Emergence of high-mass stars in complex fiber networks (EMERGE) VII. Multi-Tracer Column Density Synthesis

Obtaining accurate estimates of the gas column density $\mathrm{N}({\mathrm{H}}_2)$ is crucial to determine the physical conditions of the molecular gas in the Interstellar Medium. This paper introduces the “Multi-Tracer Column Density Synthesis” (MT-CDS) technique, a new empirical approach to derive high dynamic-range column density maps with interferometers such as ALMA. We have compared the integrated intensity maps of two density selective tracers, namely HNC (1-0) and ${\mathrm{N}}_2{\mathrm{H}}^{+}$ (1-0), obtained with IRAM-30m with ancillary Herschel column density estimates in seven star-forming regions in Orion and derived individual temperature-corrected conversion factors for each of these tracers. We applied these conversion factors obtained at low-resolution into similar high sensitivity ALMA observations to derive independent column density maps describing both the low $N_{\mathrm{H}\mathrm{N}\mathrm{C}}({\mathrm{H}}_2)$ and high $N_{{\mathrm{N}}_2{\mathrm{H}}^{+}}({\mathrm{H}}_2)$ column density components at high spatial resolution (4.5"). We also combined these independent maps into estimates for the total gas column density gas $N_{\mathrm{t}\mathrm{o}\mathrm{t}}({\mathrm{H}}_2)$ sampling more than two orders of magnitude in column density between $\sim {10}^{21.5}-{10}^{23.5}$ cm${}^{-2}$. A comparison with independent ArTeMiS column density maps at intermediate resolution (8") shows typical deviations of $\lesssim$0.2 dex from our MT-CDS gas column density estimates. We obtained individual column density maps in five of our targets at $\sim$2000 au resolution in which ALMA observations were available, that is a factor of $\sim$5 better than originally calibrated. These results demonstrate the potential of this MT-CDS technique to retrieve accurate, high dynamic range $N({\mathrm{H}}_2)$ total gas column density estimates at interferometric resolution combining multiple molecular line tracers.