Sub-arcsecond-resolution LOFAR observations of bright sub-millimetre galaxies in the North Ecliptic Pole field

Bright sub-millimetre galaxies (SMGs) contribute significantly to the star formation rate density (20–50%) and stellar mass density (∼30–50%) at redshifts 2–4 with star formation rates (SFRs) ≳ 1000 M⊙ yr−1 and stellar masses of ∼1011−1012 M⊙. The number of bright SMGs with such high SFRs is hard to reconcile with the standard models of galaxy formation and evolution. In this paper we provide evidence that, in a small sample of 12 bright SMGs, the SFRs derived from spectral energy distribution (SED) fitting are significantly higher than those obtained using low-frequency radio emission as a proxy for star formation. Using the International LOFAR Telescope (ILT), which allows imaging at 144 MHz with sub-arcsecond angular resolution, we have produced deep images of a small sample of bright SMGs in the North Ecliptic Pole (NEP) field extracted from the NEPSC2 survey. For all 12 SMGs, we find radio-emitting mid-infrared galaxies at distances from a few arcseconds down to sub-arcsecond scales from the SMG and/or the presence of a radio-emitting active galactic nucleus (AGN). The SFRs derived from the radio emission of the SMG, disentangled from the AGN-related radio emission, are systematically lower by a factor of ∼5 (median value) than those derived from the multi-band SED fitting. We discuss whether our assumptions might be, at least in part, responsible for the observed discrepancy. We argue that the radio-derived SFRs are not systematically underestimated but can be affected by a significant dispersion (0.3–0.5 dex). Considering these new SFR estimates, the offset of the specific SFR of the 12 bright SMGs from the star-forming galaxy main sequence (Δ(SSFR)) is significantly reduced. Using sub-arcsecond radio images to disentangle the contribution of the AGN and the radio emission as a proxy for the determination of the SFRs, we find that all 12 bright SMGs are found in star-forming galaxies (SFGs) or hybrid SFG–AGN systems that, on average, are only a factor of 2 more star-forming than the main sequence galaxies.