The Central 300 pc of the Galaxy probed by infrared spectra of H3+ and CO: II. Expansion and morphology of the warm diffuse gas

TL;DR

This study uses infrared spectra of H3+ to reveal that the gas in the Galaxy's Central Molecular Zone is expanding radially at high speeds, supporting a face-on view of the CMZ and suggesting recent ejection events near the galactic center.

Contribution

It provides new observational evidence of radial expansion in the CMZ using H3+ spectra, supporting a face-on morphology contrary to previous elliptical models.

Findings

Gas is expanding at up to 150 km/s within the CMZ.

The expansion likely results from ejection events 0.5-1 million years ago.

The expanding gas may infall, fueling star formation and activity near Sgr A*.

Abstract

Velocity profiles of a line of H$_3^+$ at 3.7 $\mu$m produced in warm diffuse gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of the Galaxy. Their longitude-velocity diagram indicates that the gas is radially expanding within the CMZ at speeds up to a maximum of $\sim$150 km s$^{-1}$. The current momentum and energy in the gas are $\sim 5 \times 10^8 M_\odot$ km s$^{-1}$ and $\sim 5\times 10^{53}$ erg. The motion is similar to that of the Expanding Molecular Ring (EMR) discovered in 1972 by Kaifu et al. and by Scoville. We propose that the expanding gas seen in H$_3^+$ is part of the same phenomenon, in spite of differences in estimates of density, morphology, and degree of rotation. The outward motion suggests that one or more ejection events occurred near the center of the CMZ (0.5$-$1) $\times$ 10$^6$ years ago, which may be related to creation of the recently observed microwave bubble. These observations revive the circular face-on view of the CMZ proposed in 1972, which fell out of favor after 1991 when Binney et al. proposed that a face-on view of the CMZ would show it to have an elliptical shape, with high eccentricity. While that model may apply on kiloparsec scales, we argue that it is incorrect to apply it to the much smaller CMZ. We discuss the fate of the expanding gas, which appears to be eventual infall into the center, leading to episodes of star formation and violent events associated with accretion onto Sgr~A$^\ast$.