ALMA observations of the dense and shocked gas in the nuclear region of NGC4038 (Antennae galaxies)

We present 1"(>100pc) resolution maps of millimeter emission from five molecules-CN, HCN, HCO+, CH₃OH, and HNCO-obtained towards NGC4038, which is the northern galaxy of the mid-stage merger, Antennae galaxies, with the Atacama Large Millimeter/submillimeter Array. Three molecules (CN, CH₃OH, and HNCO) were detected for the first time in the nuclear region of NGC4038. High-resolution mapping reveals a systematic difference in distributions of different molecular species and continuum emission. Active star-forming regions identified by the 3 mm and 850 μm continuum emission are offset from the gas-rich region associated with the HCN (1-0) and CO (3-2) peaks. The CN (1-0)/HCN (1-0) line ratios are enhanced (CN/HCN ≈ 0.8-1.2) in the star-forming regions, suggesting that the regions are photon dominated. The large molecular gas mass (10⁸ M) within a 0".6 (∼60pc) radius of the CO (3-2) peak and a high dense gas fraction (>20%) suggested by the HCN (1-0)/CO (3-2) line ratio may signify a future burst of intense star formation there. The shocked gas traced in the CH₃OH and HNCO emission indicates sub-kpc-scale molecular shocks. We suggest that the molecular shocks may be driven by collisions between inflowing gas and the central massive molecular complex.