Extended Data Fig. 3: The He and Hα emission-line profiles in the late spectra of SN 2020eyj show notable asymmetry.

The He i emission lines at 5,876 Å, 6,678 Å and 7,065 Å all show strong attenuation in the red wings and an apparent blue shift over time between the 131 and 251 days epochs. Such line asymmetry is commonly observed in SNe Ia-CSM (ref. ¹¹) and is interpreted as resulting from the condensation of dust in the ejecta or shocked CSM, obscuring the red wing (see ‘Dust properties’ section in Methods), but may also be a result of optical depth effects²⁰. The Hα emission line at 131 days also shows asymmetry and there is a (minor) decline in flux between the two epochs shown here. By 329 days, the Hα luminosity has dropped to the level of the line emission in the host spectrum (see ‘Optical spectroscopy’ section in Methods).