Giant molecular clouds (GMCs) are the main sites of massive star formation in galaxies, and the GMC evolution is a major driver of the galaxy evolution. To unveil the GMC evolution, we have established a scheme of identifying and classifying GMCs based on their star formation activity as follows: Type I ‘without star formation,’ Type II ‘with Hα luminosity (LHα) smaller than 10^37.5 erg/s’, and Type III ‘with LHα greater than 10^37.5 erg/s’. As we applied the scheme in LMC, M33, and M74, we interpreted GMCs to evolve from Type I to II to III, with a typical timescale of 10-20 Myr based on the associated star clusters’ age by assuming the steady state (Demachi+24; Konishi+24, etc.).
In this contribution, we focus on one of the nearest starburst galaxies, the Antennae galaxies (AG), to study how galactic collisions accelerate the GMC evolution. We identify GMCs from CO(1-0) data obtained by ALMA and HII regions from Hα data obtained by VLT with MUSE and determine their association if their boundaries overlap and their velocity centroids fall within 100 km/s. As a result, we identified nearly 1000 GMCs in total and the fractions of the three Types are found to be Type I : II : III ~ 60% : 5%: 35% , while those in M74 are 15% : 45%: 40%. In addition, the median values of the GMC mass and LHα are 10^7 Mo and 10^38 erg/s, respectively, in the AG, both of which are ten times higher than in M74. We interpret that these enhanced GMC masses and LHα are the consequence of frequent collisions and mergers of GMCs that have triggered the active formation of high-mass stars. Furthermore, from the results of age estimations of the clusters by HST, we found that 1-4 Myr clusters associated with GMCs are significantly concentrated in the overlap region, having their surface density a few times higher than the other regions. This is consistent with the simulation (Renaud+15) showing the last galactic interaction was 6 Myr ago.
