Lyman‐break galaxies at z ∼ 5 – I. First significant stellar mass assembly in galaxies that are not simply z ∼ 3 LBGs at higher redshift

We determine the ensemble properties of z ∼ 5 Lyman‐break galaxies (LBGs) selected as V ‐band dropouts to i AB < 26.3 in the Chandra Deep Field ‐South using their rest‐frame UV‐to‐visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for z ∼ 3 samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1 Gyr in look‐back time. We find that z ∼ 5 LBGs are typically much younger (<100 Myr) and have lower stellar masses (∼10 9  M ⊙ ) than their z ∼ 3 counterparts (which are typically ∼ few × 10 10  M ⊙ and ∼320 Myr old). The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous z ∼ 5 LBG population (≳70 per cent), whereas they comprise ≲30 per cent of LBG samples at z ∼ 3 . This result, which we demonstrate is robust under all reasonable modelling assumptions, shows a clear change in the properties of the luminous LBGs between z ∼ 5 and z ∼ 3 . These young and moderately massive z ∼ 5 LBGs appear to be experiencing their first (few) generations of large‐scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that z ∼ 5 witnesses a period of wide‐spread, recent galaxy formation. As such, z ∼ 5 LBGs are the likely progenitors of the spheroidal components of present‐day massive galaxies. This is supported by their high stellar mass surface densities, and is consistent with their core phase‐space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. With implied formation redshifts of z ∼ 6–7 , these luminous z ∼ 5 LBGs could have only contributed to the UV photon budget at the end of reionization. However, their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intragalactic and intergalactic media with metals, as has been established for z ∼ 3 LBGs. Their estimated young ages are consistent with inefficient metal‐mixing on galaxy‐wide scales. Therefore these galaxies may contain a significant fraction of Population III stars as proposed for z ∼ 3 LBGs by Jiminez & Haimann.