Star formation in the most distant molecular cloud in the extreme outer Galaxy: A laboratory of star formation in an early epoch of the Galaxy's formation

We report the discovery of active star formation in Digel's Cloud 2, which is one of the most distant giant molecular clouds known in the extreme outer Galaxy ( EOG). At the probable Galactic radius of 20 kpc, Cloud 2 has a quite different environment from that in the solar neighborhood, including lower metallicity, much lower gas density, and small or no perturbation from the spiral arms. With new wide- field near- infrared ( NIR) imaging that covers the entire Cloud 2, we discovered two young embedded star clusters located in the two dense cores of the cloud. Using our NIR and 12CO data, as well as H i, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidence of sequential star formation triggered by the nearby huge supernova remnant GSH 138- 01- 94. The two embedded clusters show a distinct morphology difference: the one in the northern molecular cloud core is a loose association with isolated- mode star formation, while the one in the southern molecular cloud core is a dense cluster with cluster- mode star formation. We propose that high compression due to the combination of the supernova remnant shell and an adjacent shell caused the dense cluster formation in the southern core. In view of the special environment, in particular the low- metallicity range, we suggest that the EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occurred during an early epoch of the Galaxy's formation. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range well matches that of the EOG.