Spin Temperature and Density of Cold and Warm HI in the Galactic Disk - Hidden HI -

Abstract

We present a method to determine the spin temperature $T_{\mathrm{S}}$ and volume density $n$ of HI gas simultaneously along the tangent-point circle of galactic rotation in the Milky Way by using the least-${\chi }^2$ method. The best-fit $T_{\mathrm{S}}$ is shown to range either in $T_{\mathrm{S}}=100-120$ K or in $1000-3000$ K, indicating that the gas is either in cold HI phase with high density and large optical depth, or in warm HI with low density and small optical depth. Averaged values at $3\le R\le 8$ kpc are obtained to be $T_{\mathrm{S}}=106.7\pm 16.0$ K and $n=1.53\pm 0.86$ H cm${}^{-3}$ for cold HI, and $1720\pm 1060$ K and $0.38\pm 0.10$ H cm${}^{-3}$ for warm HI, where $R=8|\sin l|$ kpc is the galacto-centric distance along the tangent-point circle. The cold HI appears in spiral arms and rings, whereas warm HI in the inter-arm regions. The cold HI is denser by a factor of $\sim 4$ than warm HI. The present analysis has revealed the hidden HI mass in cold and optically thick phase in the galactic disk. The total HI mass inside the solar circle is shown to be greater by a factor of $2-2.5$ than the current estimation by optically thin assumption.

13 pages, 18 figures, 2 tables, to appear in PASJ

References in corpus (6)

• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• Optically thick HI dominant in the local interstellar medium; an alternative interpretation to "dark gas"
• Distribution and kinematics of atomic and molecular gas inside the Solar circle
• Giant HI Hole inside the 3-kpc Ring and the North Polar Spur - The Galactic Crater -
• The validity of 21 cm spin temperature as a kinetic temperature indicator in atomic and molecular gas
• Optical Thickness, Spin Temperature, and Correction Factor for Density of the Galactic HI Gas

Cited by in corpus (5)

• FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45-m Telescope (FUGIN) IV: Galactic Shock Wave and Molecular Bow Shock in the 4-kpc Arm of the Galaxy
• Diagnostics of Gaseous Halo of the Milky Way by a Shock Wave from the Galactic Centre
• Magnetic field and ISM in the local Galactic disc
• Molecular Fraction in the Galactic Center: The Central Molecular and HI Zones
• High spin temperatures at large impact parameters: Ionisation in the outskirts of galaxies