THE CENTRAL 300 pc OF THE GALAXY PROBED BY INFRARED SPECTRA OF H₃⁺ AND CO

Oka, Takeshi

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https://hdl.handle.net/2142/122341 Copy

Description

Title

THE CENTRAL 300 pc OF THE GALAXY PROBED BY INFRARED SPECTRA OF H₃⁺ AND CO

Author(s)

Oka, Takeshi

Contributor(s)

Geballe, Thomas R.

Issue Date

2023-06-21

Keyword(s)

Astronomy

Abstract

Our 15 years’ observations of the infrared spectra of H₃⁺ and CO toward 30 young and bright stars with smooth continuum and their analyses have led to the following conclusions. I. Predominance of Warm and Diffuse Gas and High H₂ Ionization Rate: The CMZ is filled with warm (T ~ 200 K) and diffuse (n ~ 50 cm⁻³) hydrogen gas with a volume-filling factor f ~ 2/3 and atomic and molecular hydrogen number densities are comparable. The cosmic ray H₂ ionization rate, ζ ~ 2 × 10⁻¹⁴ s⁻¹, is 1000 times higher than in the solar vicinity suggesting stray magnetic field of ~ 100 µG in the CMZ if equipartition between cosmic rays and magnetic field is assumed. The X-ray-emitting ultra-hot (10⁸ K) plasma, which some thought to dominate the CMZ, does not exist. II. Expansion and Morphology of the Warm Diffuse Gas: The wide velocity profiles of H⁺ lines have allowed us to draw longitude-velocity diagrams toward 18 stars. They indicated that the diffuse molecular gas is expanding with a front speed of ~ 150 km s⁻¹. This revives the Expanding Molecular Ring proposed by Kaifu et al. and Scoville in 1972 but there are three differences: (1) the expanding gas is diffuse, (2) the expansion is radial, and (3) the gas fills the CMZ. This revives the circular geometry of the CMZ, as viewed face-on. The elliptic structure with high eccentricity is negated. III. Locations of Sgr B2 and Star Iota: The radial motion of the diffuse gas allows us to determine radial location of a star from velocity profiles of H⁺ or other molecules. Star ι, which has a strong H⁺ absorption spectrum and is at galactic longitude 0°.5476, is close to Sgr B2 (at 0°.6667). Using velocity profiles of H⁺ lines toward Star ι and of H₂O⁺, OH⁺, and ¹³CH⁺ spectra toward Sgr B2 observed by HIFI on the Herschel Space Observatory, we find that both Star ι and Sgr B2 are ~ 90 pc behind the GC’s central black hole, Sgr A*. This contradicts the previous conclusion based on trigonometric parallax, which placed Sgr B2 130 ± 60 pc in front of Sgr A*, as well as most models of the CMZ.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

Handle URL

https://hdl.handle.net/2142/122341

DOI

https://doi.org/10.15278/isms.2023.6693

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