Optical Thickness, Spin Temperature, and Correction Factor for Density
  of the Galactic HI Gas
  A method to determine the spin temperature of the local (Vlsr=0 km/s) HI gas
using saturated brightness temperature of the 21-cm line in the radial-velocity
degenerate regions (VDR) is presented. The spin temperatures is determined to
be Ts= 146.2 +/- 16.1 K by measuring saturated brightness in the VDR toward the
Galactic Center, 146.8 +/- 10.7 K by chi^2 fitting of expected brightness
distribution to observation around the VDR, and 144.4 +/- 6.8 K toward the
local arm. Assuming Ts=146 K, a correction factor Gamma for the HI density,
defined by the ratio of the true HI density for finite optical thickness to
that calculated by assuming optically thin HI, was obtained to be Gamma~1.2
(optical depth tau~0.3) in the local HI gas, ~1.8 (~1.3) toward the arm and
anti-center, and as high as ~3.6 (~2.7) in the Galactic Center direction. It is
suggested that the HI density and mass in the local arm could be ~2 times, and
that in the inner Galaxy ~3.6 times, greater than the currently estimated
values.
