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.
