Galactic "rocket engine" explains unusual stellar motion in galaxies

                                                                                                                                                                                A discovery by MPIA graduate student Athanasia Tsatsi has changed astronomers' understanding of how mergers of two galaxies can produce unusual stellar motion in the resulting elliptical galaxies, with the central region rotating in the direction opposite to that of the galaxy's other stars. Previously, such differences had been thought to be the result of an opposite ("retrograde") orientation of the galaxies prior to their merger. Looking at a simulation of a galaxy merger, Tsatsi discovered a different way of bringing about such counter-rotating cores, which involve mass loss from the bodies of these galaxies acting as a primitive galactic "rocket engine."

In so-called elliptical galaxies (which are shaped like somewhat flattened spheres), the movement of stars can show an intriguing pattern, with stars in the outer regions all rotating around the center in one way, while stars in the core region jointly rotate in a completely different direction.

Elliptical galaxies are the result of the collsion and merger of two or more disk galaxies (such as our home galaxy, the Milky Way). Previous explanations had assumed that counter-rotating (or, more generally, "kinematically decoupled") cores can form if one of the merging galaxies has a tightly bound core region, with just the right orientation relative to the galaxies' orbit during the merger. However, this explanation predicts fewer counter-rotating cores than are actually observed.

That was the situation when Athanasia Tsatsi at the Max Planck Institute for Astronomy began to look at computer simulations of galaxy mergers. Tsatsi's aim was to find out how the resulting galaxies would look through astronomical instruments – but when looking through one such "virtual instrument" at one of the simulations, she made an unexpected discovery: The elliptical galaxy that resulted from the simulated merger contained a counter-rotating core. But the merger certainly did not have the specific ("retrograde") orientation required by the usual explanation of how such cores form!

A more detailed examination showed that the counter-rotating motion is directly linked to a change of direction of the galactic central regions during the merger due to the so-called Meschchersky force, or more prosaically: due to gigantic galactic rocket engines. As the galaxies merge, the central regions lose mass which, just like the gas expelled by a rocket engine, can cause their motion to change.

The result of the simulated merger was consistent with the observed examples for such counter-rotating cores: With 130 billion times the mass of the Sun, this was one of the more massive elliptical galaxies, where such cores were known to be more common. In the simulation, the counter-rotating core remains distinct from its surroundings for 2 billion years after the coalescence of the two galaxies, making for a phenomenon sufficiently persistent as to be observable in real galaxies. Finally, the counter-rotating stars consisted mostly of older stars that had been present before the collision, not the new generation of stars produced during the merger; this, too, was what observations of such systems had shown.

Read more at: http://phys.org/news/2015-03-galactic-rocket-unusual-stellar-motion.html#jCp