Astronomers find ’naked’ galaxies devoid of dark matter

An international team of astronomers has discovered that “dark matter”, the mysterious material that seems to make up most of the mass of galaxies, is not as all-pervasive as previously believed. Surprising new results from studies of several elliptical galaxies show they are not surrounded by halos of dark matter as was expected. The findings will be presented at the UK/Ireland National Astronomy Meeting in Dublin on Wednesday April 9th by Dr Aaron Romanowsky of the University of Nottingham.

Dark matter was first discovered in galaxies in the 1970s using studies of gas in the outer parts of these systems. The high speeds at which this gas was found to be travelling implied a large gravitational pull, and hence that there must be large amounts of unexplained mass far from the centres of galaxies. Unfortunately, only the beautiful spiral galaxies contain the gas that allows such measurements to be made; the other main class of galaxy, the elliptical systems, cannot be studied in this way. It has, however, long been assumed that these galaxies are also enveloped by similar “dark halos”.

Now, though, the new study casts serious doubts on this seemingly-reasonable assumption. A team of astronomers from Australia, Italy, the Netherlands and the UK has developed and built a new instrument, the Planetary Nebula Spectrograph, which is capable of detecting and measuring the velocities of planetary nebulae in the outer parts of elliptical galaxies. Planetary nebulae are stars in the final stages of their lives. They are bright enough to be detected even in quite distant galaxies, and their motions can be used to infer the amount of mass in the previously unexplored outer parts of ordinary elliptical galaxies.

With this instrument the team have made the first systematic study of velocities in the outer parts of ordinary elliptical galaxies. They have clear results from three galaxies and supporting data from several others.

“We were expecting to find the same kinds of high velocities that are found in the outer parts of spiral galaxies,” said Dr Romanowsky. “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.”

“We were certainly surprised by the result, but there are some clues as to what might be going on,” commented team member Professor Michael Merrifield. “Elliptical galaxies are mostly found in dense galaxy clusters, and this makes for a pretty rough environment with frequent collisions between galaxies. This kind of violent interaction might well also be responsible for stripping away these galaxies’ dark halos. However, this is just speculation, and as yet we have no detailed picture as to how these naked systems of stars might have formed.”