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Description
We present the analysis of neutral hydrogen properties of 108 galaxies, based on short 21-cm observations with the Westerbork Synthesis Radio Telescope (WSRT). The results of two HI surveys are analysed to investigate the existence of relations between optical and HI properties, like diameters, hydrogen masses and average surface densities. For all galaxies in our sample we find that the HI diameter, defined at a surface density level of 1M_{sun}_/pc^2^, is larger than the optical diameter, defined at the 25^th^mag/arcsec^2^ isophotal level. The Hi-to-optical-diameter ratio does not depend on morphological type or luminosity. The strongest, physically meaningful, correlation for the sample of 108 galaxies is the one between logM_HI_ and logD_HI_, with a slope of 2. This implies that the HI surface density averaged over the whole HI disc is constant from galaxy to galaxy, independent of luminosity or type. The radial HI surface density profiles are studied using the technique of principal component analysis. We find that about 81% of the variation in the density profiles of galaxies can be explained by two dimensions. The most dominant component can be related to "scale" and the second principal component accounts for the variance in the behaviour of the radial profile in the central parts of galaxies (i.e. "peak or depression") . The third component accounts for 7% of the variation and is most likely responsible for bumps and wiggles in the observed density profiles.
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