Spiral galaxies are the most common and well-studied type of galaxies in the Universe. In spite of this, the origin of their most spectacular feature, the spiral arms, is not yet fully understood. Among many theories, it has been proposed that grand-design, double spiral arms can be created by tidal interactions with nearby objects, for example dwarf galaxies.
Marcin Semczuk and collaborators from the Nicolaus Copernicus Astronomical Center in Warsaw investigated another possibility: whether spiral arms in a Milky Way-like galaxy could be induced by tidal interactions with a galaxy cluster. They performed N-body simulations tracing the evolution of a galaxy orbiting a dark matter halo with properties similar to Virgo, a nearby galaxy cluster. A clear, two-armed spiral structure appeared in the galactic disk during the pericenter passage and then it wound up and dissolved. The new generations of spiral arms were created repeatedly during the next orbital periods. The authors were able to identify at least nine galaxies in the Virgo cluster very similar to those simulated, for which there is no evidence of an interaction with a smaller body. The upper row of the figure shows the images of three out of these nine galaxies from the Sloan Digital Sky Survey. In the lower row there are images of the morphological analogues of these galaxies simulated by the astrophysicists from the Copernicus Center.
The results were obtained as part of the project “Dynamics and morphology of interacting galaxies” funded by the National Science Centre within the Maestro program. The article "Tidal Origin of Spiral Arms in Galaxies Orbiting a Cluster", by Marcin Semczuk, Ewa L. Łokas and Andrés del Pino, describing these findings has been recently published in The Astrophysical Journal and is also available from the website.
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