An ALMA survey of submillimetre galaxies in the Extended Chandra Deep Field South: radio properties and the far-infrared/radio correlation

We present a study of the radio properties of 870μm-selected submillimetre galaxies (SMGs), observed at high resolution with ALMA in the Extended Chandra Deep Field South. From our initial sample of 76 ALMA SMGs, we detect 52 SMGs at > 3σ significance in VLA 1400 MHz imaging, of which 35 are also detected at > 3σ in new 610 MHz GMRT imaging. Within this sample of radio-detected SMGs, we measure a median radio spectral index α 610 = −0.79 ± 0.06, (with inter-quartile range α = [−1.16,−0.56]) and investigate the far-infrared/radio correlation via the parameter qIR , the logarithmic ratio of the rest-frame 8–1000μm flux and monochromatic radio flux. Our median qIR = 2.56± 0.05 (inter-quartile range qIR = [2.42, 2.78]) is higher than that typically seen in single-dish 870μm-selected sources (qIR ∼ 2.4), which may reflect the fact that our ALMA-based study is not biased to radio-bright counterparts, as previous samples were. Finally, we search for evidence that qIR and α evolve with age in a co-dependent manner, as predicted by starburst models: the data populate the predicted region of parameter space, with the stellar mass tending to increase along tracks of qIR versus α in the direction expected, providing the first observational evidence in support of these models.