Molecular Fraction in the Galactic Center: The Central Molecular and HI Zones

TL;DR

This study maps the molecular fraction in the Galactic Center, revealing high molecular content in the Central Molecular Zone and its relation to the surrounding HI gas, providing insights into the interstellar medium structure.

Contribution

It quantitatively characterizes the molecular and atomic gas fractions in the Galactic Center using archival CO and HI data, defining the CMZ and CHZ regions.

Findings

High molecular fraction ($ ext{~}0.9-0.98$) in the CMZ and GC Arms.

Defined the CMZ as regions with $f_{ m mol}^ ext{Σ} extgreater= 0.8-0.9$.

The CMZ is embedded within the Central HI Zone, with a vertical distribution of H$_2$.

Abstract

By mapping the molecular fraction of the Galactic Center (GC), we quantitatively address the question of how much molecular and central the CMZ (Central Molecular Zone) is. For this purpose we analyze the CO and HI-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 H$_2$ projected on the sky to that of total gas (H$_2$ + HI) from the line intensities, and the ratio of volume-mass densities of \Htwo\ to total gas from the brightness temperature, respectively. It is shown that $f_{\rm mol}^\Sigma$ is as high as $ \sim 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 $\sim 0.98$ toward Sgr B2. The expanding molecular ring (EMR, or the parallelogram) has a slightly smaller $f_{\rm mol}^\Sigma$ as $\sim 0.9-0.93$. We define the CMZ as the region with $f_{\rm mol}^\Sigma \ge 0.8-0.9$ between the shoulders of plateau-like distribution of H$_2$ column density from $l=-1^\circ.1$ to $+1^\circ.8$ having Gaussian vertical distribution with a half thickness of $\pm 0^\circ.2$. The CMZ is embedded in the Central HI Zone (CHZ), which is defined as an HI disc between $l\sim -2^\circ$ 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 HI gases inferred from the difference between $ f_{\rm mol}^\Sigma$ and $f_{\rm mol}^\rho$.