Fraction of recovered artificial LBGs as a function of redshift. This figure shows that the selection criteria adopted for this work cover the window in redshift that is left by the selection criteria of LBGs at z ∼ 5 and z ∼ 6. Moreover, this redshift window corresponds to the redshift of the two z ∼ 5.7 C iv absorption systems in the field. In order to consistently compare the evolution with redshift, only objects with M_UV ≤ −21.5 are included. Green dot–dashed, red dashed and dark blue solid curves correspond to objects selected with the z ∼ 5 Ri′z′-LBG selection of Ouchi et al. (2004a), the z ∼ 6 i-dropout criteria and the z ∼ 5.7 LBG selection from this work, respectively. The left-hand panel shows that the fraction of LBGs with EW(Lyα) ≤ 20 Å selected with the colour window described in Section 3.2 is maximum at the redshift of the C iv systems. The right-hand panel shows that LBGs with EW(Lyα) > 20 Å selected with the colour criteria for this work lie at higher redshift (z ∼ 5.9). This source of contaminants, however, is low and can be easy to identify from spectroscopy due to the Lyα emission line.