Illustration of redshift prediction with NMF basis vectors. (a) Rest-frame spectrum of the UDF10-4 source at redshift 0.7649 in blue. This galaxy exhibits stellar continuum, strong [O II], Hβ, and [O III] spectral emission lines (their locations are indicated on top of the figure); the best NMF reconstruction is plotted in black. (b) Corresponding χ² curve, the minimum happens at the true redshift (vertical red dashed line), hence, successfully predicting the correct redshift. The second minimum in the χ² curve at z ∼ 4.4 corresponds to a solution in which [O II] gets mistaken to be Lya. The values of the ∆χ² and R metrics are also reported in (b); their values indicate a significant minimum and a very robust redshift prediction.