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 \mu m fluxes indicative of strong H \alpha emission at z = 3.9 - 4.9 .
We find that the percentage of “ H \alpha excess ” sources reaches 37-40 % for galaxies with stellar masses log _ { 10 } ( M ^ { \ast } / M _ { \odot } ) \approx 9 - 10 , and decreases to < 20 \% at log _ { 10 } ( M ^ { \ast } / M _ { \odot } ) \sim 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 H \alpha equivalent widths ( EW ) of our “ H \alpha excess ” galaxies .
We show , for the first time , that the “ H \alpha excess ” galaxies clearly have a bimodal distribution on the SFR- M ^ { \ast } 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 \sim 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 _ { \odot } yr ^ { -1 } Mpc ^ { -3 } .
Finally , we identify an unusual > 50 \sigma overdensity of z = 3.9 - 4.9 galaxies within a 0.20 \times 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 which are fundamental to reveal new aspects of galaxy evolution in the young Universe .