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Recently, it has become familiar to follow an alternative method, which focuses on the emission properties of the galaxy population as a whole. It traces the cosmic evolution with redshift of the galaxy luminosity density and offers the prospect of an empirical determination of the global star formation history of the universe and IMF of stars independently of the merging histories, complex evolutionary phases, and possibly short-lived star formation episodes of individual galaxies. The technique relies on two basic properties of stellar populations: a) the UV-continuum emission in all but the oldest galaxies is dominated by short-lived massive stars, and is therefore a direct measure, for a given IMF and dust content, of the instantaneous star formation rate; and b) the rest-frame near-IR light is dominated by near-solar mass evolved stars, the progenitors of which make up the bulk of a galaxy's stellar mass, and is more sensitive to the past star-formation history than the blue (and UV) light. By modeling the ``emission history'' of the universe at ultraviolet, optical, and near-infrared wavelengths from the present epoch to high redshifts, one should be able to shed light on some key questions in galaxy formation and evolution studies: Is there a characteristic epoch of star and metal formation in galaxies? What fraction of the luminous baryons observed today were already locked into galaxies at early epochs? Are high-z galaxies obscured by dust? Do spheroids form early and rapidly? Is there a universal IMF?

Figure 4

Figure 4. Left: Mean comoving density of star formation as a function of cosmic time. The data points with error bars have been inferred from the UV-continuum luminosity densities of [32] (filled dots), [8] (filled squares), [37] (filled pentagons), [54] (empty dot), and [51] (empty square). The dotted line shows the fiducial rate, < rhodot* > = 0.054 Msun Mpc-3, required to generate the observed EBL. Right: dust corrected values (A1500 = 1.2 mag). The Halpha determinations of [17], [53], [19] (filled triangles), together with the SCUBA lower limit [27] (empty pentagon) have been added for comparison.

The comoving volume-averaged history of star formation follows a relatively simple dependence on redshift. Its latest version, uncorrected for dust extinction, is plotted in Figure 4 (left). The measurements are based upon the rest-frame UV luminosity function (at 1500 and 2800 Å), assumed to be from young stellar populations [34]. The prescription for a ``correct'' de-reddening of these values has been the subject of an ongoing debate. Dust may play a role in obscuring the UV continuum of Canada-France Reshift Survey (CFRS, 0.3 < z <1) and Lyman-break (z approx 3) galaxies, as their colors are too red to be fitted with an evolving stellar population and a Salpeter IMF [37]. Figure 4 (right) depicts an extinction-corrected version of the same plot. The best-fit cosmic star formation history (shown by the dashed-line) with such a universal correction produces a total EBL of 37 n W m-2 sr-1. About 65% of this is radiated in the UV+optical+near-IR between 0.1 and 5 µm; the total amount of starlight that is absorbed by dust and reprocessed in the far-IR is 13 n W m-2 sr-1. Because of the uncertainties associated with the incompleteness of the data sets, photometric redshift technique, dust reddening, and UV-to-SFR conversion, these numbers are only meant to be indicative. On the other hand, this very simple model is not in obvious disagreement with any of the observations, and is able, in particular, to provide a reasonable estimate of the galaxy optical and near-IR luminosity density.

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