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

Abstract

We present observations of the dense molecular gas tracers $\mathrm{HCN}$, $\mathrm{HNC}$, and ${\mathrm{HCO}}^{+}$ in the $J=1-0$ transition using the Atacama Large Millimeter/submillimeter Array. We supplement our data sets with previous observations of $\mathrm{CO}$ $J=1-0$, which trace the total molecular gas content. We separate the Antennae into seven bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, five super giant molecular complexes in the overlap region, and two additional bright clouds. We find that the ratio of ${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$, which traces the dense molecular gas fraction, is greater in the two nuclei (${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$ $\sim \quad 0.07-0.08$) than in the overlap region (${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$ $\lt 0.05$). We attribute this to an increase in pressure due to the stellar potential within the nuclei; a similar effect to what has been seen previously in the Milky Way and nearby spiral galaxies. Furthermore, the ratio of ${L}_{\mathrm{HNC}}/{L}_{\mathrm{HCN}}$ $\sim \quad 0.3-0.4$ does not vary by more than a factor of 1.5 between regions. By comparing our measured ratios to photon dominated region (PDR) models including mechanical heating, we find that the ratio of ${L}_{\mathrm{HNC}}/{L}_{\mathrm{HCN}}$ is consistent with mechanical heating contributing gsim5%–10% of the PDR surface heating to the total heating budget. Finally, the ratio of ${L}_{\mathrm{HCN}}/{L}_{\mathrm{HCO}+}$ 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