Estimating the kinetic temperature from H I 21-cm absorption studies: correction for turbulence broadening

The neutral hydrogen 21-cm transition is a useful tracer of the neutral interstellar medium. However, it is often not straightforward to infer physical conditions from the observed 21-cm absorption and/or emission spectra. One complication when estimating the temperature of the atomic gas is that the linewidth can have a significant contribution from non-thermal broadening. Here, we propose a formalism to separate the thermal and non-thermal broadening using a self-consistent model of turbulence broadening of the H I 21-cm absorption components. By applying this novel method, we have estimated the spin and the kinetic temperature of diffuse Galactic neutral hydrogen, and we have found that a large fraction of gas has a temperature in the unstable range. The turbulence is found to be subsonic or transonic in nature, and the clouds seem to have a bimodal size distribution. Assuming that the turbulence is magnetohydrodynamic in nature, the estimated magnetic field strength is of μG order, and is found to be uncorrelated with the H I number density.