2024ApJ...975..152Y

We present the substellar mass function of star-forming clusters (≃0.1 Myr old) in a low-metallicity environment (≃-0.7 dex). We performed deep JWST/NIRCam and MIRI imaging of two star-forming clusters in Digel Cloud 2, a star-forming region in the outer Galaxy (R_G ≳ 15 kpc). The very high sensitivity and spatial resolution of JWST enable us to resolve cluster members clearly down to a mass detection limit of 0.02 M_⊙, enabling the first detection of brown dwarfs in low-metallicity clusters. A total of 52 and 91 sources were extracted in mass-A_V-limited samples in the two clusters, from which initial mass functions (IMFs) were derived by model-fitting the F200W band luminosity function, resulting in IMF peak masses (hereafter M_C) $\mathrm{log}{M}_{C}/{M}_{\odot }\simeq -1.5\pm 0.5$ for both clusters. Although the uncertainties are rather large, the obtained M_C values are lower than those in any previous study ($\mathrm{log}{M}_{C}/{M}_{\odot }\sim -0.5$). Comparison with the local open clusters with similar ages to the target clusters (∼10⁶-10⁷ yr) suggests a metallicity dependence of M_C, with lower M_C at lower metallicities, while the comparison with globular clusters, with similarly low metallicities but considerably older (∼10¹⁰ yr), suggests that the target clusters have not yet experienced significant dynamical evolution and remain in their initial physical condition. The lower M_C is also consistent with the theoretical expectation of the lower Jeans mass owing to the higher gas density under such low metallicity. The M_C values derived from observations in such an environment would place significant constraints on the understanding of star formation.