Colliding Clouds in the Milky Way’s Bar

Molecular clouds become hotter and more turbulent as they approach the Central Molecular Zone (CMZ), and studying clouds near the Galactic Center (GC) may help us understand the physical conditions of the clouds as they enter the GC. We observed G5 in the Galactic bar near the GC at (l,b)=(+5.4°, -0.4°) with ALMA/ACA. Measurements of the spectral lines ¹²CO(2-1), ¹³CO(2-1), C¹⁸O(2-1), H₂CO(3₀₃-2₀₂), H₂CO(3₂₂-2₂₁), CH₃OH(4₂₂-3₁₂), OCS (18-17) and SiO (5-4) at 217–220GHz were taken. We studied the Galactic southern part of G5 where a cloud-cloud collision appears to take place between a cloud at -150 km s^-1 and one at -50 km s^-1. With ¹²CO(2-1) and ¹³CO(2-1), we observe a feature in velocity space with a very wide line width intersecting the two clouds where they overlap; evidence of the collision. Evidence of shocks due to the presence of SiO and enhanced abundances of the weak shock tracer CH₃OH are also present. Changes in kinetic temperature were also observed via H₂CO, with temperatures of 60 K for the -150 km s^-1 cloud and 80 K for the -50 km s^-1 cloud. The isotopologue ratios show that the -150 km s^-1 cloud is more optically thin in CO than the -50 km s^-1 cloud. The temperature, distribution of shock tracers, linewidths, and the kinematics are all consistent with Sormani et al. 2019’s model, suggesting the -50 km s^-1 cloud overshot the CMZ and is colliding with the existing cloud -150 km s^-1 on the opposite side of the galaxy.