Thursday, March 12, 2009

New Evidence of Dark Matter from Hubble?

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

Dark matter can't be directly seen or isolated in a laboratory. Yet it makes up the bulk of the matter in the universe. It is the invisible scaffolding for the formation of stars and galaxies. Dark matter is not made of the same stuff that stars, planets, and people are made of. That stuff is normal "baryonic" matter, consisting of electrons, protons, and neutrons. For 80 years astronomers have known about dark matter's "ghostly" pull on normal matter. They've known that without the gravitational "glue" of dark matter galaxy clusters would fly apart, and even galaxies would have a hard time holding together.

Now the 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's sharp view resolved 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 dwarfs' "invisible shield" is a robust halo of dark matter that keeps them intact despite a several-billion-year-long bumper-car game inside the massive galaxy cluster.

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 those 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 mass 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 outside Tucson, Ariz. Those 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 March 1 issue of the Monthly Notices of the Royal Astronomical Society.

Astronomer Christopher Conselice of the University of Nottingham, U.K., and leader of the Hubble observations. Other team members are Samantha J. Penny of the University of Nottingham; Sven De Rijcke of the University of Ghent in Belgium; and Enrico Held of the University of Padua in Italy.


Friday, March 06, 2009

Trio of Galaxies mix it up

This NASA Hubble Space Telescope image shows three galaxies playing a game of gravitational tug-of-war that may result in the eventual demise of one of them.

Located about 100 million light-years away in the constellation Piscis Austrinus (the Southern Fish), the galaxy interaction may ultimately lead to the three reforming into two larger star cities.

The three galaxies—NGC 7173 (middle left), NGC 7174 (middle right), and NGC 7176 (lower right)—are part of Hickson Compact Group 90, named after astronomer Paul Hickson, who first cataloged these small clusters of galaxies in the 1980s.

NGC 7173 and NGC 7176 appear to be smooth, normal elliptical galaxies without much gas and dust.

In stark contrast, NGC 7174 is a mangled spiral galaxy that appears as though it is being ripped apart by its close neighbors. The galaxies are experiencing a strong gravitational interaction, and as a result, a significant number of stars have been ripped away from their home galaxies. These stars are now spread out, forming a tenuous luminous component in the galaxy group.

Ultimately, astronomers believe that NGC 7174 will be shredded and only the two "normal" elliptical galaxies (NGC 7173 and NGC 7176) will remain.