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Galaxy shapes explained by collisions and mergers
Posted: 12 January 2010

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The menagerie of shapes and sizes of galaxies in the Universe can be explained by whether they have endured collisions and mergers with other galaxies, with spiral galaxies experiencing barely any collisions at all in “quiet’ regions of the Universe, according to a new and detailed computer model.

The Hubble sequence, or tuning fork diagram, that classifies galaxies according to their shape – ellipticals, spirals and barred spirals. Irregular galaxies tend to be malformed thanks to a merger that is in process. AN graphic by Greg Smye–Rumsby.

Galaxies can be classified into ellipticals, spirals and barred spirals according to Edwin Hubble’s galaxy sequence (sometimes referred to as the tuning fork diagram). The prevailing model of how these galaxies formed is known as hierarchical formation – the idea that galaxies build up gradually from larger and larger mergers, eventually culminating in a major merger that creates a giant elliptical galaxy. We certainly see galaxies actively colliding in space, and find large ellipticals at the centres of galaxy clusters where hundreds or even thousands of galaxies are crammed into a relatively small space. Until now, however, the evolution of galaxies had never been shown quantitatively.

Elliptical galaxies, like M84 pictured here, formed from many mergers with other galaxies. Image: SDSS.

This has changed thanks to the work of Dr Andrew Benson of the California Institute of Technology and Dr Nick Devereux of the Embry-Riddle University in Arizona, who have used galaxy data collected by the Two Micron All Sky Survey (2MASS), which is an infrared survey of galaxies using two 1.3-metre telescopes in Arizona and Chile that cover both hemispheres. This data was then fed into a powerful computer model called GALFORM (developed in collaboration with the galaxy formation group at the University of Durham, UK), which is designed to simulate galaxy formation based on the physics of a Universe dominated by dark matter and dark energy (the so-called Lambda Cold Dark Matter model that best describes the Universe that we inhabit). Remarkably, GALFORM was able to simulate the entire 13.7 billion year history of galaxy formation in the Universe, ending up with a Universe filled with different types of galaxies that represent the variety that we see in the galactic population today. Furthermore, Benson and Devereux were able to scrutinize the simulation to determine what conditions drove galaxies to take on the shapes that they do.

Spiral galaxies, in general, populate “quiet’ region of the Universe where they are allowed time to develop. In spirals such as M101 pictured here, the central bulge will have formed first from minor mergers with dwarf galaxies. Image: NASA/ESA/K Kuntz (JHU)/F Bresolin (University of Hawaii)/J Trauger (JPL)/J Mould (NOAO)/Y-H-Chue (university of Illinois)/STScI.

For example, large elliptical galaxies such as M84 or M87 in the Virgo Cluster of galaxies are the children of large disc galaxies that smash together in what we call ‘major mergers’. This has been known for some time; indeed in cases such as the Centaurus A elliptical galaxy we can even still see the dusty remnants of the spirals. What turned out to be the most surprising result of the simulation however was that spiral galaxies endure barely any collisions, and that some spirals can go ten billion years without a single merger.

In their research, published in the Monthly Notices of the Royal Astronomical Society, Benson and Devereux point out the two types of spirals – those with classical central bulges and those without or with bars – at best experience a couple of minor mergers with small dwarf satellite galaxies. The classical bulges are generally made of older stars and were the first part of the galaxy to form from small mergers, while the younger stars in the spiral arms forming later. However, not all spiral galaxies have this style of bulge.

Barred spiral disc galaxies, such as NGC 1300 pictured here, experience few galaxy mergers and form their bars and bulges via disc instabilities. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA).

“Not all bulges of galaxies are made via mergers,” Benson told Astronomy Now. “In many cases the bulge formed when a disc became too massive and buckled under the influence of its own gravity. The bulges created in this way are expected to look subtly different from those formed via mergers, so future surveys of galaxies may be able to separate these two classes of bulge.” The search for these bulges could also potentially be incorporated into the Galaxy Zoo project, wherein hundreds of thousands of users log on to help classify galaxies.

Our own Galaxy, the Milky Way, is a spiral galaxy with a bar running through its centre. According to the GALFORM simulation, the Milky Way must have experienced only a few minor mergers, and some kind of disc instability that created the bar, which is a 27,000 light year long stream of stars and gas (see our related news story here). Often in barred galaxies, the bar leads to what is termed a pseudobulge. This bulge is much smaller than the classical bulge you might find in a galaxy like M31, the Andromeda Galaxy, and is younger too, formed from stars made from gas funnelled into the centre of the Galaxy from the disc via the bar. A classic example of a galaxy with a pseudobulge is the edge-on disc galaxy NGC 4565 in the constellation of Coma Berenices. And yes, there are indications that our own Milky Way Galaxy may also have a pseudobulge.

The collisions of large spiral galaxies in ‘major mergers’, such as this cosmic coming together in NGC 6050, drive the creation of elliptical galaxies. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA)/K Noll (STScI)/ESA–Hubble Collaboration.

“Disc galaxies tend to live in quiet regions of the Universe, with not much going on around them,” says Benson. However, our Local Group of galaxies is going to become rather noisy within the next few billion years when the Andromeda Galaxy collides with the Milky Way, with the consequent creation of a new ‘phoenix’ elliptical galaxy from their ‘ashes’.

This future merger will be just one of many mergers that are ongoing in the Universe. “Our results correctly predict the observed galaxy merger rate, and the specific star formation rate up to a redshift of 1 [around seven billion years ago], and we are currently working on verifying other observables,” adds Devereux. The other feather in the cap for the GALFORM model is its successful integration of the Lambda Cold Dark Matter model, boosting the reputation of that particular theory, which is crucial for cosmologists attempting to map the large scale structure and evolution of the Universe as a whole.