Astronomy Now Home

Spheroid galaxies are "spirals without spirals"
Posted: 23 June 2011

Bookmark and Share

New observations of galaxies originally identified as spheroids according to Hubble's 85-year old classification scheme show that the majority are in fact more closely related to spiral galaxies than previously believed.

Spiral galaxies and spheroid galaxies have been regarded as two distinct families since Edwin Hubble devised his Tuning Fork diagram of galaxy classification in 1926. In this scheme spirals are characterized by the presence of discs of stars and gas in rapid rotation, whereas early ellipticals, or spheroids, have little gas and less rotation. An intermediate between the two at the central node of the the fork lie lenticulars, which are likened in terms of morphology to an elliptical galaxy, but with dynamics similar to spirals.

Hubble's "Tuning Fork" diagram of galaxy classification. AN graphic by Greg-Smye Rumsby.

But these initial classifications relied on optical observations of galaxy shape, making it difficult to distinguish thin face-on discs of spiral galaxies from rounder edge-on spheroids. To probe this issue, the international ATLAS3D team of astronomers selected 260 galaxies to study with the SAURON spectrograph on the 4.2-metre William Herschel Telescope on La Palma, which allows observations of stellar motions. Since the stars in a thin disc rotate much faster than those in a rounder spheroid, studying the kinematics made it possible to recognize a disc from a spheroid at any inclination.

"The galaxies were selected based on their optical appearance," ATLAS3D team member Tom Oosterloo who is based at the Netherlands Institute for Radio Astronomy, ASTRON, tells Astronomy Now. "We considered all known galaxies out to a distance of 42 mega-parsecs [approximately 137 million light years] and brighter than a certain luminosity, and selected those which showed no sign of spiral structure on optical images."

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 disc-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.

The astronomers found that around 85 percent of the target galaxies turned out to be so-called fast rotators, that is, their dynamical characteristics are very similar to spiral galaxies, and only a tiny fraction of the early-type galaxies – the “slow rotators” – are genuine spheroids.

"We propose a revision of the classification scheme," he says. "Instead of lenticulars being an intermediate class between non-rotating ellipticals and rotating spirals, we propose that instead they form a parallel sequence to the spiral galaxies. That is, their dynamical similarity to spirals is more important than their morphological similarity to ellipticals. One reason is that in the past, morphology was the main property on which galaxies were classified but it turns out that many galaxies were misclassified as being elliptical because of this."

The team liken the new diagram to a comb-like structure, where elliptical galaxies are parallel to spirals and linked to them along the teeth of the comb, while only a few true ellipticals are separated into the handle.

The newly proposed comb diagram for galaxy classification. Image courtesy Michele Cappellari.

The implication is that there is a much closer connection between early-type and spiral galaxies than previously thought, and models will have to consider how these lenticular galaxies form and evolve. "In a way, these lenticular galaxies (which we now call 'Fast Rotators') are spirals galaxies without spirals arms – 'spirals without spirals'," says Oosterloo. "We suspect that interactions with neighbouring galaxies are more important in shaping a galaxy than previously thought."

ATLAS3D is an international collaboration of researchers across Europe and North America. The UK's Principal Investigator, Michele Cappellari of the University of Oxford, is lead author of the paper describing the new findings in an upcoming issue of the Monthly Notices of the Royal Astronomical Society. "It’s an exciting moment, after four years of work in the project, we have the final piece of the puzzle which enables us to say that text-books used to teach astronomy for over 70 years now need to be revised," he says.