SEDIGISM-ATLASGAL: dense gas fraction and star formation efficiency across the Galactic disc

By combining two surveys covering a large fraction of the molecular material in the Galactic disc, we investigate the role spiral arms play in the star formation process. We have matched clumps identified by APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) with their parental giant molecular clouds (GMCs) as identified by SEDIGISM, and use these GMC masses, the bolometric luminosities, and integrated clump masses obtained in a concurrent paper to estimate the dense gas fractions (DGF(gmc) = Sigma M-clump/M-gmc) and the instantaneous star formation efficiencies (i.e. SFEgmc = Sigma L-clump/M-gmc). We find that the molecular material associated with ATLASGAL clumps is concentrated in the spiral arms (similar to 60 per cent found within +/- 10 km s(-1) of an arm). We have searched for variations in the values of these physical parameters with respect to their proximity to the spiral arms, but find no evidence for any enhancement that might be attributable to the spiral arms. The combined results from a number of similar studies based on different surveys indicate that, while spiral-arm location plays a role in cloud formation and HI to H-2 conversion, the subsequent star formation processes appear to depend more on local environment effects. This leads us to conclude that the enhanced star formation activity seen towards the spiral arms is the result of source crowding rather than the consequence of any physical process.

Automated summary

By studying the role of spiral arms in the star formation process, it was found that the molecular material associated with ATLASGAL clumps is mostly concentrated in the arms, but there was no evidence of enhanced physical parameters near the arms. The results suggest that star formation activities depend more on local environment effects, rather than the influence of spiral arms.