instituto de Astrofisica de Canarias (IAC)
in the Canary Islands, Spain.

Astronomers at the Instituto de Astrofisica de Canarias (IAC) —
Alister Graham, Peter Erwin, Nicola Caon, and Ignacio Trujillo — have
discovered a fundamental connection between supermassive black holes —
the most massive singular objects in our Universe — and the galaxies
at whose centers these objects reside. This connection was found by
studying the global distribution of stars within galaxies, the
concentration of which is related to the mass of the central
supermassive black hole. The work is to be published in The
Astrophysical Journal Letters.

Supermassive black holes are one million to over a billion times more
massive than our Sun. Even so, the ‘event horizon’ (the surface from
which no light can escape because of the strength of the gravitational
field) of a one million solar mass black hole is only about 4 times
larger than our Sun — they are very dense objects. The Sun weighs
2 times 10^(30) kg (almost five hundred thousand times heavier than
the Earth), and is 1.4 million kilometers in diameter. These
supermassive black holes have been found in a large number of
galaxies, including our own.

The global distribution of stars within both elliptical galaxies and
the bulges of spiral galaxies has been found to be directly related to
the mass of a galaxy’s central supermassive black hole. More massive
galaxies are not simply bigger versions of less massive galaxies, as
has been frequently assumed in the past. Their entire structure is
different: more massive galaxies are more centrally concentrated.
The precise degree of concentration has now been shown to correlate
extremely well with the mass of the central supermassive black hole.
“This is an important realization which provides further insight into
the formation of both galaxies and their central black holes. We now
know that any viable theory of supermassive black hole growth must be
connected with the (eventual) global structure of the host galaxy,”
according to project leader Dr Alister Graham.

It would appear quite natural that more centrally concentrated
galaxies — which are those which have stronger gravitational potential
wells — can more efficiently supply gas and material to fuel their
central black holes. The researchers note, however, that it is possible
that the processes which shaped the galaxy and built the supermassive
black hole operated in tandem. Whether or not smaller primordial black
holes existed before galaxies formed around them remains an open

Observational implications

This discovery also has important practical advantages. The discovery
last year of a relation between central black hole mass and galaxy
velocity dispersion (a measure of average stellar motions within a
galaxy) meant that it was possible to estimate a galaxy’s central black
mass from the galaxy’s velocity dispersion. Unfortunately, this is a
time-consuming process: to obtain a velocity dispersion measurement,
the light from a galaxy must first be dispersed into its constituent
wavelengths (colors). This dilutes the galaxy light and thus requires
long exposure times. With this latest discovery, astronomers can now
predict black hole masses directly from the images of galaxies, by
measuring the radial fall-off in luminosity and thus determining its
concentration. This way, thousands of very distant, high-redshift
galaxies can be studied cheaply and effectively. Astronomers hope
to gain a deeper understanding of the evolution of these enigmatic
objects, which appear prevalent throughout our Universe.

This work is based on archival data obtained with the NASA/ESA Hubble
Space Telescope, and the Isaac Newton Group of Telescopes at the
Spanish Observatorio del Roque de Los Muchachos (La Palma).

Notes for editor

The scientific article will be published in a December 2001 edition
of the Peer Reviewed international journal “The Astrophysical Journal”.
The full scientific article can however be viewed immediately at:

[Small image (60KB)

Large image (1MB)

Artist’s impression of a supermassive black hole at the center of a
galaxy, and a star in it’s proximity. Credit: Gabriel Pérez Díaz,
SMM del IAC.


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IAC Press Office

Phone: +34 922 605 206 or +34 922 605 371

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Dr Alister Graham

Phone: +34 922 605 252

Fax: +34 922 605 210

email: agraham@ll.iac.es