Relations between ionising photon production efficiences ξ_ion and stellar masses M_⋆ (a) and SFR^−0.9M_⋆ (b) for samples of CSFGs and high-z SFGs. The solid line in (b) is the maximum likelihood relation whereas stacks for z ∼ 1.4–2.6 SFGs (Shivaei et al. 2018) for Calzetti et al. (1994, 2000) and SMC reddening laws are shown in (a) by green and magenta lines, respectively. Mean ξ_ion values of z = 4–5 galaxies (Lam et al. 2019b) in the three stellar mass bins are shown in (a) by asterisks, whereas z ∼ 6.6 SFGs with Lyα emission by Endsley et al. (2020) are represented in both panels by filled circles. Values of ξ_ion assumed in canonical Universe reionisation models are shown with a thick horizontal grey line (e.g., Bouwens et al. 2016). The meanings of symbols for the SDSS CSFGs are the same as in Fig. 1.