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Session 65 - ISM in Nearby Galaxies.
Oral session, Tuesday, January 16
Salon del Rey South, Hilton
The resolved neutral hydrogen emission properties of a sample of eleven of the nearest spiral galaxies is analyzed. Between 60 and 90% of the total HI line flux within R_25 is due to a super-cloud network with a face-on covering factor of about 15%. Averaged line profiles of this component are systematically non-Gaussian with a narrow core (less than about 6 km s^-1 FWHM) superposed on a broad pedestal (30 km s^-1 FWHM). The fractional line flux due to super-clouds plummets abruptly near R_25 where a diffuse outer disk sets in. All galaxies display a systematic increase in the super-cloud brightness temperature with radius followed by a flattening and a subsequent decline before reaching the diffuse outer disk. The qualitative and quantitative properties of these two components can be understood in terms of the phase transition between cool, dense gas in the super-clouds and warm, diffuse gas in inter-arm, out-of-plane and outer disk environments. A physical model is developed which can successfully account for the observed distribution of HI brightness temperature with radius in the various systems. The basic assumption is that the thermal pressure in the mid-plane is proportional to the hydrostatic pressure due to the overlying ISM. Good correspondence with the data is achieved with smoothly decreasing profiles of the mid-plane pressure, and a corresponding increase in the HI kinetic temperature. The HI brightness temperature of super-clouds increases with radius while the gas is relatively opaque and then declines as it becomes transparent. Local opacity corrections to the column density of super-clouds are typically a factor of two, but may exceed a factor of five, while total HI mass corrections vary from a factor of less than 1.1 in the SBm galaxy NGC 2366 to 2.2 in the ScI galaxy NGC 5457.
