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

Abstract

ABSTRACT 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 submillimetre-galaxy regime ($S_{850\, \mu \rm m}\lesssim 6~\rm mJy$ beam−1) 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 submillimetre-galaxies fall within 3σ of the main sequence, but 40 per cent are classified as starbursts. Faint submillimetre 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 disc galaxies and mergers. Both populations show an increment towards lower redshifts (z < 2) of their concentration in H-band morphology, but faint submillimetre galaxies on average show larger concentration values at later times. These findings support the claim that faint submillimetre galaxies are mostly a population of massive dust-obscured disc-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 submillimetre galaxies differ from those of optically selected star-forming galaxies of the same stellar mass.