ALMA CO Observations of a Giant Molecular Cloud in M33: Evidence for High-Mass Star Formation Triggered by Cloud-Cloud Collisions
Abstract
We report the first evidence for high-mass star formation triggered by collisions of molecular clouds in M33. Using the Atacama Large Millimeter/submillimeter Array, we spatially resolved filamentary structures of giant molecular cloud 37 in M33 using 12CO(J = 2-1), 13CO(J = 2-1), and C18O(J = 2-1) line emission at a spatial resolution of 2 pc. There are two individual molecular clouds with a systematic velocity difference of 6 km s-1. Three continuum sources representing up to 10 high-mass stars with the spectral types of B0V-O7.5V are embedded within the densest parts of molecular clouds bright in the C18O(J = 2-1) line emission. The two molecular clouds show a complementary spatial distribution with a spatial displacement of 6.2 pc, and show a V-shaped structure in the position-velocity diagram. These observational features traced by CO and its isotopes are consistent with those in high-mass star-forming regions created by cloud-cloud collisions in the Galactic and Magellanic Cloud HII regions. Our new finding in M33 indicates that the cloud-cloud collision is a promising process to trigger high-mass star formation in the Local Group.