IRX as a function of the slope difference Δβ for the HerS/SDSS quasars by Dong & Wu (2016, blue dots), the Seyfert galaxies by García-González et al. (2016, blue crosses), and the high-z galaxies by Barišić et al. (2017). The high-z galaxies are classified according to the statistical analysis presented in Sect. 3 (red triangles: actual galaxies, yellow squares: unclassifiable objects, green stars: actual quasars). The curves available in the literature that describe the IRX-to-Δβ relation for various classes of galaxies (Pettini et al. 1998; Meurer et al. 1999; Penner et al. 2012; Casey et al. 2014; Talia et al. 2015; Forrest et al. 2016) are also shown.