(Abridged) Type I superluminous supernovae (SLSNe I) are rare, powerful
explosions whose mechanism and progenitors remain elusive. SLSNe I show a
preference for low-metallicity, actively star-forming dwarf galaxies. We
investigate whether the hosts of SLSNe I show increased evidence for
interaction. We use a sample of 42 SLSN I images obtained with HST
and measure the number of companion galaxies by counting the objects detected
within a given radius from the host. As a comparison, we used two Monte
Carlo-based methods to estimate the expected average number of companion
objects in the same images, as well as a sample of 32 galaxies that have hosted
long gamma-ray bursts (GRBs). About 50% of SLSN I hosts 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 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 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. 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. 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.Comment: Accepted for publication in A&A. In v2 replaced graphs with higher
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