Since Edwin Hubble introduced his famous tuning fork diagram more than 70 years ago, spiral galaxies and early-type galaxies have been regarded as being two distinct families.
A known issue of Hubble’s classification, however, is that it mostly relies on optical images, from which it is nearly impossible to recognize thin face-on disks of stars from much rounder edge-on spheroids.
For this reason the fraction of disks-like systems hidden in the early-type category has been a matter of debate for decades. Using SAURON integral-field spectrograph at the William Herschel Telescope, astronomers of the ATLAS3D collaboration
were able to derive maps of the stellar motions for 260 galaxies. For the first time, it was found that the overwhelming majority of the early-type galaxies in the nearby Universe does not consist of roundish spheroidal objects,
but instead has disks and mostly resembles spiral galaxies with the gas and dust removed. Only a tiny fraction of the early-type galaxies – the “slow rotators” – are genuine spheroids.