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

Abstract

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 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 HI, radio continuum, and IRAS data in the archives, we discuss the detailed star formation processes in this unique environment. We show clear evidences of a 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 other in the southern molecular cloud core is a dense cluster with cluster-mode star formation. We propose that high compression by the combination of the SNR shell and an adjacent shell caused the dense cluster formation in the southern core. Along with the low metallicity range of the EOG, we suggest that EOG could be an excellent laboratory for the study of star formation processes, such as those triggered by supernovae, that occured 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 matches with that of the EOG well.

Accepted by The Astrophysical Journal (18 pages, 9 figures; a version w/full-resolution color figures is available at http://www.ioa.s.u-tokyo.ac.jp/~naoto/papers/apj.cl2_quirc/ms2p_final.pdf)

References in corpus (26)

• Embedded Clusters in Molecular Clouds
• The Formation of the First Star in the Universe
• The New Galaxy: Signatures of its Formation
• The Formation of the First Stars. I. The Primordial Star Forming Cloud
• A stellar relic from the early Milky Way
• The most metal-poor galaxies
• On the Initial Mass Function of Population III Stars
• Star Formation from Galaxies to Globules
• Protostellar Collapse with Various Metallicities
• Star Formation Properties of a Large Sample of Irregular Galaxies
• Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The HI Disk
• Broad-Band Imaging of a Large Sample of Irregular Galaxies
• The Extreme Outer Regions of Disk Galaxies: I. Chemical Abundances of HII Regions
• Tracing the Galactic thick disk to Solar metallicities
• The first generation of stars: first steps towards chemical evolution of galaxies
• Fossil Imprints of the First Generation Supernova Ejecta in Extremely Metal-Deficient Stars
• Chemical Evolution of the Galactic Halo through Supernova-Induced Star Formation and Its Implication for Population III Stars
• A Comparison of the Chemical Evolutionary Histories of the Galactic Thin Disk and Thick Disk Stellar Populations
• Star Formation in the Extreme Outer Galaxy: Digel Cloud 2 Clusters
• Mid-Infrared Images of Stars and Dust in Irregular Galaxies
• Discovery of Young Stellar Objects at the Edge of the Optical Disk of Our Galaxy
• A star cluster at the edge of the Galaxy
• GSH 138-01-94, an old supernova remnant in the far outer Galaxy
• Deep Near-Infrared Imaging af an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernovae Triggered Star Formation
• Galactic Edge Clouds I: Molecular Line Observations and Chemical Modelling of Edge Cloud 2
• Chemical Evolution of Odd Elements in an Inhomogeneous Early Galaxy

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• Molecular clouds in the extreme outer Galaxy between $l=34.75^{\circ }$ to $45.25^{\circ }$
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• Low-metallicity Young Clusters in the Outer Galaxy. II. Sh 2-208
• Discovery of Star Formation in the Extreme Outer Galaxy Possibly Induced by a High-velocity Cloud Impact
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• Overview Results of JWST Observations of Star-Forming Clusters in the Extreme Outer Galaxy
• GSH 90-28-17: a Possible Old Supernova Remnant
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• A New Search for Star Forming Regions in the Southern Outer Galaxy
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• Cosmic Rays and Non-thermal Emission Induced by Accretion of Cool Gas onto the Galactic Disk
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