New evidence for dark matter around small galaxies

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 12 March, 2009

Astronomers studying the Perseus galaxy cluster with the Hubble Space Telescope have uncovered a new line of evidence that suggests galaxies are embedded in halos of dark matter.
 
Hubble was used to peer into the heart of the Perseus galaxy cluster, where it discovered a large population of small galaxies that have remained intact while larger galaxies around them are being ripped apart by the gravitational pull of neighbouring galaxies. This provides strong evidence that the dwarf galaxies are being cushioned by a protective sheath of dark matter.

“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 lead astronomer Christopher Conselice of the University of Nottingham. “These dwarfs are very old galaxies that have been in the cluster 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.”

Four dwarf galaxies observed in the Perseus Galaxy Cluster. They appear smooth and symmetrical, suggesting they have not been tidally disrupted - evidence that dark matter may be present. Image: NASA, ESA, and C. Conselice and S. Penny (University of Nottingham).

Dark matter, although invisible, accounts for the majority of the Universe’s mass. It’s existence is inferred from the gravitational influence it exerts on normal matter, like stars, gas and dust, by looking at the velocities and motions of stars as they move randomly through a galaxy. Dark matter is also thought to act like glue, binding galaxies together and providing the Universe with a framework for which they can form and grow.

The Perseus Cluster, however, at a distance of 250 million light years, is too far away for telescopes to resolve individual stars and measure their motions. Conselice and his team therefore came up with the idea of determining the minimum mass the dwarfs must have to protect them from being disrupted by the strong tidal pull of gravity from larger galaxies, in order to determine if dark matter was present in the galaxies.

Conselice had already observed the galaxies using the WIYN Telescope at the Kitt Peak National Observatory in Arizona, but those results only hinted that many of the galaxies were smooth and therefore possibly dark matter dominated. It took the powerful eyes of Hubble’s Advanced Camera for Surveys to find the evidence Conselice was looking for.

The location of four of the dwarf galaxies within the Perseus Galaxy Cluster. Image: NASA, ESA, and Z. Levay (STScI), D. De Martin (ESA/Hubble), and C. Conselice (University of Nottingham).

Hubble made observations of 29 dwarf elliptical galaxies in the Perseus Cluster, 17 of which were new discoveries. The results revealed that the dwarf galaxy population may have higher amounts of dark matter than the spiral galaxies. “With these results, we cannot say whether the dark matter content of the dwarfs is higher than in the Milky Way Galaxy,” says Conselice. “Although, the fact that spiral galaxies are destroyed in clusters, while the dwarfs are not, suggests that is indeed the case.”

Previous studies with Hubble and NASA’s Chandra X-ray Observatory found evidence of dark matter in entire clusters of galaxies such as the Bullet Cluster, and the new Hubble observations continue the search for dark matter in individual galaxies.

The results of the study are reported in the 1 March edition of the journal Monthly Notices of the Royal Astronomical Society.