The temperature and opacity of atomic hydrogen in spiral galaxies

We analyze the resolved neutral hydrogen emission properties of a sample of 11 of the nearest spiral galaxies. Between 60% and 90% of the total H I line flux within the optical disk is due to a high-brightness network (HBN) of emission features that are marginally resolved in their narrow dimension at about 150 pc and have a face-on covering factor of about 15%. Averaged line profiles of this component are systematically non-Gaussian, with a narrow core (less than or similar to 6 km s(-1) FWHM) superposed on broad Lorentzian wings (30 km s(-1) FWHM). An upper limit to the gas temperature of 300 K follows directly from the narrow line profiles, while simple modeling suggests kinetic temperatures equal to the peak emission brightness temperature (80-200 K) in all cases but the outer disks of low-mass galaxies, where the HBN becomes optically thin to the 21 cm line. Positive radial gradients in the derived kinetic temperature are found in all spiral galaxies. The distributions of brightness temperature with radius in our sample form a nested system with galaxies of earlier morphological type systematically displaced to lower temperatures at all radii. The fractional line flux due to the HBN plummets abruptly near the edge of the optical disk, where a diffuse outer gas disk takes over. We identify the HBN with the cool neutral medium.