In this figure, the top panels show three spiral galaxies in the Virgo cluster, imaged with the Sloan Digital Sky Survey. The bottom panels provide a comparison with three morphologically similar galaxies generated in simulations. The simulations — run by Marcin Semczuk, Ewa Łokas, and Andrés del Pino (Nicolaus Copernicus Astronomical Center, Poland) — were designed to examine how the spiral arms of galaxies like the Milky Way may have formed. In particular, the group explored the possibility that so-called “grand-design spiral arms” are caused by tidal effects as a Milky-Way-like galaxy orbits a cluster of galaxies. The authors show that the gravitational potential of the cluster can trigger the formation of two spiral arms each time the galaxy passes through the pericenter of its orbit around the cluster. Check out the original paper below for more information!
Marcin Semczuk et al 2017 ApJ 834 7. doi:10.3847/1538-4357/834/1/7