SHORT 21-CM WSRT OBSERVATIONS OF SPIRAL AND IRREGULAR GALAXIES - HI PROPERTIES

We present the analysis of neutral hydrogen properties of 108 galaxies , based on short 21-cm observations with the Westerbork Synthesis Radi o Telescope (WSRT). The results of two H I surveys are analysed to inv estigate the existence of relations between optical and H I properties , like diameters, hydrogen masses and average surface densities. For a ll galaxies in our sample we find that the H I diameter, defined at a surface density level of 1 M. pc(-2), is larger than the optical diame ter, defined at the 25(th) mag arcsec-2 isophotal level. The H I-to-op tical-diameter ratio does not depend on morphological type or luminosi ty. The strongest, physically meaningful, correlation for the sample o f 108 galaxies is the one between log M-H I and log D-H I, With a slop e of 2. This implies that the H I surface density averaged over the wh ole H I disc is constant from galaxy to galaxy, independent of luminos ity or type. The radial H I 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 ''sc ale'' 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 t he variation and is most likely responsible for bumps and wiggles in t he observed density profiles.