The Central 300 pc of the Galaxy Probed by Infrared Spectra of and CO. I. Predominance of Warm and Diffuse Gas and High H 2 Ionization Rate

Abstract

The molecular gas in the central molecular zone (CMZ) of the Galaxy has been studied using infrared absorption spectra of H 3 + lines at 3.5–4.0 μ m and CO lines near 2.34 μ m. In addition to the previously reported spectra of these lines toward eight stars located within 30 pc of Sgr A*, there are now spectra toward ∼30 bright stars located from 140 pc west to 120 pc east of Sgr A*. The spectra show the presence of warm ( T  ∼ 200 K) and diffuse ( n  < 100 cm −3 ) gas with N (H 3 + ) ∼ 3 × 10 15 cm −2 on the majority of sight lines. Instead of our previous analysis, in which only electrons from photoionization of carbon atoms were considered, we have developed a simple model calculation in which the cosmic-ray ionization of H 2 and H is also taken into account. We conclude the following: (1) Warm and diffuse gas dominates the volume of the CMZ. The volume filling factor of dense gas must be much less than 0.1, and the CMZ is not as opaque as previously considered. The X-ray-emitting ultrahot 10 8 K plasma, which some thought to dominate the CMZ, does not exist over extended regions. (2) The cosmic-ray ionization rate is ζ  ∼ 2 × 10 −14 s −1 , higher than in Galactic dense clouds and diffuse clouds by factors of ∼1000 and ∼100, respectively. If the equipartition law stands, this suggests a pervading magnetic field on the order of ∼100 μ G.