Context :

Aims : We use high-resolution continuum images obtained with the Atacama Large Millimeter Array ( ALMA ) to probe the surface density of star-formation in z \sim 2 galaxies and study the different physical properties between galaxies within and above the star-formation main sequence of galaxies .

Methods : We use ALMA images at 870 \mu m with 0.2 arcsec resolution in order to resolve star-formation in a sample of eight star-forming galaxies at z \sim 2 selected among the most massive Herschel galaxies in the GOODS- South field .
This sample is supplemented with eleven galaxies from the public data of the 1.3 mm survey of the Hubble Ultra-Deep Field , HUDF .
We derive dust and gas masses for the galaxies , compute their depletion times and gas fractions and study the relative distributions of rest-frame UV and far-infrared light .

Results : ALMA reveals systematically dense concentrations of dusty star-formation close to the center of the stellar component of the galaxies .
We identify two different starburst regimes : ( i ) the classical population of starbursts located above the SFR-M _ { \star } main sequence , with enhanced gas fractions and short depletion times and ( ii ) a sub-population of galaxies located within the scatter of the main sequence that experience compact star formation with depletion timescales typical of starbursts of \sim 150 Myr .
In both starburst populations , the far infrared and UV are distributed in distinct regions and dust-corrected star formation rates estimated using UV-optical-NIR data alone underestimate the total star formation rate .
Starbursts hidden in the main sequence show instead the lowest gas fractions of our sample and could represent the last stage of star-formation prior to passivization .
Being Herschel -selected , these main sequence galaxies are located in the high-mass end of the main sequence , hence we do not know whether these ” starbursts hidden in the main sequence ” also exist below 10 ^ { 11 } M _ { \odot } .
Active galactic nuclei are found to be ubiquitous in these compact starbursts , suggesting that the triggering mechanism also feeds the central black hole or that the active nucleus triggers star formation .

Conclusions :