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
micron fluxes indicative of strong Halpha emission at z=3.9-4.9. We find that
the percentage of "Halpha excess" sources reaches 37-40% for galaxies with
stellar 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 is
likely due to AGN contamination. We derive star formation rates (SFR) and
specific 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 on
the main sequence of star formation (with log10(sSFR/yr^{-1})<-8.05) or in a
starburst cloud (with log10(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 Msun yr^{-1}
Mpc^{-3}. Finally, we identify an unusual >50sigma overdensity of z=3.9-4.9
galaxies within a 0.20 x 0.20 sq. arcmin 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 which are fundamental to
reveal new aspects of galaxy evolution in the young Universe.Comment: 18 pages, 11 figures, 1 table. Re-submitted to the ApJ, after
addressing referee report. Main changes with respect to v1: a new section and
a new appendix have been added to investigate further the origin and
robustness of the sSFR bimodality. No conclusion change