Star Formation in Galaxies at z ∼ 4–5 from the SMUVS Survey: A Clear Starburst/Main-sequence Bimodality for Hα 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 H α emission at z = 3.9 – 4.9 . We find that the percentage of “H α excess” sources reaches 37%–40% for galaxies with stellar masses log 10 ( M * / M ⊙ ) ≈ 9 – 10 and decreases to < 20 % at log 10 ( M * / M ⊙ ) ∼ 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 H α equivalent widths of our “H α excess” galaxies. We show, for the first time, that the “H α 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 ⊙ yr − 1 Mpc − 3 . Finally, we identify an unusual > 50 σ overdensity of z = 3.9 – 4.9 galaxies within a 0.20 × 0.20 arcmin 2 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.