2007ApJ...669..749S

We investigate the properties of 1088 Lyman break galaxies (LBGs) at z∼3 selected from a ∼2.63 deg² subregion of the First Look Survey field using the ground-based multicolor data and the Spitzer Space Telescope mid-infrared data at 3-8 and 24 µm. With the wide area and the broad wavelength coverage, we sample a large number of ``rare'' u-band dropouts that are massive (M_*>10¹¹ M_☉), allowing us to perform a statistical analysis of these subsets of LBGs which have not been studied in detail. Optically bright (R_AB≤24.5 mag) LBGs detected in mid-infrared (S_3.6µm≥6 µJy) reside at the most massive and dusty end of the LBG population, with relatively high and tight M/L in rest-frame near-infrared. Most infrared-luminous LBGs (S_24µm≥100 µJy) are dusty star-forming galaxies with star formation rates of 100-1000 M_☉/yr, total infrared luminosity of >10¹² L_☉. By constructing the UV luminosity function of massive LBGs, we estimate that the lower limit for the star formation rate density from LBGs more massive than 10¹¹ M_☉ at z∼3 is ≥3.3x10^–3 M_☉^–1/yr/Mpc3, showing for the first time that the UV-bright population of massive galaxies alone contributes significantly to the global star formation rate density at z∼3. When combined with the star formation rate densities at z<2, our result reveals a steady increase in the contribution of massive galaxies to the global star formation from z=0 to z∼3, providing strong support to the downsizing of galaxy formation.