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We report the discovery of active star formation in Digel's Cloud 2, which isone of the most distant giant molecular clouds known in the extreme outerGalaxy (EOG). At the probable Galactic radius of ~20 kpc, Cloud 2 has a quitedifferent environment from that in the solar neighborhood, including lowermetallicity, much lower gas density, and small or no perturbation from spiralarms. With new wide-field near-infrared (NIR) imaging that covers the entireCloud 2, we discovered two young embedded star clusters located in the twodense cores of the cloud. Using our NIR and 12CO data as well as HI, radiocontinuum, and IRAS data in the archives, we discuss the detailed starformation processes in this unique environment. We show clear evidences of asequential star formation triggered by the nearby huge supernova remnant, GSH138-01-94. The two embedded clusters show a distinct morphology difference: theone in the northern molecular cloud core is a loose association withisolated-mode star formation, while the other in the southern molecular cloudcore is a dense cluster with cluster-mode star formation. We propose that highcompression by the combination of the SNR shell and an adjacent shell causedthe dense cluster formation in the southern core. Along with the lowmetallicity range of the EOG, we suggest that EOG could be an excellentlaboratory for the study of star formation processes, such as those triggeredby supernovae, that occured during an early epoch of the Galaxy's formation. Inparticular, the study of the EOG may shed light on the origin and role of thethick disk, whose metallicity range matches with that of the EOG well.

astrophysical journal/the astrophysical journal

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