...)

Misleading velocity curves can also be a result of instrumental effects. If the seeing disc is noticeably smaller than the slit width and our object is clumpy (which is the case of HzRG) the clumps occupy different spatial positions in the direction perpendicular to the slit. The centroid of the emission lines will be shifted in the spectral direction. Another negative effect is that the sky or arc lines may not represent the true instrumental profile and therefore we need to derive it from the seeing value or, better, from a point source along the slit.

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