  Lyman Break Galaxies (LBGs) are widely thought to be prototypical young
galaxies in the early universe, particularly representative of those undergoing
massive events of star formation. Therefore, LBGs should produce significant
amounts of X-ray emission. We aim to trace the X-ray luminosity of Lyman Break
Galaxies across cosmic time and from that derive constraints on their star
formation history. We utilize the newly released 4 Ms mosaic obtained with the
Chandra X-ray Observatory, the deepest X-ray image to date, alongside with the
superb spectroscopic data sets available in the CDF-S survey region to
construct large but nearly uncontaminated samples of LBGs across a wide range
of redshift (0.5 < z < 4.5) which can be used as input samples for stacking
experiments. This approach allows us to trace the X-ray emission of Lyman Break
Galaxies to even lower, previously unreachable, flux density limits (~10^-18 mW
m^-2) and therefore to larger redshifts. We reliably detect soft-band X-ray
emission from all our input redshift bins except for the highest redshift (z~4)
one. From that we derive rest-frame 2-10 keV luminosities and infer star
formation rates and stellar masses. We find that star formation in LBGs peaks
at a redshift of z_peak~3.5 and then decreases quickly. We also see a
characteristic peak in the specific star formation rate (sSFR=SFR/M_*) at this
redshift. Furthermore, we calculate the contribution of LBGs to the total
cosmic star formation rate density (SFRD) and find that the contribution of
LBGs is negligible. Therefore, we conclude that most of the star formation in
the early universe takes place in lower luminosity galaxies as suggested by
hierarchical structure formation models.
