The rest-frame ultraviolet luminosity density of star-forming galaxies at redshifts z>3.5

We have measured the rest-frame lambdasimilar to1500 Angstrom comoving specific luminosity density of star-forming galaxies at redshift 3.5<x<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 similar to0.09 deg(2) and are also relatively deep, reaching between 0.2L*(3) and 0.5L*(3), depending on the redshift, where L*(3) is the characteristic UV luminosity of LBGs at zsimilar to3. The specific luminosity density appears to be nearly constant with redshift over the range 3<z<6, although the measure at zsimilar to6 remains relatively uncertain, because it depends on the accurate estimate of the faint counts of the zsimilar to6 sample. If LBGs are fair tracers of the cosmic star formation activity, our results suggest that at zsimilar to6, namely, at less than similar to7% of the current cosmic age, the universe was already producing stars as vigorously as it did near its maximum several gigayears later, at 1less than or similar tozless than or similar to3.