Dark Matter in Galaxies
For some time now, it has been known that most of the matter in galaxies is invisible--so called dark matter. Dark matter cannot be detected directly, its effects can only be inferred gravitationally. However, "weighing" a galaxy is no simple task. The phenomenon of gravitational lensing provides a powerful way to measure the dark matter in galaxies. Gravitational lensing occurs when the radiation from a distant source is bent by the gravity of a massive object, like a galaxy, that lies between us and the source. When this happens, the source appears highly distorted and multiple images of it can be seen around the lensing galaxy.
Since gravitational lensing is sensitive to all the matter in a galaxy, including the dark matter, it tells us about the total mass in a galaxy. By subtracting the mass in stars and gas, it is possible to measure the properties of the dark matter in galaxies.
RG astronomers are using this technique to study the basic morphology of the dark matter in galaxies to test theories of galaxy formation. Since gravitational lensing is achromatic, it affects radiation from the source across the electromagnetic spectrum, from x-rays down to radio waves. Although many of the known gravitational lenses have been discovered at optical wavelengths, an advantage of using radio wavelengths is that the radio waves pass virtually untouched through the lensing galaxy. This allows us to study the lensing effect, and hence dark matter properties, over a range of distances from the center of the lensing galaxy.
A high redshift radio lobe is gravitationally lensed into an "Einstein Ring" by a galaxy. The image shows radio emission at 3cm wavelength in contours overlaid on a greyscale image from the Hubble Space Telescope's NICMOS near-infrared camera. The radio image was made using data from the Very Large Array (VLA) radio telescope. The point-like source to the lower right is the core of the distant radio galaxy that produced the radio lobe that is lensed. It is not significantly lensed itself because it is too far from the center of the lensing galaxy.
