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

Obtaining accurate estimates of the gas column density N(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 N 2H+ (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 HNC(H 2) and high N N 2H+(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 tot(H 2) sampling more than two orders of magnitude in column density between about 10 21.5-10 23.5 cm-2. A comparison with independent ArTeMiS column density maps at intermediate resolution (8") shows typical deviations of 0.2 dex from our MT-CDS gas column density estimates. We obtained individual column density maps in five of our targets at about 2000 au resolution in which ALMA observations were available, that is a factor of about 5 better than originally calibrated. These results demonstrate the potential of this MT-CDS technique to retrieve accurate, high dynamic range N(H 2) total gas column density estimates at interferometric resolution combining multiple molecular line tracers.