Deep Near‐Infrared Imaging of an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernova‐Triggered Star Formation

Abstract

While conducting a near-infrared (NIR) survey of ''Digel clouds,'' which are thought to be located in the extreme outer Galaxy (EOG), Kobayashi & Tokunaga found two embedded young clusters in ''Cloud 2,'' a giant molecular cloud at the Galactic radius of $20 kpc. Because the molecular cloud is located in the vicinity of a supernova remnant (SNR) H i shell, GSH 138À01À94, it was suggested that the star formation activity in Cloud 2 was triggered by this expanding H i shell. We obtained deep NIR images of one of the embedded clusters in Cloud 2 with high sensitivity (K $ 20 mag, 10). We identified 52 cluster members. The estimated stellar density ($10 pc À2) suggests that the cluster is a T association. This is the deepest NIR imaging of an embedded cluster in the EOG. The observed K-band luminosity function (KLF) suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit of $0.1 M is not significantly different from the typical IMFs in the field and in the nearby star clusters. The overall characteristics of this cluster appear to be similar to those of other embedded clusters in the far outer Galaxy. The estimated age of the cluster from the KLF, which is less than 1 Myr, is consistent with the view that the star formation was triggered by the H i shell whose age was estimated at 4.3 Myr (Stil & Irwin). The three-dimensional geometry of the SNR shell, the molecular cloud, and the embedded cluster, which is inferred from our data, as well as the cluster's age, strongly suggest that the star formation in Cloud 2 was triggered by the SNR shell.