Unveiling the most elusive star-forming galaxies

Our knowledge of the cosmic star formation history is crucially dependent on completeness of the observed samples, and the best efforts must be made to ensure the best completeness levels against two classes of sources: intrinsically faint galaxies and heavily dust-obscured ones.
In the first part of the talk I will show recent results on the relation between the SFR and the stellar mass, i.e. the Main Sequence (MS) relation of star-forming galaxies, in the HST Frontier Fields. Gravitational lensing allows us to extend the analysis of the MS down to stellar masses as low as 10^7.5 Msun at z<4 and 10^8 Msun at higher redshift, i.e. a factor of ~10 below previous results. I will show that the scatter around the MS increases at such low stellar masses, suggesting a larger variety of star formation histories, in agreement with theoretical predictions.
In the second part of the talk I will present a detailed study of a representative of the elusive dust-enshrouded star-forming population that is typically not included in the measured SFH nor in the stellar mass density. Thanks to ALMA data, an apparently extremely dust-rich galaxy turned out to be a faint submillimeter galaxy (SMG) lensed by a foreground source and completely obscured at optical wavelengths. A photo-z of z~3.3 and the physical parameters are estimated thanks to an accurate reconstruction of the SED. With a SFR lower than 300 Msun/yr, this galaxy belongs to the class of faint SMGs (S870um~2.5mJy) that are important contributors to the cosmic SFH. This faint galaxy population thus likely represents an important and missing piece of our understanding of the cosmic SFH and galaxy evolution.