As a first step for studying star formation in the extreme outer Galaxy ( EOG ) , we obtained deep near-infrared ( J,H,K -bands ) 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 outer Galaxy ( galactic radius R _ { g } \sim 19 kpc ) .
With high spatial resolution ( FWHM \sim 0 \farcs 3 – 0 \farcs 35 ) and deep imaging ( K \sim 21 mag , 5 \sigma ) with the IRCS imager at the Subaru telescope , we detected cluster members with a mass detection limit of < 0.1 M _ { \odot } , which is well into the substellar regime .
These high quality data enables a comparison of EOG to those in the solar neighborhood on the same basis for the first time .
Before interpreting the photometric result , we have first constructed the NIR color-color diagram ( dwarf star track , classical T Tauri star ( CTTS ) locus , reddening law ) in the Mauna Kea Observatory filter system and also for the low metallicity environment since the metallicity in EOG is much lower than those in the solar neighborhood .
The estimated stellar density suggests that an “ isolated type ” star formation is ongoing in Cloud 2-N , while a “ cluster type ” star formation is ongoing in Cloud 2-S .
Despite the difference of the star formation mode , other characteristics of the two clusters are found to be almost identical : ( 1 ) K -band luminosity function ( KLF ) of the two clusters are quite similar , as is the estimated IMF and ages ( \sim 0.5 –1 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 ( \sim 10 % ) .
The similarity of two independent clusters with a large separation ( \sim 25 pc ) strongly suggest that their star formation activities were triggered by the same mechanism , probably the supernova remnant ( GSH 138-01-94 ) as suggested in Kobayashi et al .
and Yasui et al .