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

Abstract

Supernova (SN) 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 disc, 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 analyse 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). Our results show that when the PAs of the clouds are all mapped to values into the [0 • , 90 • ] interval, there is a significant degree of spatial correlation between the PAs 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 ultraviolet emission and low levels of star formation observed with the Galaxy Evolution Explorer (GALEX) satellite in the outer regions of local galaxies.

Supernova feedback in a local vertically stratified medium: interstellar turbulence and galactic winds

“…In the Milky Way, the driving scale can be probed by measuring the correlation scale of molecular clouds with similar inclinations with respect to the Galactic disc. Using this method, Dib et al (2009) estimated a driving scale 100 pc comparable to that found in our simulations. In external galaxies, the driving scale can be estimated by measuring the ISM velocity power spectrum, for example using HI gas observations (e.g.…”

The RMS survey: galactic distribution of massive star formation★

“…Lin & Shu (1964) predict that the influence of spiral-arm shocks is significantly weaker in the outer Galaxy beyond the corotation radius (R GC ∼ 8 kpc). This would lead to less confined and smooth spiral features outside this radius, with supernovae probably playing a more important role than the spiral structure in determining the state of the ISM (Dib et al 2009). Our RMS sources are young enough that they are still embedded in their natal molecular clouds and have not been dispersed by the Galactic gravitational potential, and are thus a good probe of changes in the molecular ISM.…”

ATLASGAL – properties of compact H ii regions and their natal clumps★

“…This drop in the metallicity gradients is interpreted by the authors as being due to a gap in the dense gas at the co-rotation radius, with smaller gas flows and consequently fewer mergers producing massive clumps outside the co-rotation radius than inside (see Lumsden et al 2013 for a more detailed discussion). While spiral arms are thought to play an important role in forming dense molecular structures within the co-rotation radius (e.g., Terebey 1998 andDobbs et al 2006) it is likely that this process is regulated by a different mechanism in the outer Galaxy, such as supernovae (Dib et al 2009). Alternatively, the lack of massive clumps in the outer Galaxy could simply be due to statistics rather than any physics since there is a powerlaw mass function for clumps (CMF) and so a smaller population of clumps will have far fewer massive clumps, and none, if the clump population density is below some level (e.g., Snell et al 2002).…”