Upper limits (we note that only the first four spectra are shown here; the full set is available in Appendix B). Shown is the flux density of the 21-cm H I line emission, S_HI, in Jy, as a function of the radial velocity in the LSR reference frame, V_LSR, in km s⁻¹. All spectra are peak profiles (see Section 3.2). Spectra are shown only for the 70 objects with upper limits to their H I line emission for which digital versions of the observations are available, that is, those observed from 2001 onwards with the renovated NRT (thus excluding the 25 objects with the ‘old data’ note in online Table 5). The red vertical and horizontal lines indicate respectively the central CO or OH line velocity from the literature and its corresponding expansion velocity, whereas green lines indicate other types of literature velocities (e.g. optical or SiO lines) and an indicative expansion velocity of 10 km s⁻¹, that is, the average measured value. The flat blue horizontal lines show the 0 Jy flux density level. The plotted total velocity range is the full observed range for each object.