N2H+(1-0) as a tracer of dense gas in and between spiral arms
Abstract
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 CHIMPS 13CO(3-2) survey, one in the Sagittarius-arm and one in an inter-arm region, were mapped with dense gas tracers inside a 0.06 deg2 area and with a spatial resolution of around 0.4 and 0.65 pc at the distance of the targets using the IRAM 30m telescope, 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% 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 inter-arm 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.
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