Left panel: theoretical IRX-to-β relations for mixed AGN and host-galaxy UV contributions. A AGN dominance of 30% (solid line) to 70% (dashed line) has been adopted. For comparison, the theoretical IRX-to-β relation by Meurer et al. (1999) with used to evaluate the UV host-galaxy contribution (dot-dashed line) and the relation by Pettini et al. (1998) for the UV AGN contribution with (dotted line) are also shown superimposed to the data sets by Barišić et al. (2017 black squares), Dong & Wu (2016, cyan dots) and García-González et al. (2016, magenta crosses). Right panel: least-squares fit of Eq. (8) (solid line) to the high-z galaxy data by Barišić et al. (2017) and the AGN samples by Dong & Wu (2016) and García-González et al. (2016), along with its boundaries at 1σ (dashed lines) and 2σ level (dotted lines).