Diffuse HI Disks in Isolated Galaxies

TL;DR

This study investigates the diffuse HI components in isolated galaxies using high-precision flux measurements from two telescopes, revealing that most galaxies' HI distributions align with simple models, with some exceptions showing more extensive or concentrated HI regions.

Contribution

The paper introduces a beam matching technique to statistically assess diffuse HI components in galaxy disks, providing new insights into HI distribution structures.

Findings

Approximately 75% of galaxies' HI distributions match simple models.

Some galaxies have more extended HI than predicted.

Others show more centrally concentrated HI distributions.

Abstract

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