Black line: integrated spectrum of J1000+0234 SMG, extracted inside the white squares shown in right panel of Fig. 1, added by a constant (2 × 10^-16erg s^-1 cm^-2 Å^-1), for visual purposes. Red line: Sky spectra, which were derived as the median of the sky spectrum extracted from six different extraction regions located outside the overall structure of J1000+0234. Green line shows the residual between object and sky spectra and 1σ uncertainty of the sky spectrum is displayed with a blue line. Gray shaded area displays the 5σ level. [O ii] doublet wavelengths using redshifts from Lyα and ¹²CO(4−3) emission-lines are represented by dashed and dot-dashed vertical black lines, respectively (Capak et al. 2008; Schinnerer et al. 2008, see Table 1). Dotted vertical lines correspond to sky line wavelengths.