tokens: Charlot ( 2003 ) 5 for the MILES spectral library (Snchez-Blzquez etal. 2006 ) using a Chabrier ( 2003 ) stellar initial mass function (IMF) over the range 0.1100M . The components for the base models comprised 16 ages from 1Myr to 13Gyr for each of four metallicities ( Z 0.004, 0.008, 0.02, 0.05). We note that tests with somewhat different base model sets and choice of IMF did not significantly affect continuum subtraction and so do not affect the emission-line results on which our analyses rely. Our procedure largely mirrors that performed on MUSE data of supernova hosts elsewhere (e.g. Galbany etal. 2016a ). 3.3 Emission-line fitting Emission-line fitting was performed on the spectra after subtracting the best-fitting continuum, as determined in Section 3.2 . Prior to fitting, the continuum-subtracted spectra were median filtered with a width of 120 this width was chosen in order to remove any broad residuals left by the imperfect continuum fitting, whilst leaving the narrow emission lines unaffected. The emission-line model was a composite model of single Gaussians initially centred at the wavelengths of H , [O iii ] 4959,5007, H, [N ii ] 6548,6583 and [S ii ] 6716,6731 (the two exceptions to this set-up, SNe 2002bp and 2008ge, are detailed later). The fitting was performed via the weighted LevenbergMarquardt least-squares algorithm. Such fitting is, however, susceptible to finding local minima solutions that are dependent on the initial guesses for the Gaussian parameters. In order to circumvent the dependency on initial guesses, and to restrict to physically plausible solutions we added the following caveats and steps to our fitting routine: The velocity offset of the Gaussian means were restricted to within 500km s 1 relative to the galaxys rest frame (i.e. after accounting for the redshift of the galaxy nucleus). This comfortably encloses the limits of rotational velocities seen in late-type galaxies (Sofue Rubin 2001 ) whilst preventing misidentification of lines. The velocity offset of the Balmer and forbidden lines were each tied, i.e. a single velocity offset for H and H was fit and a separate offset was fit for the other lines. The standard deviations of the Gaussians were restricted to 130km s 1 and pairs of lines for [O iii ], [N ii ] and [S ii ] had tied values for the standard deviation (in km s 1 ). In order to semibrute force locate the global minimum, a list of several initial guesses for each of the means, standard deviations and amplitudes to be fit was made. A grid was then formed of all possible combinations of these parameters to use as initial guesses for the LevenbergMarquardt fitter, which found the local minimum for each initial guess. The residual between the data and model was used as an estimator of the goodness of fit for each grid position and used to identify the best overall fit. Where the fitting process failed, we manually inspected the emission-line spectrum. In most cases, low signal to noise or
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