Dark matter has never been directly detected--scientists infer its existence from other measurements. In particular, the speed at which gas moves in the outer parts of spiral galaxies implies a large gravitational pull and points to the existence of unseen matter. In elliptical galaxies, however, there is no such gas to track. Thus, although researchers have assumed that these galaxies, too, are surrounded by halos of dark matter, they have been unable to test that idea--until now.
A collaboration led by Aaron J. Romanowsky of the University of Nottingham designed and built a new instrument, the Planetary Nebula Spectrograph, to locate and obtain spectra from aging stars located on the outskirts of elliptical galaxies. By analyzing the wavelength of light coming from the planetary nebulae, the researchers could determine how fast they were moving. (In the image above, red dots indicate planetary nebulae moving away from the telescope, blue dots represent movement toward it. The size of the dot denotes the relative speeds of the nebulae.) If elliptical galaxies and their spiral counterparts contained similar amounts of dark matter, the team would have detected very high velocity measurements. "Instead, the relatively low speeds of planetary nebulae we actually observed are what we would expect if there were little or no dark matter around these galaxies," Romanowsky says. So far, of the three elliptical galaxies tested, none appear to house dark matter. According to team member Michael R. Merrifield, also of the University of Nottingham, measurements of 20 more elliptical galaxies are planned. The additional findings should help clarify the dark matter picture. "It's a bit of a leap to say all elliptical galaxies in the universe don't have dark matter," he notes. "But if we find some elliptical galaxies have dark matter and some don't, perhaps we can determine what the deciding factor is."