5. A CONSTANT STAR-FORMATION DENSITY?

Based on the agreement between the z 3 and z 4 luminosity functions at the bright end, it has been recently argued by [54] that the decline in the luminosity density of faint HDF Lyman-break galaxies observed in the same redshift interval [33] may not be real, but simply due to sample variance in the HDF. When extinction corrections are applied, the emissivity per unit comoving volume due to star formation may then remain essentially flat for all redshift z 1 (see Fig. 2). While this has obvious implications for hierarchical models of structure formation, the epoch of first light, and the reionization of the intergalactic medium (IGM), it is also interesting to speculate on the possibility of a constant star-formation density at all epochs 0 z 5, as recently advocated by [41]. Figure 3 (left panel) shows the time evolution of the near-IR rest-frame luminosity density of a stellar population characterized by a Salpeter IMF, solar metallicity, and a (constant) star-formation rate of _* = 0.054 M yr^-1 Mpc^-3 (needed to produce the observed EBL) ⁽¹⁾. The predicted evolution appears to be a poor match to the observations: it overpredicts the local K-band luminosity density [16] and undepredicts the 1 µm emissivity at z 1 from the CFRS survey [31].