Early Low-Mass Galaxies and Star-Cluster Candidates at z~6-9 Identified by the Gravitational Lensing Technique and Deep Optical/Near-Infrared Imaging
Abstract
We present very faint dropout galaxies at z~6-9 with a stellar mass M* down to M*~106Mo 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 357 galaxies with M*~106-109Mo, and find that a stellar mass to UV luminosity LUV ratio M*/LUV is nearly constant at M*~106-109Mo. Our best-estimate M*/LUV function is comparable to a model of star-formation duration time of 100 Myr, but 2-7 times higher than the one of 10 Myr assumed in a previous study (at the 5sigma level) that would probably underestimate M* of faint galaxies. We derive the galaxy stellar mass functions (GSMFs) at z~6-9 that agree with those obtained by previous studies with no M*/LUV assumptions at M*>~108Mo, and that extends to M*~106Mo. Estimating the stellar mass densities rho* with the GSMFs, we find that rho* smoothly increases from log(rho*/[Mo Mpc(-3)])=5.91 +0.75/-0.65 at z~9 to 6.21 +0.39/-0.37 at 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 Re on the source plane, we have pinpointed two objects with low stellar masses (M*<=107Mo) and very compact morphologies (Re<=40 physical pc) that are comparable with those of globular clusters (GCs) in the Milky Way today. These objects are candidates of star clusters that should be a part or a dominant component of high-redshift low-mass galaxy, some of which may be related to GCs today.
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