2020ApJ...893...60K

We present very faint dropout galaxies at z ∼ 6-9 with a stellar mass M_* down to M_*∼10⁶M_☉ that are found in deep optical/near-infrared (NIR) images of the full data sets of the Hubble Frontier Fields (HFF) program in conjunction with deep ground-based and Spitzer images and gravitational-lensing magnification effects. We investigate stellar populations of the HFF dropout galaxies with the optical/NIR photometry and BEAGLE models made of self-consistent stellar population synthesis and photoionization models, carefully including strong nebular emission impacting on the photometry. We identify 453 galaxies with M_*∼10⁶–10⁹M_☉. Our best-estimate M_*/L_UV function is comparable to a model of star formation duration time of 100 Myr that is assumed in Bouwens et al. We derive the galaxy stellar mass functions (GSMFs) at z ∼ 6-9 that agree with those obtained by previous studies at M_*≳10⁸M_☉, and that extend to M_*∼10⁶M_☉. Estimating the stellar mass densities ρ_* with the GSMFs, we find a very slow evolution from z ∼ 9 to z ∼ 6-7, which is consistent with the one estimated from star formation rate density measurements. In conjunction with the estimates of the galaxy effective radii R_e on the source plane, we have pinpointed four objects with low stellar masses (M_*≤ 10⁷M_☉) and very compact morphologies (R_e≤ 40 pc) that are comparable with those of globular clusters (GCs) in the Milky Way today. These objects are candidates of star clusters, some of which may be related to GCs today.