Fig. 7

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Effect of changing the resolution on the σ measurement. First panel (from the left): comparison between the velocity dispersion values obtained from the combined spectrum and those obtained from the three single arms at their original resolution. The solid black line shows the one-to-one identity relation. Second panel: difference between the velocity dispersion computed from the combined and smooth spectra and those measured from the single-arm spectra as a function of the mean S/N of each system. The horizontal lines represent the mean offset found for the three arms and correspond to the vertical lines in the following panel (we use a different line style for each different arm). The y-axis is flipped to better compare it with the x-axis in the third panel. No clear correlation with the S/N is found. Third panel: distribution of the σ differences between the arms, drawn from the histograms, assuming a Gaussian profile. A small offset for the UVB (VIS) is visible, which systematically underestimates (overestimates) the σ by 14 km s−1 (−8 km s−1) on average. For the NIR, the distribution peaks at around 3 km s−1, but a much larger scatter is found. Fourth panel: relative shift in σ against the σ measured from the single-arm spectra.
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