2017MNRAS.468.4030S

This paper presents a method to determine the spin temperature of the local (V_LSR = 0 km s^–1) H I gas using the saturated brightness temperature of the 21-cm line in radial velocity-degenerate regions (VDRs). The spin temperature is determined to be: T_S = 146.2 ± 16.1 K by measuring saturated brightness in the VDR towards the Galactic Centre; 146.8 ± 10.7 K by χ² fitting of the expected brightness distribution to observations around the VDR; and 144.4 ± 6.8 K towards the local arm. Assuming T_S = 146 K, a correction factor Γ for the H I density, defined by the ratio of the true H I density for finite optical thickness to that calculated by assuming optically thin H I, was obtained as Γ ∼ 1.2 (optical depth τ ∼ 0.3) in the local H I gas, ∼1.8 (∼1.3) towards the arm and anti-Centre, and as high as ∼3.6 (∼2.7) in the Galactic Centre direction. It is suggested that the H I density and mass in the local arm could be ∼2 times greater than currently estimated values, and that in the inner Galaxy could be ∼3.6 times greater.