IRAM PdBI continuum-subtracted spectrum of the [C ii] 158 μm emission line of SDSS J1148+5251. The three spectra shown have been extracted using different apertures, namely circular apertures with diameters of 2 arcsec (left panel), 4 arcsec (central panel) and 8 arcsec (right panel). Note the different scales on the y-axes in the three panels. For display purposes, the spectra have been re-binned by a factor of 2, corresponding to channels of 46.8 km s^-1. The Gaussian fits to the line profiles (using a narrow and a broad Gaussian component to fit respectively the narrow core, tracing quiescent gas, and the broad wings, tracing the outflow) are performed on the original non-binned spectra and the fits results are reported in Table 1.