* Astronomy

The ATLAS3D Project: Replacing the Handle of Hubble's Tuning Fork

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 spheroidal systems, with less rotation and often non-axisymmetric shapes. This clear distinction is emphasised in Hubble's tuning-fork diagram, where early-type galaxies (E0-E7 and S0) lie on the handle of the fork, well separated from spiral galaxies (Sa-Sc). 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 recognise 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 recognise a disk from a spheroid at any inclination. However it requires complex and time-consuming observations. To obtain these observations, the ATLAS3D team was created: a collaboration of about 25 astrophysicists from 6 countries in Europe and North America, led by Michele Cappellari (Oxford, UK), Eric Emsellem, Davor Krajnovic (ESO, Germany) and Richard McDermid (Gemini, USA).

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