We compiled from the literature and our own observations HI single dish spectra of over 800 CIG galaxies. After after making cuts for the quality of the spectra and AMIGA’s isolation criteria there are 544 spectra of well isolated galaxies (399 detections and 145 marginals and non-detections). As AMIGA represents a sample of galaxies with minimal impact from environment this sample can be used to define a baseline of HI-normalcy in the absence of interactions, which has not been updated for the most isolated galaxies since Haynes & Giovanelli 1984 (HG84).

We fit two scaling relations between HI mass and optical diameter and optical luminosity using a maximum likelihood regression model that includes the contribution of upper limits (marginals and non-detections) and measurement uncertainties in both dependent and independent variables (including distance uncertainties). We find that our HI mass to optical diameter relation is almost identical to that of HG84, but our relation with optical luminosity is considerably steeper. The lack of accounting for measurement uncertainty (in the independent variable) in the OLS regression method used by HG84 and others explains some, but not all, of this difference.

The remaining difference is attributed to the slightly earlier morphology distribution of our sample compared to HG84 as later types are found to have flatter gradient relations and higher intercept values, i.e. late-type, low-luminosity galaxies are more HI-rich than their earlier type equivalents, and later type galaxies are generally more HI-rich. We also noted that when split by morphological type the luminosity-based relations had lower estimates of intrinsic scatter than the diameter-based ones. Although diameter has typically been found to be the best predictor of HI mass, this might indicate that once morphology is completely accounted for, luminosity could be an intrinsically better predictor.