2024MNRAS.530.1311F

Recent advances in identifying giant molecular filaments in Galactic surveys allow us to study the interstellar material and its dense, potentially star forming phase on scales comparable to resolved extragalactic clouds. Two large filaments detected in the ¹³CO/C¹⁸O(J = 3-2) Heterodyne Inner Milky Way Plane Survey (CHIMPS) survey, one in the Sagittarius-arm and one in an interarm region, were mapped with dense gas tracers inside a 0.06 square degrees area and with a spatial resolution of around 0.4 and 0.65 pc at the distance of the targets using the 30 m telescope of the Institut de Radioastronomie Millimétrique (IRAM) to investigate the environmental dependence of the dense gas fraction. The N₂H⁺(1 - 0) transition, an excellent tracer of the dense gas, was detected in parsec-scale, elliptical clumps and with a filling factor of around 8.5 per cent in our maps. The N₂H⁺-emitting areas appear to have higher dense gas fraction (e.g. the ratio of N₂H⁺ and ¹³CO emission) in the interarm than in the arm which is opposite to the behaviour found by previous studies, using dust emission rather than N₂H⁺ as a tracer of dense gas. However, the arm filament is brighter in ¹³CO and the infrared emission of dust, and the dense gas fraction determined as above is governed by the ¹³CO brightness. We caution that measurements regarding the distribution and fraction of dense gas on these scales may be influenced by many scale- and environment-dependent factors, as well as the chemistry and excitation of the particular tracers, then consider several scenarios that can reproduce the observed effect.