Observed [NII] 205 μm spectra of the sources in our sample. The orange histogram represents the 1D spectra of submillimeter galaxies, while the red histogram represents the spectra of quasar hosts. The best Gaussian plus continuum fit is shown in solid blue. The dashed blue line represents the mean dust continuum flux for sources undetected in the [NII] 205 μm emission. The gray bar represents the mean error (RMS) of the spectrum, while the black bar represents the width of the [CII] 158 μm line used to estimate the upper limit on the integrated line flux for the non-detections (see Sect. 3.1). The zero velocity is with respect to the reference redshift in Table 1.