Athanasia Tsatsi, a graduate student at Max Planck Institute for Astronomy (MPIA), has redefined astronomers' understanding of galaxy mergers that result in unusual stellar motion, according to a press release. Astronomers were working with the idea that retrograde - or opposite - orientations of the merging galaxies caused their differences. However, Tsatsi found that mass loss from the galactic bodies was acting as a "rocket engine."
Elliptical galaxies (shaped like semi-flattened spheres) feature stars in the outer regions that rotate around the center while stars at the core rotate in a different direction. Galaxies like our Milky Way were formed by a collision of two or more disk galaxies. Previously, "kinematically decoupling" - or counter-rotation - was blamed for the core formation, but as Tsatsi looked at galaxy mergers through a computer simulation, she found, "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," according to the press release.
"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," the press release continued. "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."
Tsatsi's findings involved a solitary case, but it is considered sufficient proof of concept that Meshchersky mechanism is possible.
