Observing strategy & Data Reduction

The data were first reduced using the usual ESOREFLEX (Freudling et al. 2013) pipeline routine; the steps included bias and overscan subtraction, flat-fielding correction, wavelength calibration, determination of the line spread function (LSF), illumination correction, sky background subtraction, and flux calibration. Once the final combined cube was obtained, we performed a second data reduction including a few changes in the main workflow:

• Custom mask: we extracted the white image of each target galaxy. We built a custom mask to remove the light contribution of foreground/background/spurious sources, including the light contribution of the target. This will help to improve the sky background evaluation.

• Normalization of the exposures:  for each target galaxy, we normalised the pixtables and FOVs of each exposure, rescaling them with specific multiplicative factors before the exposure alignment step. This will help to decrease the flux fluctuations across the FOV of the final combined datacube.

• ZAP:  the final sky-subtracted datacube is characterised by the presence of sky residuals; thus, it is necessary to further clean the cube by applying the Zurich Atmospheric Purge algorithm (ZAP,  Soto et al. 2016). We introduced a custom mask in the ZAP routine to improve the detection of the sky background by filtering all the light contributions in the FOV.

The flowchart of the data reduction is shown in the left Figure.