Spin temperature and density of cold and warm H i in the Galactic disk: Hidden H i

Abstract

Abstract We present a method to determine the spin temperature TS and volume density n of H i gas simultaneously along the tangent-point circle of Galactic rotation in the Milky Way by using the χ2 method. The best-fit TS is shown to range either in TS ∼ 100–120 K or in 1000–3000 K, indicating that the gas is in the cold H i phase with high density and large optical depth, or in warm H i with low density and small optical depth. Averaged values at 3 ≤ R ≤ 8 kpc are obtained to be TS = 106.7 ± 16.0 K and n = 1.53 ± 0.86 H cm−3 for cold H i, and 1720 ± 1060 K and 0.38 ± 0.10 H cm−3 for warm H i, where R = 8 |sinl| kpc is the galacto-centric distance along the tangent-point circle. The cold H i appears in spiral arms and rings, whereas warm H i appears in the inter-arm regions. The cold H i is denser by a factor of ∼4 than warm H i. The present analysis has revealed the hidden H i mass in the cold and optically thick phase in the Galactic disk. The total H i mass inside the solar circle is shown to be greater by a factor of 2–2.5 than the current estimation by the optically thin assumption.