Rapid build-up of the stellar content in the protocluster core SPT2349−56 at z = 4.3

The protocluster SPT2349$-$56 at $z\,{=}\,4.3$ contains one of the most actively star-forming cores known, yet constraints on total stellar mass this system are highly uncertain. We have therefore carried out deep optical and infrared observations system, probing rest-frame ultraviolet to wavelengths. Using positions spectroscopically-confirmed members, we identify counterparts perform detailed source deblending, allowing us fit spectral energy distributions in order estimate masses. show that galaxies masses proportional their high star-formation rates, consistent with other field submillimetre (SMGs) around redshift 4. average lower molecular gas-to-stellar fractions depletion timescales than SMGs, although considerable scatter. construct stellar-mass function for compare it $z\,{=}\,1$ galaxy clusters, finding shapes between two. measure half-light radii from our HST-F160W imaging, sample similar size typical these redshifts. However, brightest HST-detected sample, found near luminosity-weighted centre core, remains unresolved wavelength. Hydrodynamical simulations predict core will quickly merge into a cluster galaxy, thus provide direct view early formation mechanisms class object.