Cloud Collision Signatures in the Central Molecular Zone

TL;DR

This study uses simulations to analyze how different molecular lines reveal cloud collision signatures in the Central Molecular Zone, showing that alternative tracers are more effective than CO under CMZ conditions.

Contribution

It provides the first combined hydrodynamical, chemical, and radiative transfer simulations of cloud collisions specifically tailored to CMZ conditions, validating alternative tracers for collision detection.

Findings

CO bridging feature is less distinct in CMZ conditions.

Alternative tracers like CS, HCO+, N2H+ effectively reveal collision signatures in CMZ.

Simulations confirm the suitability of non-CO tracers for CMZ cloud collision studies.

Abstract

Molecular cloud collisions are a prominent theory for the formation of stars. Observational studies into cloud collisions identify the collision via a bridging feature: a continuous strip of line emission that connects two intensity peaks that are related in position space and separated in velocity space. Currently, most observations of collisions and these bridging features take place in the Milky Way disc. They are also theorized to take place in the Central Molecular Zone (CMZ), where temperatures and densities are both significantly higher than in the disc. For studies in the Milky Way Disc, the most commonly-used tracer tends to be CO. However, for studies in the CMZ, where the density and temperature are significantly higher, low-J CO lines lose their ability to adequately highlight the bridging feature of cloud collisions. As a result, studies have begun using other tracers, whose physical and chemical behavior has not been studied under CMZ conditions. We perform combined hydrodynamical, chemical and radiative transfer simulations of cloud collisions under both disc- and CMZ-like conditions, and investigate collision signatures in a number of commonly-observed molecular lines. Under the Milky Way disc conditions CO has the standard bridging feature; however, the other tracers, CS, HCO$^+$, N$_2$H$^+$ only emit in the intermediate-velocity bridge region, making the feature itself challenging to detect. In the CMZ, the higher density and temperature make the bridging feature far more indistinct for CO, but the other tracers have morphologically similar bridging features to the CO disc model, validating their use as tracers of cloud collisions under these conditions.