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Fig. 4.


Distribution of the spectral data points of the ALMA 267 GHz observations shown in Fig. 3 (histogram) with the expected Gaussian distribution for the measured standard deviation of the data overplotted. The vertical dotted lines indicate 1 and 2σ limits. The data are clearly non-Gaussian, showing a bimodal distribution that would be expected for a spectrum dominated by systematics such as instrumental ripples. This means that low S/N signals cannot be reliably linked to a false positive probability.

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