A New Search for Star Forming Regions in the Southern Outer Galaxy

TL;DR

This study investigates the structure and star-forming potential of the outer Galaxy using CO observations, revealing its complex three-dimensional structure and a low likelihood of high-mass star formation in this region.

Contribution

It provides detailed physical properties of star-forming clumps in the outer Galaxy and analyzes their relation to large-scale structures, a novel comprehensive survey in this region.

Findings

Strong correlation between CO clouds and high-density Hi emission regions

Maximum 6% of clumps can form high-mass stars

No correlation between physical parameters and expanding supershells

Abstract

Star-formation in the outer Galaxy is thought to be different from the inner Galaxy, as it is subject to different environmental parameters such as metallicity, interstellar radiation field, or mass surface density that all change with Galactocentric radius. We therefore aimed at getting a more detailed view on the structure of the outer Galaxy, determining physical properties for a large number of star forming clumps and understanding star-formation outside the Solar circle. We use pointed $^{12}$CO(2-1) observations conducted with the APEX telescope to determine the velocity components towards 830 dust clumps identified from 250 $\mu$m Herschel/Hi-GAL SPIRE emission maps in the outer Galaxy between $225\deg<\ell<260\deg$. We determined kinematic distances from the velocity components, in order to analyze the structure of the outer Galaxy and to estimate physical properties such as dust temperatures, bolometric luminosities, clump masses, and H2 column densities for 611 clumps. We find the CO clouds to be strongly correlated with the highest column density parts of the Hi emission distribution, spanning a web of bridges, spurs and blobs of star forming regions between the larger complexes, unveiling the complex three-dimensional structure of the outer Galaxy in unprecedented detail. Using the physical properties of the clumps, we find an upper limit of 6% (40 sources) to be able to form high-mass stars. This is supported by the fact that only 2 methanol Class II masers or 34 known or candidate Hii regions are found in the whole survey area, indicating an even lower fraction to be able to form high-mass stars in the outer Galaxy. We fail to find any correlation of the physical parameters of the identified (potential) star forming regions with the expanding supershell, indicating that although the shell organizes the interstellar material into clumps, their properties are unaffected.