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 COHRS ${}^{12}$CO(3$-$2), the CHIMPS ${}^{13}$CO(3$-$2), and $\text{Herschel}$-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 multi-scale view of different environments inside a galaxy regarding molecular emissions, ratios, and thus the estimated star formation activity.

28 pages, 26 figures, accepted by MNRAS (30 May 2025)