The ATLAS3D Project: Calling A Different Tune

Article written: 21 Jun , 2011
Updated: 24 Dec , 2015


In 1926, astronomer Edwin Hubble gave us our first basic galaxy classification scenario – the Hubble Sequence. Using photographic plates, Hubble derived a simplistic system based on three visually known structures: elipitical, spiral and lenticular. This sequence, when plotted out, gave the appearance of a common object and eventually became known as the “Hubble Tuning Fork” (as seen above). For many decades, this served as a standard. Now the ATLAS3D Project is calling a different tune…

Just who is the pied piper in this merry band? The ATLAS3D project is a multiwavelength survey combined with a theoretical modelling effort. The observations it takes spans from the radio to the millimetre and optical. It provides multicolor imaging, as well as two-dimensional kinematics of the atomic, molecular and ionized gases, together with the kinematics and population of the stars, Where does it dance? Only around a carefully selected, volume-limited sample of 260 early-type galaxies.

Heading up the project is a team of 25 astronomers from Europe and Northern America, including ASTRON astronomers Morganti, Oosterloo, and Serra – and all with a mission – to update and revise our understanding of galactic evolution. Employing the SAURON spectrograph on the 4.2-meter William Herschel Telescope on La Palma, the team was able to distinguish stellar movement in the pre-determined galactic candidates. These new assessments show that spheroid galaxies belong to the spiral galaxy classification. How did they come to that conclusion? The largest portion of spheroids – or early types – are basically the same family as spirals and evolve along a similar line. But with ATLAS3D findings, we’re looking at new concepts.

Maps of the observed velocity of the stars in the volume-limited sample of 260 early-type galaxies of the ATLAS3D survey. Red/blue colours indicate stars moving away/towards us respectively. Fast rotating and disk-like galaxies are characterized by two large and symmetric red/blue peaks at the two sides of the centre. This figure shows that this class of objects constitutes the vast majority of the sample. Credit: ATLAS3D Project

We’re seeing beyond the optical (photographic plates) which founded Hubble’s original diagram – where once galaxies were separated by their distinct characteristics such as rapid rotators rich in stars and gas – or as slowly moving, gas-poor models. Up until now, it was also next to impossible to distinguish sparse “face-on” structure from edge-on spheroids. With the aid of kinematic data astronomers can “see” rotation – allowing observation of all galaxy types from any angle.

“Slow and fast rotators tend to be classified as ellipticals and lenticulars, respectively, but the contamination is strong enough to affect results solely based on such a scheme: 20 per cent of all fast rotators are classified as ellipticals, and more importantly 66 per cent of all ellipticals in the ATLAS3D sample are fast rotators.” says the team. “Our complete sample of 260 ETGs leads to a new criterion to disentangle fast and slow rotators which now includes a dependency on the apparent ellipticity. It separates the two classes significantly better than the previous prescription.”

While it will take many years and many more observations to sort out all the new data, it would seem that our current understanding of galactic evolution just might need a “tune up”.

Oringinal Story Source: ASTRON.

4 Responses

  1. Member

    Yo Tammy, referring to the “multicolorr” in the third line of the second paragraph, did you intend that to be the British spelling “multicolour” or the American spelling “multicolor”?

  2. Jon Hanford says

    Readers may want to compare the Hubble “fork” diagram above with that proposed. Fig. 1 of the paper presents their ‘comb-like’ diagram and features several galaxies selected from this study:

    The authors note that this new diagram is similar to a classification scheme first proposed by Sidney van den Bergh (waaay back in 1976) whereby S0 galaxies are seen to mirror the spiral sequence of Sa-Sb-Sc and introduces a class of intermediate ‘anemic’ spirals (Aa-Ab-Ac) as transition objects:…206..883V&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

    The current paper also notes that their derived relation breaks down when applied to dense galaxy clusters, suggesting a different formation mechanism shapes these cluster galaxies.

  3. Torbjörn Larsson says

    This article put me in a fast rotation mode (head wise) and I was oscillating as a tuning fork until I saw this in the paper:

    “From a purely empirical point of view the difference between fast rotator ETGs and spiral galaxies is in their dust content, visible as spiral arms in optical photometry, and their cold gas content, as detected via molecular lines, which is significantly lower than spiral galaxies (Young et al. 2011, hereafter Paper IV).”

    So it is less a matter of which music moves the sphericals and non-sphericals, and more a matter of cleaning your room!? I feel like a teenager under parents all over again…

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