A new search for star forming regions in the southern outer Galaxy^★

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: koenig@mpifr-bonn.mpg.de
² School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK

Received: 24 September 2020
Accepted: 16 October 2020

Abstract

Context. Star formation in the outer Galaxy is thought to be different from that in the inner Galaxy, as it is subject to different environmental parameters such as metallicity, interstellar radiation field, or mass surface density, which all change with galactocentric radius. Extending our star formation knowledge, from the inner to the outer Galaxy, helps us to understand the influences of the change of the environment on star formation throughout the Milky Way.

Aims. We aim to obtain 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. As one of the largest expanding Galactic super-shells is present in the observed region, a unique opportunity is taken here to investigate the influence of such an expanding structure on star formation as well.

Methods. We used pointed ¹²CO(2–1) observations conducted with the APEX telescope to determine the velocity components towards 830 dust clumps identified from 250 μm Herschel/Hi-GAL SPIRE emission maps in the outer Galaxy between 225° < ℓ < 260°. 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. For this, we determined the dust spectral energy density distributions from archival mid-infrared to submillimeter (submm) emission maps.

Results. We find the identified CO clouds to be strongly correlated with the highest column density parts of the H I 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) capable of forming high-mass stars. This is supported by the fact that only two methanol Class II masers, or 34 known or candidate H II regions, are found in the whole survey area, indicating an even lower fraction that are 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, the properties of the latter are unaffected.

Conclusions. Using the APEX telescope in combination with publicly available Hi-GAL, MSX, and Wise continuum emission maps, we were able to investigate the structure and properties of a region of the Milky Way in unprecedented detail.