2022MNRAS.516.3911S

By mapping the molecular fraction of the Galactic Centre (GC), we quantitatively address the question of how much molecular and central the CMZ (central molecular zone) is. For this purpose, we analyse the CO and H I-line archival data, and determine the column- (surface-) and volume-molecular fractions, $f_{\rm mol}^\Sigma$ and $f_{\rm mol}^\rho$, which are the ratio of column-mass density of H₂ projected on the sky to that of total gas (H₂ + H I) from the line intensities, and the ratio of volume-mass densities of H₂ to total gas from the brightness temperature, respectively. It is shown that $f_{\rm mol}^\Sigma$ is as high as ∼0.9-0.95 in the CMZ, and $f_{\rm mol}^\rho$ is 0.93-0.98 in the GC Arms I and II attaining the highest value of ∼0.98 toward Sgr B2. The expanding molecular ring (EMR, or the parallelogram) has a slightly smaller $f_{\rm mol}^\rho$ as ∼0.9-0.93. We define the CMZ as the region with $f_{\rm mol}^\Sigma \ge 0.8\rm {-\!\!-}0.9$ between the shoulders of plateau-like distribution of H₂ column density from l = -1${_{.}^{\circ}}$1 to +1${_{.}^{\circ}}$8 having Gaussian vertical distribution with a half thickness of ±0${_{.}^{\circ}}$2. The CMZ is embedded in the Central H I Zone (CHZ), which is defined as an H I disc between l ∼ -2° and +2${_{.}^{\circ}}$5, b = -0${_{.}^{\circ}}$5 and +0${_{.}^{\circ}}$5. Based on the analysis, we discuss the origin of CMZ and interstellar physics such as the volume filling factors of molecular and H I gases inferred from the difference between $f_{\rm mol}^\Sigma$ and $f_{\rm mol}^\rho$.