The formation of young massive clusters triggered by cloud-cloud collisions in the Antennae galaxies NGC 4038/NGC 4039

The formation mechanism of super star clusters (SSCs), present-day analogs of the ancient globulars, still remains elusive. The major merger that is the Antennae galaxies is forming SSCs and is one of the primary targets to test the cluster formation mechanism. We reanalyzed the archival ALMA CO data of the Antennae and found three typical observational signatures of a cloud-cloud collision toward SSC B1 and other SSCs in the overlap region: (i) two velocity components with ∼100 km s^-1 velocity separation, (ii) bridge features connecting the two components, and (iii) a complementary spatial distribution between them, lending support to collisions of the two components as a cluster formation mechanism. We present a scenario that two clouds with 100 km s^-1 velocity separation collided, and SSCs having ∼10⁶-10⁷ M_⊙ were formed rapidly during that time scale. We compared the present results with the recent studies of star-forming regions in the Milky Way and the Large Magellanic Cloud, where the SSCs having ∼10⁴-10⁵ M_⊙ are located. As a result, we found that there is a positive correlation between the compressed gas pressure generated by collisions and the total stellar mass of an SSC, suggesting that the pressure may be a key parameter in SSC formation.