Evidence of a Cloud-Cloud Collision from Overshooting Gas in the Galactic Center

Abstract

The Milky Way is a barred spiral galaxy with "bar lanes" that bring gas towards the Galactic Center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone, it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud which we believe is the site of one such collision, near the Galactic Center at (l,b) = (+5.4, -0.4) with the ALMA/ACA. We took measurements of the spectral lines ${}^{12}$CO J=2-1, ${}^{13}$CO J=2-1, C${}^{18}$O J=2-1, H${}_2$CO J=3${}_{03}$-2${}_{02}$, H${}_2$CO J=3${}_{22}$-2${}_{21}$, CH${}_3$OH J=4${}_{22}$-3${}_{12}$, OCS J=18-17 and SiO J=5-4. We observed a velocity bridge between two clouds at $\sim$50 km/s and $\sim$150 km/sin our position-velocity diagram, which is direct evidence of a cloud-cloud collision. We measured an average gas temperature of $\sim$60 K in G5 using H${}_2$CO integrated intensity line ratios. We observed that the ${}^{12}$C/${}^{13}$C ratio in G5 is consistent with optically thin, or at most marginally optically thick ${}^{12}$CO. We measured 1.5 x 10${}^{19}$ cm${}^{-2}$(K km/s)${}^{-1}$ for the local X${}_{CO}$, 10-20x less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the Central Molecular Zone (CMZ) and colliding with a bar lane on the opposite side of the Galactic center.

Accepted to ApJ. 27 pages, 19 figures

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Cited by in corpus (6)

• 3-D CMZ IV: Distinguishing Near vs. Far Distances in the Galactic Center Using Spitzer and Herschel
• Revealing Gas Inflows towards the Galactic Central Molecular Zone
• Star Formation in Extreme Environments: A 200 pc High Velocity Gas Stream in the Galactic Centre
• Gas Transfer Between the Inner 3-kpc Disk and the Galactic Central Molecular Zone
• Turbulent Pressure Heats Gas and Suppresses Star Formation in Galactic Bar Molecular Clouds
• 3D CMZ V: A new orbital model of our Galaxy's Center, informed by data across the electromagnetic spectrum