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 bars^1–3. These bars are thought to form either through a physical process inherent in galaxies^4–6, or through an external process such as galaxy–galaxy interactions^7–9. 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.