The SCUBA-2 Cosmology Legacy Survey: The EGS deep field -- III. The evolution of faint submillimeter galaxies at $z<4$

We present a demographic analysis of the physical and morphological properties of $450/850~\mu\rm m$-selected galaxies from the deep observations of the SCUBA-2 Cosmology Legacy Survey in the Extended Groth Strip that are detected below the classical submillimeter-galaxy regime ($S_{850 \mu\rm m}\lesssim 6~\rm mJy$/beam) and compare them with a sample of optically-selected star-forming galaxies detected in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey in the same field. We derive the evolution of the main sequence of star-forming galaxies, finding a steeper specific star formation rate versus stellar mass at $z>2.5$ than previous studies. Most faint submillimeter-galaxies fall within $3\sigma$ of the main sequence, but 40~per cent are classified as starbursts. Faint submillimeter galaxies have 50~per cent larger sizes at $2<z<3$ than optically-selected star-forming galaxies of the same mass range. This is also the redshift bin where we find the largest fraction of starbursts, and hence we could be witnessing merging processes, as confirmed by the preference for visual-morphology classifications of these systems as irregular disk galaxies and mergers. Both populations show an increment towards lower redshifts ($z<2$) of their concentration in $H$-band morphology, but faint submillimeter galaxies on average show larger concentration values at later times. These findings support the claim that faint submillimeter galaxies are mostly a population of massive dust-obscured disk-like galaxies that develop larger bulge components at later epochs. While the similarities are great, the median sizes, starburst numbers and $H$-band concentration of faint submillimeter galaxies differ from those of optically-selected star-forming galaxies of the same stellar mass.