The orientations of molecular clouds in the outer Galaxy: Evidence for the scale of the turbulence driver ?

TL;DR

This study investigates the orientations of molecular clouds in the outer Galaxy and finds evidence that supernova explosions influence the turbulence and structure of the interstellar medium even in regions with low star formation rates.

Contribution

It provides observational evidence linking supernova-driven turbulence to molecular cloud orientations in the outer Galaxy, extending understanding of ISM dynamics beyond star-forming regions.

Findings

Significant spatial correlation of cloud PAs on 100-800 pc scales.

Supernova explosions influence molecular cloud orientations in the outer Galaxy.

Outer Galaxy supernovae impact ISM structure despite lower star formation rates.

Abstract

Supernova explosions inject a considerable amount of energy into the interstellar medium (ISM) in regions with high to moderate star formation rates. In order to assess whether the driving of turbulence by supernovae is also important in the outer Galactic disk, where the star formation rates are lower, we study the spatial distribution of molecular cloud (MC) inclinations with respect to the Galactic plane. The latter contains important information on the nature of the mechanism of energy injection into the ISM. We analyze the spatial correlations between the position angles (PAs) of a selected sample of MCs (the largest clouds in the catalogue of the outer Galaxy published by Heyer et al. 2001). Our results show that when the PAs of the clouds are all mapped to values into the [0,90]degrees interval, there is a significant degree of spatial correlation between the $PA$s on spatial scales in the range of 100-800 pc. These scales are of the order of the sizes of individual SN shells in low density environments such as those prevailing in the outer Galaxy and where the metallicity of the ambient gas is of the order of the solar value or smaller. These findings suggest that individual SN explosions, occurring in the outer regions of the Galaxy and in likewise spiral galaxies, albeit at lower rates, continue to play an important role in shaping the structure and dynamics of the ISM in those regions. The SN explosions we postulate here are likely associated with the existence of young stellar clusters in the far outer regions of the Galaxy and the UV emission and low levels of star formation observed with the GALEX satellite in the outer regions of local galaxies.