Rethinking Galaxies

In 1936 Edwin Hubble came up with the idea of using a tuning fork to classify galaxies. He placed the elliptical (E) galaxies along the handle and normal and barred spirals (S and Sb) on the two prongs. At the junction were lens-shaped "lenticular" systems (S0), which he considered intermediate in structure.

Hubble's classification scheme has remained popular among amateurs and professionals for more than 70 years. But the two dozen international researchers who've been working on the Atlas3D project think it might be time for change.

Atlas3D began in 2006 as an ambitious project to map 260 elliptical and lenticular galaxies (commonly called early-type galaxies) in three dimensions, using integral-field spectroscopy to obtain two-dimensional maps of the motions of their stars. Over the years, the team headed by Michele Cappellari (Oxford University), Davor Krajnović, Eric Emsellem (ESO) and Richard McDermid (Gemini Observatory) has published many papers that have projected these early-type galaxies in new light.

Early-type galaxies have an interesting history. When Hubble began his observations, he assumed that the featureless ellipticals were infant galaxies that would gradually amass dust and gas, grow into adolescent lenticular systems and then evolve into complex spirals. So the phrase "early-type galaxy" is actually a misnomer. Later research showed that the opposite is more likely true: ellipticals and lenticulars are actually old galaxies that likely evolved from aging spiral systems.

"The reason we are focusing on early-type galaxies is that they are thought to constitute the endpoint of galaxy evolution," said Cappellari. "They have already exhausted their gas from which stars were formed, and are now 'red and dead'. For these reasons they constitute an ideal place to look for the fossil record of billions of years of galaxy evolution."

In optical images, these early systems (built of old stars and little gas or dust)appear smooth and plain, while spiral galaxies show arms bursting with areas of active star formation, and disks rich in gas and dust. Also while most stars in a spiral rotate in the same direction within its central plane, yielding a high net rotation, stars in early-type galaxies are generally assumed to move randomly in a rounder shape, canceling each other's angular momentum and giving such systems very little net rotation.

But by combing images and rotational data, the Atlas3D team found that the majority of early-type galaxies, even ellipticals that were thought to be slowly rotating spheroids, rotate almost as fast as spirals do. Based on this finding, the team suggests that these early-type galaxies be reclassified as "slow rotators" and "fast rotators". Before the study, the 260 early-type galaxies in the sample divided into 68 ellipticals and 192 lenticulars. However, from its velocity mapping, the team believes that they could be better classified as 36 slow rotators and 224 fast rotators.

"These are not physical distinctions," said team member Anne-Marie Weijmans, (University of Toronto). "The fast-rotating early-type galaxies still look like ellipticals and lenticulars. They are not spirals because they do not have spiral arms and a disk."

"Appearances can be deceiving," said Tom Oosterloo (ASTRON, The Netherlands), another member of the team. "Even objects that appear elliptical, rather round and featureless on the sky, can be inclined and fast-rotating disks," Cappellari explains. "Our surprising new result is that the fraction of misclassification is as high as 66% for galaxies classified as elliptical by professional astronomers!"

The team has used these findings to suggest an alternative to Hubble's tuning fork: the Atlas3D comb diagram. It positions early-type galaxies along the handle in increasing order of their rotational speed, and spirals will occupy three teeth instead of two prongs. The fast-rotating galaxies, placed at the junction of a tooth and the handle, are likely to have evolved from that family of spirals.

Cappellari believes that this new approach should be kept in mind, for example, by the citizen-science project Galaxy Zoo, which teamed researchers with interested lay individuals to classify galaxies based on their shapes in optical images. And ultimately the Atlas3D team hope that its "comb" diagram will replace Hubble's beloved tuning fork.