2024ApJ...962....4M

The tip of the tidal tail, resulting from an encounter between galaxies, features gas concentrations and some star-forming regions, such as tidal dwarf galaxies (TDGs). This region provides a unique laboratory for examining the star formation process in a dynamical environment distinct from that of disk galaxies. Using the Nobeyama 45 m telescope, we conducted ¹²CO(1-0) position-switching observations at the tips of the southern tidal tail in the Antennae galaxies. We detected CO emission not only from the two star-forming TDG candidates but also in regions with no significant star formation. Adopting a Galactic CO-to-H₂ conversion factor without helium correction, the H₂ gas surface density is ∼5-12 M_⊙ pc^–2. In most regions, the molecular-to-atomic gas ratio is around unity (0.6-1.9), but we find a region with a high ratio with a 3σ lower limit of >7.2. The star formation efficiency (SFE) of molecular gas is notably low (<0.15 Gyr^–1), indicating less active star formation than in both nearby disk galaxies (∼0.5-1.0 Gyr^–1) and other TDGs previously observed. Including previous observations, the molecular gas SFEs vary widely among TDGs/tidal tails, from 10^–2 to 10 Gyr^–1, demonstrating significant variations in star formation activity. Potential factors contributing to the low SFE in the Antennae tail tips include extensive tides and/or the young age of the tail.