Observational evidence for bar formation in disk galaxies via cluster–cluster interaction

Bars are elongated structures that extend from the centre of galaxies, and about one-third of disk galaxies are known to possess bars1–3. These bars are thought to form either through a physical process inherent in galaxies4–6, or through an external process such as galaxy–galaxy interactions7–9. However, there are other plausible mechanisms of bar formation that still need to be observationally tested. Here we present the observational evidence that bars can form via cluster–cluster interaction10. We examined 105 galaxy clusters at redshift 0.015 < z < 0.060 that are selected from the Sloan Digital Sky Survey data, and identified 16 interacting clusters. We find that the barred disk-dominated galaxy fraction is about 1.5 times higher in interacting clusters than in clusters with no clear signs of ongoing interaction (42% versus 27%). Our result indicates that bars can form through a large-scale violent phenomenon, and cluster–cluster interaction should be considered an important mechanism of bar formation.