STAR FORMATION IN THE EXTREME OUTER GALAXY: DIGEL CLOUD 2 CLUSTERS

摘要

We obtained deep near-infrared (NIR) images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star-forming regions in the extreme outer Galaxy, at the Galactic radius (R_g) of ～19 kpc. We detected cluster members with a mass detection limit of ＜0.1 solar mass , which is well into the substellar regime (K ～ 21 mag, 5 σ). These high-quality data enable a comparison of EOG values to those in the solar neighborhood on the same basis for the first time. We first constructed the NIR color-color diagram in the Mauna Kea Observatory (MKO) filter system and also for the low-metallicity environment, since the metallicity in the EOG is much lower than it is in the solar neighborhood. The estimated stellar density suggests that "isolated-type" star formation is ongoing in Cloud 2-N, while "cluster-type" star formation is ongoing in Cloud 2-S. Despite this different star formation mode, other characteristics of the two clusters are almost identical: (1) the K-band luminosity functions (KLFs) of the two clusters are quite similar, as are the estimated IMFs and ages (～0.5-l Myr) from the KLF fitting; (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood. The similarity of two independent clusters with a large separation (～25 pc) strongly suggests that their star formation activities were triggered by the same mechanism, probably a supernova remnant (GSH 138-01-94), as suggested in our earlier works.