Cloud–cloud collisions in the Antennae galaxies: Does high-speed collision suppress star formation?

Abstract Cloud–cloud collision (CCC) has been proposed as a mechanism for triggering massive star formation. Observations in the Milky Way and nearby galaxies have revealed the presence of CCCs with collision velocities ($v_{\mathrm{col}}$) of 1–40 km s$^{-1}$, and the connection between star formation activity and the properties of colliding clouds has been investigated. In this study, we expand the study to much faster (${\sim}100\:$km s$^{-1}$) CCCs in a nearby colliding galaxies’ system, the Antennae galaxies. We examine how star formation rate (SFR) on a sub-kpc scale depends on the $v_{\mathrm{col}}$ and mass ($M_{\mathrm{mol}}$) of giant molecular clouds (GMCs) across the Antennae galaxies, which show diverse star formation activity. Furthermore, to examine the star formation process at a more fundamental level, we also investigate how the star formation efficiency (SFE) of a colliding GMC depends on its $v_{\mathrm{col}}$ and $M_{\mathrm{mol}}$. SFR is calculated using H$\alpha$ and mid-infrared data. From ${\sim}2000$ GMCs identified in the CO(1–0) data cube using the ALMA archival data, collision velocities are estimated based on the velocity dispersion among GMCs in a sub-kpc scale region, assuming random motion in three-dimensional space. GMCs are considered to be colliding at a velocity of ${\sim}10$–150 km s$^{-1}$. We find that regions where high-speed collisions ($v_{\mathrm{col}}\sim 100\:$km s$^{-1}$) of massive ($M_{\mathrm{mol}}\sim 10^{7-8}\, M_\odot$) GMCs are seen show the highest surface density of SFR. Particularly, in the region with $v_{\mathrm{col}}\sim 100\:$km s$^{-1}$, we find that SFR on a sub-kpc scale increases with increasing $M_{\mathrm{mol}}$ in the range of ${\sim}10^{6}$–$10^{8}\, M_\odot$. The SFE of a colliding cloud is estimated to be $0.1\%$–$3.0\%$ without clear $M_{\mathrm{mol}}$ dependence, and the SFE is the lowest at the $v_{\mathrm{col}}\sim 100$–150 km s$^{-1}$. These results suggest that the most active star formation in the Antennae galaxies seems to occur due to large GMC mass.