ALMA CO Observations of a Giant Molecular Cloud in M33: Evidence for High-Mass Star Formation Triggered by Cloud-Cloud Collisions

TL;DR

This study provides the first observational evidence in M33 that high-mass star formation can be triggered by collisions between molecular clouds, using high-resolution ALMA data to identify collision signatures and associated star formation.

Contribution

It demonstrates that cloud-cloud collisions are a viable mechanism for high-mass star formation in the Local Group, based on detailed ALMA observations of a giant molecular cloud in M33.

Findings

Identification of two colliding molecular clouds with velocity difference of ~6 km/s.

Detection of embedded high-mass stars within the collision region.

Observation of complementary spatial distribution and V-shaped velocity structure.

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 $^{12}$CO($J$ = 2-1), $^{13}$CO($J$ = 2-1), and C$^{18}$O($J$ = 2-1) line emission at a spatial resolution of $\sim$2 pc. There are two individual molecular clouds with a systematic velocity difference of $\sim$6 km s$^{-1}$. Three continuum sources representing up to $\sim$10 high-mass stars with the spectral types of B0V-O7.5V are embedded within the densest parts of molecular clouds bright in the C$^{18}$O($J$ = 2-1) line emission. The two molecular clouds show a complementary spatial distribution with a spatial displacement of $\sim$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.