June 22, 2011
TORONTO, ON – Astronomers usually only see flat versions of the real galaxies in the Universe, as pictures projected on their computer screens. But researchers in the international collaboration ATLAS3D have used a clever trick to figure out what 260 galaxies do in the third dimension missing from their images.
They added motion to the picture by measuring the velocities of stars going through the plane of the image. Using this method, the ATLAS3D researchers were able to tell whether stars were part of a flattened spinning disk or a motionless balloon-shaped blob (called elliptical galaxy).
This advance should help them figure out how galaxies change as they age and collide with one another.
Spirals, Disks, and Blobs
We’ve all seen pictures of spinning galaxies with elegant spiral arms and dust lanes in-between their stars.
Even when seen edge-on, the dust lanes are visible, so it’s easy to tell that these galaxies have disks.
But some disk galaxies have little gas or dust, or lack spiral arms. Without such structures, they appear as rounded shapes when seen face-on, or oval ones when seen at an angle.
These featureless galaxies can easily be confused with elliptical galaxies, which are football-shaped, but barely spin at all.
Taking a Hit
By telling us how fast stars in a galaxy rotate around their galaxy’s centre, the ATLAS3D result changes our understanding of galaxies and how they evolve over time. Not only could the astronomers find out whether a football-shaped blob is a motionless galaxy or a spinning disk, they can learn more about what happens when galaxies hit each other.
Young galaxies seem to have lots of gas that hasn’t yet been turned into burning stars —and they spin fast, so they look like the poster-child galaxy with spiral arms and dust lanes.
As galaxies collide or combine, they cause each other’s gas to spark into new stars. Older galaxies have less gas, little dust, and no spiral arms.
More Than Meets the Eye
The ATLAS3D researchers found that, of those older galaxies without spiral arms or dust lanes, many more did have a spinning disk structure than previously thought —even though they had been classified as motionless elliptical galaxies from their two-dimensional picture.
These galaxies have already been through collisions, and if they have kept spinning, it may mean that collisions do not ruin the orderly rotation of their disks so much. The ATLAS3D team has already prepared computer simulations and performed more telescope observations to test this idea.
The ATLAS3D researchers used the 4.2-m William Herschel Telescope on La Palma, Canary Islands, during 40 nights to make their observations. They relied on the SAURON integral-field spectrograph to map out the speed of stars in 260 nearby galaxies.
Dr. Anne-Marie Weijmans
Dunlap Institute for Astronomy & Astrophysics
University of Toronto
Dr. Michele Cappellari
Department of Physics
University of Oxford