Molecular line signatures of cloud–cloud collisions

F D Priestley,A P Whitworth

doi:10.1093/mnras/stab1777

F D Priestley, A P Whitworth

Open Access

https://doi.org/10.1093/mnras/stab1777

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Abstract

ABSTRACT Collisions between interstellar gas clouds are potentially an important mechanism for triggering star formation. This is because they are able to rapidly generate large masses of dense gas. Observationally, cloud collisions are often identified in position–velocity (PV) space through bridging features between intensity peaks, usually of CO emission. Using a combination of hydrodynamical simulations, time-dependent chemistry, and radiative transfer, we produce synthetic molecular line observations of overlapping clouds that are genuinely colliding, and overlapping clouds that are just chance superpositions. Molecules tracing denser material than CO, such as NH3 and HCN, reach peak intensity ratios of 0.5 and 0.2 with respect to CO in the ‘bridging feature’ region of PV space for genuinely colliding clouds. For overlapping clouds that are just chance superpositions, the peak NH3 and HCN intensities are co-located with the CO intensity peaks. This represents a way of confirming cloud collisions observationally and distinguishing them from chance alignments of unrelated material.

Highlights

Collisions between molecular clouds may trigger or accelerate star formation, and galactic-scale simulations suggest that cloud collisions are relatively common events (Tasker & Tan 2009; Dobbs, Pringle & Duarte-Cabral 2015)
Regardless of viewing angle, the gas density is dominated by the dense material in the shock-compressed layer between the colliding clouds. This results in significant differences in the appearance of the clouds when mapped in different molecular lines, depending on the abundance of the molecule in question and the excitation conditions of the transition
Since the enhanced dense gas emission from NH3 and HCN occurs in the same region of PV space as the CO bridging feature, identifying this enhanced dense gas emission requires that a bridging feature is

Summary

Introduction

Collisions between molecular clouds may trigger or accelerate star formation, and galactic-scale simulations suggest that cloud collisions are relatively common events (Tasker & Tan 2009; Dobbs, Pringle & Duarte-Cabral 2015). Cloud collisions with relative velocities of ∼10 km s−1 appear to be fast enough to result in significant compression at the interface between the clouds, thereby promoting star formation, without being so fast that the clouds are entirely disrupted (Takahira, Tasker & Habe 2014; Balfour et al 2015; Balfour, Whitworth & Hubber 2017; Liow & Dobbs 2020). While the presence of multiple velocity components in molecular line emission may be suggestive of an ongoing cloud collision, it is not conclusive proof, and additional signatures such as cloud-scale emission from shock tracers (Jimenez-Serra et al 2010; Cosentino et al 2018, 2020) are necessary to distinguish collisions from chance line-of-sight alignments. First identified in combined hydrodynamical-radiative transfer modelling of cloud collisions by Haworth et al (2015a), bridging features result from the deceleration of material at the interface between

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Conclusion