The interacting nature of dwarf galaxies hosting superluminous supernovae

Abstract

Context. Type I superluminous supernovae (SLSNe I) are rare, powerful explosions whose mechanism and progenitors remain elusive. Several studies have shown a preference for SLSNe I to occur in low-metallicity, actively star-forming dwarf galaxies. Aims. We investigate whether the host galaxies of SLSNe I show increased evidence for interaction. Galaxy interaction can trigger star formation and provide favourable conditions for these exceptional explosions to take place. Methods. Based on SLSN host galaxy images obtained with the Hubble Space Telescope (HST), we narrowed down a sample of 42 images obtained in the rest-frame ultraviolet over the redshift range between 0 < z < 2. The number of host galaxy companions was measured by counting the number of objects detected within a given projected radius from the host. As a comparison, we used two different Monte Carlo-based methods to estimate the expected average number of companion objects in the same HST images, as well as a sample of 32 dwarf galaxies that have hosted long gamma-ray bursts (GRBs). Results. About 50% of SLSN I host galaxies have at least one major companion (within a flux ratio of 1:4) within 5 kpc. The average number of major companions per SLSN I host galaxy is 0.70−0.14+0.19. Our two Monte Carlo comparison methods yield a lower number of companions for random objects of similar brightness in the same image or for the SLSN host after randomly redistributing the sources in the same image. The Anderson-Darling test shows that this difference is statistically significant (p-value < 10−3) independent of the redshift range. The same is true for the projected distance distribution of the companions. The SLSN I hosts are, thus, found in areas of their images, where the object number density is greater than average. The SLSN I hosts have more companions than GRB hosts (0.44−0.13+0.25 companions per host distributed over 25% of the hosts) but the difference is not statistically significant. The difference between their separations is, however, marginally significant with SLSN companions being closer, on average, than those of GRBs. Conclusions. The dwarf galaxies hosting SLSNe I are often part of interacting systems. This suggests that SLSNe I progenitors are formed after a recent burst of star formation. Low metallicity alone cannot explain this tendency.