3rd August 2005

Voracious black holes hide their appetite in dusty galaxies 

A UK-led team of astronomers reports today (August 4th) in Nature that they have tracked 
down an elusive population of black holes growing rapidly hidden behind clouds of dust. 
Their results suggest that most black hole growth takes place in dusty galaxies, solving 
astronomer's headaches, as until now, the cosmic x-ray background suggested the existence 
of more growing black holes than they could find.

Growing black holes, known as quasars, are some of the brightest objects in the Universe 
and are seen by the light emitted as gas and dust spiral into the black hole. Quasars are 
situated in the inner-most regions of galaxies and can consume the equivalent mass of between 
ten and a thousand stars in one year! Astronomers believe that all quasars are surrounded 
by a dusty ring which hides them from sight on Earth in about half of cases.

However, examining the cosmic x-ray background, which is made up primarily of the emissions 
from quasars, astronomers realised that there should be many more obscured quasars than 
currently known. Objects surrounded by dust are hard to see with visible light, so the 
astronomers looked at infrared wavelengths, which are less likely to be reflected away. 
Using NASA's Spitzer Space Telescope's First Look Data, they were able to find a new 
population of obscured quasars. The new quasars have no spectra that can be seen and are 
thought to be hidden behind the dust of the galaxy itself rather than just a dust ring. The 
presence of lots of dust in a galaxy indicates that stars are still forming there. The 
researchers found 21 examples of these lost quasars in a relatively small patch of sky. All 
of the objects were confirmed as quasars by the National Radio Astronomy Observatory's Very 
Large Array radio telescope, New Mexico, and the Particle Physics and Astronomy Research 
Council's William Hershel Telescope on La Palma.

Alejo Martinez-Sansigre from the University of Oxford explains "We were missing a large 
population of obscured quasars, which had been inferred from studies at X-ray frequencies. 
This newly discovered population is large enough to account for the X-ray background, and 
now we wish to find out why there are more obscured quasars than unobscured ones".

>From their study, the team believes that there are more quasars hidden by dust than not 
and that most black holes grow in short, efficient bursts at the heart of growing galaxies.

Professor Richard Wade, Chief Executive of the Particle Physics and Astronomy Research 
Council which supports the University of Oxford Astrophysics group said "The new population 
of Quasars suggest that throughout cosmic history most black holes grow in the heart of 
dusty active galaxies with stars still forming."

Notes for Editors

1. NASA's Jet Propulsion Laboratory (JPL) manages the Spitzer Space Telescope mission 
for NASA's Science Mission Directorate, Washington. Science operations are conducted at 
the Spitzer Science Center at the California Institute of Technology (Caltech) in Pasadena, 
Calif. JPL is a division of Caltech. Additional information about the Spitzer Space Telescope 
is available on the Internet, at: http://www.spitzer.caltech.edu/spitzer 

2. The Isaac Newton Group of Telescopes (ING) is an establishment of the Particle Physics 
and Astronomy Research Council (PPARC) of the United Kingdom, the Nederlandse Organisatie 
voor Wetenschappelijk Onderzoek (NWO) of the Netherlands and the Instituto de Astrofísica 
de Canarias (IAC) in Spain. The ING operates the 4.2-metre William Herschel Telescope, the 
2.5-metre Isaac Newton Telescope, and the 1.0-metre Jacobus Kapteyn Telescope. The telescopes 
are located in the Spanish Roque de Los Muchachos Observatory on La Palma, Canary Islands, 
which is operated by the Instituto de Astrofísica de Canarias (IAC).

2. The paper can be viewed at http://arxiv.org/abs/astro-ph/0505486
3. A scientific image showing one of the newly discovered Quasars are available from www.pparc.ac.uk/Nw/hidden_quasars


Julia Maddock
PPARC Press Office
Email Julia.maddock@pparc.ac.uk
Tel +44 1793 442094

Alejo Martinez-Sansigre
University of Oxford
Email: ams@astro.ox.ac.uk
Tel +44 1865 273 337

NASA Contact
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, Calif.

Chris Willott
National Research Council Canada
Email: chris.willott@nrc-cnrc.gc.ca
Tel +1 250 363 8103

The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science 
investment agency. It funds research, education and public understanding in four areas of 
science - particle physics, astronomy, cosmology and space science. 

PPARC is government funded and provides research grants and studentships to scientists in 
British universities, gives researchers access to world-class facilities and funds the UK 
membership of international bodies such as the European Laboratory for Particle Physics (CERN), 
and the European Space Agency. It also contributes money for the UK telescopes overseas on 
La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal 
Observatory, Edinburgh and the MERLIN/VLBI National Facility, which includes the Lovell 
Telescope at Jodrell Bank observatory.

PPARC's Public Understanding of Science and Technology Awards Scheme funds both small 
local projects and national initiatives aimed at improving public understanding of its 
areas of science.