The sizes, masses and specific star formation rates of massive galaxies at 1.3 < z < 1.5: strong evidence in favour of evolution via minor mergers

We report the results of a comprehensive study of the relationship between galaxy size, stellar mass and specific star-formation rate (sSFR) at redshifts 1. 3= 6x10¹0 Msun), spectroscopic sample from the UKIDSS Ultra-deep Survey (UDS), with accurate stellar-mass measurements derived from spectro photometric fitting, we find that at z~1. 4 the location of massive galaxies on the size-mass plane is determined primarily by their sSFR. At this epoch we find that massive galaxies which are passive (sSFR <= 0. 1 Gyr^-1) follow a tight size-mass relation, with half-light radii a factor f=2. 4+/-0. 2 smaller than their local counterparts. Moreover, amongst the passive sub-sample we find no evidence that the off-set from the local size-mass relation is a function of stellar population age. Based on a sub-sample with dynamical mass estimates we also derive an independent estimate of f=2. 3+/-0. 3 for the typical growth in half-light radius between z~1. 4 and the present day. Focusing on the passive sub-sample, we conclude that to produce the necessary evolution predominantly via major mergers would require an unfeasible number of merger events and over populate the high-mass end of the local stellar mass function. In contrast, we find that a scenario in which mass accretion is dominated by minor mergers can produce the necessary evolution, whereby an increase in stellar mass by a factor of ~2, accompanied by an increase in size by a factor of ~3. 5, is sufficient to reconcile the size-mass relation at z~1. 4 with that observed locally. Finally, we note that a significant fraction (44+/-12%) of the passive galaxies in our sample have a disk-like morphology, providing additional evidence that separate physical processes are responsible for the quenching of star-formation and the morphological transformation of massive galaxies (abridged).