Diffuse H I Disks in Isolated Galaxies

In order to investigate the contribution of diffuse components to their total H I emission, we have obtained high-precision H I line flux densities with the 100 m Green Bank Telescope for a sample of 100 isolated spiral and irregular galaxies which we have previously observed with the 43 m telescope. A comparison of the observed H I line fluxes obtained with the two different telescopes, characterized by half-power beam widths of 9' and 21', respectively, exploits a "beam-matching" technique to yield a statistical determination of the occurrence of diffuse H I components in their disks. A simple model of the H I distribution within a galaxy well describes ~75% of the sample and accounts for all of the H I line flux density. The remaining galaxies are approximately evenly divided into two categories: those which appear to possess a significantly more extensive H I distribution than the model predicts, and those for which the H I distribution is more centrally concentrated than predicted. Examples of both extremes can be found in the literature, but little attention has been paid to the centrally concentrated H I systems. Our sample has demonstrated that galaxies do not commonly possess extended regions of low surface brightness H I gas which is not accounted for by our current understanding of the structure of H I disks. Eight H I-rich companions to the target objects are identified, and a set of extragalactic H I line flux density calibrators is presented.