Astronomer Edwin Hubble gave us our first basic galaxy classification. Using photographic plates, Hubble derived a simplistic system based on three visually known structures: elipitical, spiral and lenticular. For many decades, this served as a standard. Now the ATLAS3D Project is calling a different tune.
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 4.2-meter 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.
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