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-$0), and N$_2$H$^+(1-$0) towards two of these filaments, one associated with the Sagittarius arm and one with an interarm area. Using the data alongside the CO High-Resolution Survey (COHRS) $^{12}$CO(3–2), the 13CO/C18O (J= 3-2) Heterodyne Inner Milky Way Survey (CHIMPS) $^{13}$CO(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, N$_2$H$^+$ is the best at tracing the truly dense gas. Significant differences can be seen between the $^{13}$CO, HCN, and N(H$_2)_{\mathrm{dust}}$ levels of the arm and interarm, while the N$_2$H$^+$ emission is more uniform regardless of location, meaning that the observed variations in line ratios like N$_2$H$^+$/HCN or N$_2$H$^+$/$^{13}$CO 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 multiscale view of different environments inside a galaxy regarding molecular emissions, ratios, and thus the estimated star formation activity.