Massive Lyman Break Galaxies at Z ∼ 3 in the Spitzer Extragalactic First Look Survey

We investigate the properties of 1088 Lyman Break Galaxies (LBGs) at z ∼ 3 selected from a ∼ 2.63 deg2 sub-region of the First Look Survey field using the ground-based multi-color 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 which are massive (M∗ > 10M⊙), allowing us to perform a statistical analysis of these subsets of LBGs that have not been studied in detail. Optically bright (RAB ≤ 24.5 mag) LBGs detected in mid-infrared (S3.6μm ≥ 6μJy) 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 (S24μm ≥ 100μJy) are dusty star-forming galaxies with star formation rates of 100–1000 M⊙/yr, total infrared luminosity of> 10L⊙. 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 10M⊙ at z ∼ 3 is ≥ 3.3× 10M⊙/yr/Mpc, 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. Subject headings: cosmology: observations – galaxies: evolution – galaxies: high-redshift – galaxies: stellar content – galaxies: starburst – infrared: galaxies