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High spin temperatures at large impact parameters: Ionisation in the outskirts of galaxies

By including the most recent observations of HI 21-cm absorption through near-by galactic disks, we confirm our previous assertion that there is an anti-correlation between the abundance of cool neutral atomic gas and impact parameter. Comparing the measured neutral hydrogen column densities of the sample with the absorption strength, we find a peak in the mean spin temperature of ~2310 K at an impact parameter of ~ 14 kpc, with > 1000 K in the remainder of the disk. This is significantly different to the spin temperature distribution in the Milky Way, which exhibits a constant ~ 250 - 400 K over 8 - 25 kpc. The measured column densities may, however, suffer from beam dilution, which we show appears to be the case for the observations of HI 21-cm emission in which the beam subtends radii of > 10 kpc. We therefore apply the column density profile of the Milky Way, in addition to the mean of the sample, observed at sufficiently high resolution, and the mean profile for the near-by galaxies in the IllustrisTNG simulations. All of the models yield a peak in the mean spin temperature at similar radii/impact parameters (10 - 15 kpc) as the measured column densities. These radii are similar to those of the spiral arms where HII regions are often concentrated. We therefore suggest that the elevated spin temperatures trace the HII regions observed in the outer disk of many spiral galaxies.