A team of 25 astronomers from Europe and Northern America, including ASTRON astronomers Morganti, Oosterloo, and Serra, has shown that many galaxies, which are normally classified as spheroid galaxies according to the 70 year old Hubble classification scheme, are in fact spiral galaxies.

The so-called ATLAS3D team observed a sample of 260 galaxies with the SAURON spectrograph on the 4,2m William Herschel Telescope on La Palma, which allowed them to determine the movements of the stars in these carefully selected galaxies. The results are important because it gives astronomers more information about the way galaxies form.

The team proposed a revised scheme in which the vast majority of spheroid galaxies, also known as early-type galaxies, are close relatives of spiral galaxies and for this reason form a parallel sequence to them. The new paradigm highlights a much closer connection between early-type and spiral galaxies than previously thought, and this will need to be considered in future models of how galaxies form. The above results were presented in three ATLAS3D team papers which will appear this month on the journal Monthly Notices of the Royal Astronomical Society.

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. The spirals are characterised by the presence of disks of stars and gas in rapid rotation, while the early-types are gas poor and described as spheroid systems, with less rotation and often non-axisymmetric shapes. This clear distinction is emphasized in Hubble's tuning-fork diagram, where early-type galaxies lie on the handle of the fork, well separated from spiral galaxies. The separation is physically relevant as it implies a distinct path of formation for the two classes of objects.

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. The solution to the problem comes from observations of the stellar kinematics: the stars in a thin disk rotate much faster than those in a rounder spheroid. This implies that the kinematics makes it possible to recognize a disk from a spheroid at any inclination. However it requires complex and time-consuming observations.

The new results were unexpected and reveal a new paradigm for early-type 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. This indicates that Hubble's classic tuning-fork gives a misleading description of galaxy structure.

Source: Astron Netherlands Institute for Radio Astronomy