The Rest-Frame Ultraviolet Luminosity Density of Star-forming Galaxies at Redshifts z > 3.5

We have measured the rest-frame λ ~ 1500 Å comoving specific luminosity density of star-forming galaxies at redshift 3.5 < z < 6.5 (Lyman break galaxies [LBGs]) selected from deep, multiband images taken with the Hubble Space Telescope and the Advanced Camera for Surveys, obtained as part of the Great Observatories Origins Deep Survey (GOODS). The samples cover ~0.09 deg2 and are also relatively deep, reaching between 0.2L and 0.5L, depending on the redshift, where L is the characteristic UV luminosity of LBGs at z ~ 3. The specific luminosity density appears to be nearly constant with redshift over the range 3 < z < 6, although the measure at z ~ 6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the z ~ 6 sample. If LBGs are fair tracers of the cosmic star formation activity, our results suggest that at z ~ 6, namely, at less than ~7% of the current cosmic age, the universe was already producing stars as vigorously as it did near its maximum several gigayears later, at 1 ≲ z ≲ 3.