We determine the ensemble properties of z \sim 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 \sim 3 samples , we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1 \mathrm { Gyr } in lookback time .
We find that z \sim 5 LBGs are typically much younger ( < 100 \mathrm { Myr } ) and have lower stellar masses ( \sim 10 ^ { 9 } \mathrm { M _ { \odot } } ) than their z \sim 3 counterparts ( which are typically \sim few \times 10 ^ { 10 } \mathrm { M _ { \odot } } and \sim 320 \mathrm { 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 \sim 5 LBG population ( \ga 70 per cent ) , whereas they comprise \la 30 per cent of LBG samples at z \sim 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 \sim 5 and z \sim 3 .
These young and moderately massive z \sim 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 \sim 5 witnesses a period of wide-spread , recent galaxy formation .
As such , z \sim 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 \sim 6 - 7 , these luminous z \sim 5 LBGs could have only contributed to the UV photon budget at the end of reionisation .
However , their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intra- and inter-galactic media with metals , as has been established for z \sim 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 metal-free stars as has been previously proposed for z \sim 3 LBGs ( ) .