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

Abstract

Bars are an elongated structure that extends from the centre of galaxies, and about one-third of disk galaxies are known to possess bars 1,2,3 . These bars are thought to form either through a physical process inherent in galaxies 4,5,6 , or through an external process such as galaxygalaxy interactions 7,8,9 . 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 interaction 10 . 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 diskdominated 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.We used a volume-limited sample of galaxies with log(M star /M ) ≥ 10.0 in the MPA-JHU catalogue, and selected 105 clusters with M 200 > 7 × 10 13 M in the redshift range of 0.015 < z < 0.060. Here, M star is the galaxy stellar mass, M the mass of Sun and M 200 the cluster halo mass (see Methods). Of these, 16 clusters were found to be in pairs or have substructures (see Methods) and are defined here as interacting clusters. Figure 1 shows examples of the surface number density maps, velocities, and spatial distributions for galaxies of clusters in isolation, in pairs, and with substructures.As detailed in Methods, we detected bars in the cluster member galaxies using a quantitative method that searches for an elongated structure that has a large ellipticity for several consecutive isophotal ellipses but a sudden drop beyond a certain radius, and a nearly constant position angle over only the high-ellipticity region. We complemented the bar classification with a visual inspection to exclude false classifications and add bar galaxies that were not detected by the automated method. This process also examines the radial surface brightness profiles of galaxies, from which we derived the bulge-tototal light ratio (B/T ) of each galaxy. Example images of barred and non-barred galaxies are presented in Fig. 2.The bar fraction of interacting clusters and noninteracting clusters as a function of B/T is shown in Fig. 3a. We find that the bar fraction is several times higher for galaxies with B/T ≤ 0.5 (hereafter, disk-dominated galaxies) than those with B/T > 0.5, in agreement with results from previous studies 2,11,12 . The results of previous numerical simulations also support the finding that the formation or maintenance of bars is prohibited in galaxies with high B/T or high central mass concentrations 13,14,15 . It is also known that bar formation can be suppressed in galaxies that are dynamically hot (random motions of stars are dominant) or have high velocity dispersions 4,5 . Given that bars are...

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