The way galaxies have been classified for decades has been questioned by an international team of astronomers. After revealing that two-thirds of local elliptical galaxies are actually fast-spinning discs, the team has suggested that the Hubble "tuning fork" – the long-standing method for classifying galaxies – may need retuning.

Galaxies come in all shapes and sizes: from flat spinning discs to almost-stationary blob-like elliptical galaxies. However, accurately classifying these huge objects can sometimes be tricky due to the angle from which they are observed. When seen face-on, older disc galaxies that have lost their distinctive dust lanes and spirals can masquerade as equally featureless, but spherical, elliptical galaxies. Elliptical galaxies are thought to have very little net rotation whereas disc galaxies rotate much faster. Measuring their rotation speed can therefore help distinguish between them.

Such a test has been performed using the ATLAS3D survey, led by Michele Cappellari at the University of Oxford, UK. The survey consists of 260 non-spiral galaxies in the nearby universe. "We divided each galaxy up into a grid and took spectra for each individual section," Cappellari told physicsworld.com. "By analysing these spectra we could measure the red-shift, or the blue-shift, of each section," he adds. If an area shows a red-shift, it is moving away from us; if it shows a blue-shift, it is coming towards us. If one limb of a galaxy is red-shifted and the opposite limb is blue-shifted then the galaxy must be rotating, and you can measure how fast.

More rotating discs

What surprised Cappellari and his colleagues was that 66% of the galaxies previously classified as elliptical were now shown by ATLAS3D to be fast-rotating discs. "Two-thirds of these galaxies are essentially no different from spirals that have had the gas and dust removed – they are 'naked' spirals," Cappellari explains. "Such a large fraction is not something one can ignore; it brings a significant change to our understanding of galaxy formation," he continues. This had led the team to make a distinction between non-spiral galaxies: the conventional ellipticals are "slow rotators" and the naked spirals are "fast rotators".

The result is threatening to overturn more than 80 years of conventional wisdom. Astronomers currently classify galaxies using a "tuning fork" diagram constructed by Edwin Hubble in the mid-1920s. His fork has non-spiral galaxies forming the handle, with the two different flavours of spiral galaxies – those with and those without barred centres – constituting the prongs. However, Cappellari's result shows that fast rotators may be more closely related to spirals than previously thought. "We feel our result could re-write the way textbooks on galaxy structure are written," he says.

The iconic Hubble tuning fork image could be replaced by the ATLAS3D "comb". The handle of the comb is formed by non-spiral galaxies in the order of their rotation speed, from slowest to fastest. The spiral galaxies then form three teeth, which attach to the handle at the end containing their fast-rotating, but naked, cousins. "In future when classifying galaxies in projects such as Galaxy Zoo, this is the picture that needs to be kept in mind," explains Cappellari.

Extending the survey

"It is an important result," says Karen Masters of the University of Portsmouth, UK, who uses data from Galaxy Zoo in her research. "Currently these fast rotators would likely be classified on Galaxy Zoo as non-disc galaxies because they are smooth and featureless. This research is showing that a large fraction of these galaxies do have a spinning disc, just one that can't be seen from the image. You have to go in and look at the dynamics," she told physicsworld.com. "It is a beautiful data set and it will be very interesting to see what happens if the survey is extended to include a larger sample," she adds.

Cappellari is planning just that. "I am involved in a proposal to increase our sample size by a factor of 100," he explains. But he is keen to stress that future results should not distract from these current findings. "No matter what we find in the future, we already have a major reinterpretation of the structure of local galaxies," he says.

The findings are published in Monthly Notices of the Royal Astronomical Society and on the arXiv preprint server.