ALMA observations of the dense and shocked gas in the nuclear region of
  NGC 4038 (Antennae galaxies)
  We present 1" (<100 pc) resolution maps of millimeter emission from five
molecules-CN, HCN, HCO+, CH3OH, and HNCO-obtained towards NGC 4038, 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 NGC 4038.
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 um 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 (10^8 Msun) within a 0.6" (~60 pc) 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.
