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 870 mu m-selected submillimetre galaxies (SMGs), observed at high resolution with Atacama Large Millimeter Array (ALMA) in the Extended Chandra Deep Field South. From our initial sample of 76 ALMA SMGs, we detect 52 SMGs at >3 sigma significance in Karl G. Jansky Very Large Array 1400 MHz imaging, of which 35 are also detected at >3 sigma in new 610 MHz Giant Metre-Wave Radio Telescope imaging. Within this sample of radio-detected SMGs, we measure a median radio spectral index a alpha(1400)(610) = -0.79 +/- 0.06, (with inter-quartile range alpha = [-1.16, -0.56]) and investigate the far-infrared/radio correlation via the parameter q(IR), the logarithmic ratio of the rest-frame 8-1000 mu m flux and monochromatic radio flux. Our median q(IR) = 2.56 +/- 0.05 (inter-quartile range q(IR) = [2.42, 2.78]) is higher than that typically seen in single-dish 870 mu m-selected sources (q(IR) similar to 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 q(IR) and alpha evolve with age in a codependent 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 q(IR) versus alpha in the direction expected, providing the first observational evidence in support of these models.