Total mass against the spectral index for the RPS sample (orange markers and labels) and the other galaxies in the sample (blue markers). For clarity, uncertainties are only displayed for the RPS sample, they are of comparable size (σ_α ∼ 0.1) for the other galaxies. Downward pointing arrows correspond to 4σ upper limits. The orange and blue line show the corresponding best-fitting relations, and the red dashed line shows the fit from Heesen et al. (2022).