Resolved Atomic Super-clouds in Spiral Galaxies

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High quality data are presented of neutral hydrogen emission and absorption in the fields of eleven of the nearest spiral galaxies. Multi-configuration VLA observations have provided angular resolution of 6 arcsec (corresponding to about 100 pc at the average galaxy distance of 3.5 Mpc) and velocity resolution of 6 km s⁻¹, while accurately recovering the total line flux detected previously with filled apertures. Previous experience suggests that this physical resolution is sufficient to at least marginally resolve the H1 super-cloud population which delineates regions of active star formation. A high brightness filamentary network of H1 super-clouds is seen in each galaxy. Emission brightness temperatures in excess of 200 Kelvin are sometimes detected at large radii, even in relatively face-on systems. All galaxies display a systematic increase in the observed brightness temperature of super-clouds with radius, followed by a flattening and subsequent decline. In the few instances where background continuum sources allow detection of H1 absorption, the indicative spin temperatures are consistent with the super-cloud brightness temperature seen in emission at similar radii. These data suggest substantial opacity of the H1 in the super-cloud network.