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We study a large galaxy sample from the Spitzer Matching Survey of theUltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced 3.6micron fluxes indicative of strong Halpha emission at z=3.9-4.9. We find thatthe percentage of "Halpha excess" sources reaches 37-40% for galaxies withstellar masses log10(M*/Msun) ~ 9-10, and decreases to <20% at log10(M*/Msun) ~10.7. At higher stellar masses, however, the trend reverses, although this islikely due to AGN contamination. We derive star formation rates (SFR) andspecific SFR (sSFR) from the inferred Halpha equivalent widths (EW) of our"Halpha excess" galaxies. We show, for the first time, that the "Halpha excess"galaxies clearly have a bimodal distribution on the SFR-M* plane: they lie onthe main sequence of star formation (with log10(sSFR/yr^{-1})<-8.05) or in astarburst cloud (with log10(sSFR/yr^{-1}) >-7.60). The latter contains ~15% ofall the objects in our sample and accounts for >50% of the cosmic SFR densityat z=3.9-4.9, for which we derive a robust lower limit of 0.066 Msun yr^{-1}Mpc^{-3}. Finally, we identify an unusual >50sigma overdensity of z=3.9-4.9galaxies within a 0.20 x 0.20 sq. arcmin region. We conclude that the SMUVSunique combination of area and depth at mid-IR wavelengths provides anunprecedented level of statistics and dynamic range which are fundamental toreveal new aspects of galaxy evolution in the young Universe.

Star formation in galaxies at z~4-5 from the SMUVS survey: a clear starburst/main-sequence bimodality for Halpha emitters on the SFR-M* plane