Extreme Outer Galaxy: A Laboratory of Star Formation in an Early Epoch of Galaxy Formation

TL;DR

This paper investigates star formation in the extreme outer Galaxy, a low-metallicity, low-density environment, to understand early galaxy formation processes and the origin of the thick disk.

Contribution

It provides detailed analysis of star formation in the EOG, especially supernova-triggered processes, using multi-wavelength data on a molecular cloud beyond the Outer arm.

Findings

Star formation occurs in low-metallicity, low-density environments.

Supernova-triggered star formation is significant in the EOG.

Insights into early galaxy formation and thick disk origin.

Abstract

The extreme outer Galaxy (EOG) has a very different environment from that in the solar neighborhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. The EOG is an excellent laboratory for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. We show an example of a molecular cloud in the EOG (Digel's Cloud 2), which is located at R_g ~ 20 kpc beyond the Outer arm. Based on our NIR and 12CO data as well as HI, radio continuum, and IRAS data in the archives, we examined the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.