HST WFC3 image in the F606W filter in the main panel. The image spans approximately 60′′ by 60′′, which is 220 kpc by 220 kpc at the redshift z = 0.232 of ASASSN-15lh and corresponds to the field of view of our MUSE integral field spectroscopy (Sect. 2.3). The six panels above and to the right of the WFC3 image show extracted MUSE spectra of the host (plus transient) and the five additional galaxies at a similar redshift. These galaxies are denoted SFI, SFII, and SFIII for the three star-forming galaxies and EI and EII for two passive ellipticals in the main image. Other galaxies visible in the image are either fore- or background sources. The size of the MUSE point spread function and a physical scale at z = 0.232 are indicated in the top left and bottom right corner of the image, respectively.