Dense gas tracers in and between spiral arms: from Giant Molecular Filaments to star-forming clumps
Abstract
Giant Molecular Filaments are opportune locations in our Galaxy to study the star-forming interstellar matter and its accumulation on spatial scales comparable to those now becoming available for external galaxies. We mapped the emission of HCN(1-0), HCO+(1-0), and N2H+(1-0) towards two of these filaments, one associated with the Sagittarius arm and one with an interarm area. Using the data alongside the COHRS 12CO(3-2), the CHIMPS 13CO(3-2), and Herschel-based column density maps, we evaluate the dense gas tracer emission characteristics and find that although its filling factor is the smallest among the studied species, N2H+ is the best at tracing the truly dense gas. Significant differences can be seen between the 13CO, HCN, and N(H2)dust levels of the arm and interarm, while the N2H+ emission is more uniform regardless of location, meaning that the observed variations in line ratios like N2H+/HCN or N2H+/13CO are driven by species tracing moderate-density gas and not the star-forming gas. In many cases, greater variation in molecular emission and ratios exist between regions inside a filament than between the arm and interarm environments. The choice of measure of the dense gas and the available spatial resolution have deep impact on the multi-scale view of different environments inside a galaxy regarding molecular emissions, ratios, and thus the estimated star formation activity.
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