Star Formation in Galaxies at z similar to 4-5 from the SMUVS Survey: A Clear Starburst/Main-sequence Bimodality for H alpha Emitters on the SFR-M* Plane

We study a large galaxy sample from the Spitzer Matching Survey of the UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced 3.6 μ {{m}} fluxes indicative of strong Hα emission at z=3.9{--}4.9. We find that the percentage of “Hα excess” sources reaches 37%–40% for galaxies with stellar masses {{log}}10({M}* /{M}ȯ )≈ 9{--}10 and decreases to <20 % at {{log}}10({M}* /{M}ȯ )∼ 10.7. At higher stellar masses, however, the trend reverses, although this is likely due to active galactic nucleus contamination. We derive star formation rates (SFR) and specific SFR (sSFR) from the inferred Hα equivalent widths of our “Hα excess” galaxies. We show, for the first time, that the “Hα excess” galaxies clearly have a bimodal distribution on the SFR–M* plane: they lie on the main sequence of star formation (with {{log}}10({sSFR}/{{yr}}-1)<-8.05) or in a starburst cloud (with {{log}}10({sSFR}/{{yr}}-1)> -7.60). The latter contains ∼ 15 % of all the objects in our sample and accounts for > 50 % of the cosmic SFR density at z=3.9{--}4.9, for which we derive a robust lower limit of 0.066 {M}ȯ {{yr}}-1 {{Mpc}}-3. Finally, we identify an unusual > 50σ overdensity of z=3.9{--}4.9 galaxies within a 0.20× 0.20 arcmin2 region. We conclude that the SMUVS unique combination of area and depth at mid-IR wavelengths provides an unprecedented level of statistics and dynamic range that are fundamental to revealing new aspects of galaxy evolution in the young universe.