HEIC0903: FOR IMMEDIATE RELEASE 14:00 (CET)/09:00 am EDT 12 March, 2009
http://www.spacetelescope.org/news/html/heic0903.html

News Release: The NASA/ESA Hubble Space Telescope provides new evidence 
for dark matter around small galaxies

12-March 2009 The NASA/ESA Hubble Space Telescope has uncovered a 
strong new line of evidence that galaxies are embedded in halos of dark 
matter.

Peering into the tumultuous heart of the nearby Perseus galaxy cluster, 
Hubble discovered a large population of small galaxies that have 
remained intact while larger galaxies around them are being ripped 
apart by the gravitational tug of other galaxies.

The Hubble images provide further evidence that the undisturbed 
galaxies are enshrouded by a “cushion” of dark matter that protects 
them from their rough-and-tumble neighbourhood.

Dark matter is an invisible form of matter that accounts for most of 
the Universe’s mass. Astronomers have deduced the existence of dark 
matter by observing its gravitational influence on normal matter, such 
as stars, gas and dust.

 “We were surprised to find so many dwarf galaxies in the core of this 
cluster that were so smooth and round and had no evidence at all of any 
kind of disturbance,” says astronomer Christopher Conselice of the 
University of Nottingham, UK, and leader of the team that made the 
Hubble observations. “These dwarfs are very old galaxies that have been 
in the cluster for a long time. So if something was going to disrupt 
them, it would have happened by now. They must be very, very dark-
matter-dominated galaxies.”

The dwarf galaxies may have an even higher amount of dark matter than 
spiral galaxies. “With these results, we cannot say whether the dark 
matter content of the dwarfs is higher than in the Milky Way Galaxy,” 
Conselice says. “Although, the fact that spiral galaxies are destroyed 
in clusters, while the dwarfs are not, suggests that this is indeed the 
case.”

First proposed about 80 years ago by Swiss astronomer Fritz Zwicky, 
dark matter is thought to be the glue that holds galaxies together. 
Astronomers suggest that dark matter provides a vital scaffolding for 
the Universe, forming a framework for the formation of galaxies through 
gravitational attraction. 

Observations by Hubble’s Advanced Camera for Surveys spotted 29 dwarf 
elliptical galaxies in the Perseus Cluster, located 250 million light-
years away and one of the closest galaxy clusters to Earth. Of these 
galaxies, 17 are new discoveries.

Because dark matter cannot be seen, astronomers detected its presence 
through indirect evidence. The most common method is by measuring the 
velocities of individual stars or groups of stars as they move randomly 
in the galaxy or as they rotate around the galaxy. The Perseus Cluster 
is too far away for telescopes to resolve individual stars and measure 
their motions. So Conselice and his team derived a new technique for 
uncovering dark matter in these dwarf galaxies by determining the 
minimum additional mass contribution from dark matter that the dwarfs 
must have to protect them from being disrupted by the strong, tidal 
pull of gravity from larger galaxies.

Studying these small galaxies in detail was possible only because of 
the sharpness of Hubble’s Advanced Camera for Surveys. Conselice and 
his team first spied the galaxies with the WIYN Telescope at Kitt Peak 
National Observatory. These observations, Conselice says, only hinted 
that many of the galaxies were smooth and therefore dark matter 
dominated. “Those ground-based observations could not resolve the 
galaxies, so we needed Hubble imaging to nail it,” he says.

The Hubble results appeared in the 1 March issue of Monthly Notices of 
the Royal Astronomical Society.

###

Notes for editors
The Hubble Space Telescope is a project of international cooperation 
between ESA and NASA.

Image credit: NASA, ESA and C. Conselice (University of Nottingham)
 
If you wish to no longer receive these News and Photo Releases, please 
send an e-mail to distribution@spacetelescope.org with your name.


For more information, please contact:

Colleen Sharkey
Hubble/ESA, Garching, Germany
Tel: +49-89-3200-6306
Cell: +49 0151 1537 3591
E-mail: csharkey@eso.org

Christoher J. Conselice
School of Physics and Astronomy
University of Nottingham
Tel: +44 0115 951 5137
E-mail: conselice@nottingham.ac.uk

Samantha J. Penny
School of Physics and Astronomy
University of Nottingham
Tel: +44 115 951 5183
E-mail: ppxsp@nottingham.ac.uk

Sven de Rijcke
Tel: +32 9 264 47 91
Fax: +32 9 264 49 89
E-mail: sven.derijcke@ugent.be

Enrico V. Held
Osservatorio Astronomico di Padova
Vicolo dell'Osservatorio 5
Tel: +39 049 829 3409
E-mail: held@oapd.inaf.it