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The dense gas in the largest molecular complexes of the Antennae: HCN and HCO+ observations of NGC 4038/39 using ALMA

We present observations of the dense molecular gas tracers HCN, HNC, and HCO+ in the J=1-0 transition using ALMA. We supplement our datasets with previous observations of CO J=1-0, which traces the total molecular gas content. We separate the Antennae into 7 bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, 5 super giant molecular complexes in the overlap region and 2 additional bright clouds. We find that the ratio of L(HCN)/L(CO), which traces the dense molecular gas fraction, is greater in the two nuclei (L(HCN)/L(CO) ~ 0.07 - 0.08) than in the overlap region (L(HCN)/L(CO) <0.05). We attribute this to an increase in pressure due to the stellar potential within the nuclei, similar to what has been seen previously in the Milky Way and nearby spiral galaxies. Furthermore, the ratio of L(HNC)/L(HCN) ~ 0.3-0.4 does not vary by more than a factor of 1.5 between regions. By comparing our measured ratios to PDR models including mechanical heating, we find that the ratio of L(HNC)/L(HCN) is consistent with mechanical heating contributing >= 5% - 10% of the PDR surface heating to the total heating budget. Finally, the ratio of L(HCN)/L(HCO+) varies from ~1 in the nucleus of NGC 4038 down to ~0.5 in the overlap region. The lower ratio in the overlap region may be due to an increase in the cosmic ray rate from the increased supernova rate within this region.