2017MNRAS.467.4304V

We report on five compact, extremely young (<10 Myr) and blue (β_UV < -2.5, F_λ = λ^β) objects observed with VLT/Multi Unit Spectroscopic Explorer at redshifts 3.1169 and 3.235, in addition to three objects at z = 6.145. These sources are strongly magnified (3-40 times) by the Hubble Frontier Field galaxy clusters MACS J0416 and AS1063. Their delensed half-light radii (R_e) are between 16 and 140 pc, the stellar masses are ≃1-20 x 10⁶ M_☉, the magnitudes are m_UV = 28.8-31.4 (-17 < M_UV < -15) and specific star formation rates can be as large as ∼800 Gyr^–1. Remarkably, the inferred physical properties of two objects are similar to those expected in some globular cluster formation scenarios, representing the best candidate proto-GCs discovered so far. Rest-frame optical high-dispersion spectroscopy of one of them at z = 3.1169 yields a velocity dispersion σ_v ≃ 20 km s^–1, implying a dynamical mass dominated by the stellar mass. Another object at z = 6.145, with delensed M_UV ≃ -15.3 (m_UV ≃ 31.4), shows a stellar mass and a star formation rate surface density consistent with the values expected from popular GC formation scenarios. An additional star-forming region at z = 6.145, with delensed m_UV ≃ 32, a stellar mass of 0.5 x 10⁶ M_☉ and a star formation rate of 0.06 M_☉ yr^–1 is also identified. These objects currently represent the faintest spectroscopically confirmed star-forming systems at z > 3, elusive even in the deepest blank fields. We discuss how proto-GCs might contribute to the ionization budget of the Universe and augment Lyα visibility during reionization. This work underlines the crucial role of JWST in characterizing the rest-frame optical and near-infrared properties of such low-luminosity high-z objects.