# Optical Thickness, Spin Temperature, and Correction Factor for Density of the Galactic HI Gas

## TL;DR

This paper presents a method to determine the spin temperature of Galactic HI gas using saturated 21-cm line brightness, and introduces a correction factor for HI density estimates, revealing higher actual densities in the Galaxy.

## Contribution

It introduces a new technique to measure HI spin temperature from saturated brightness and calculates correction factors for HI density, improving mass estimates of Galactic HI gas.

## Key findings

- Spin temperature of local HI gas is approximately 146 K.
- Correction factor for HI density is about 1.2 locally and up to 3.6 in the Galactic Center.
- Estimated HI mass in the local arm and inner Galaxy is significantly higher than previous estimates.

## Abstract

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.