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

Abstract

We report the results of a comprehensive study of the relationship between\ngalaxy size, stellar mass and specific star-formation rate (sSFR) at redshifts\n1.3<z<1.5. Based on a mass complete (M_star >= 6x10^10 Msun), spectroscopic\nsample from the UKIDSS Ultra-deep Survey (UDS), with accurate stellar-mass\nmeasurements derived from spectro photometric fitting, we find that at z~1.4\nthe location of massive galaxies on the size-mass plane is determined primarily\nby their sSFR. At this epoch we find that massive galaxies which are passive\n(sSFR <= 0.1 Gyr^-1) follow a tight size-mass relation, with half-light radii a\nfactor f=2.4+/-0.2 smaller than their local counterparts. Moreover, amongst the\npassive sub-sample we find no evidence that the off-set from the local\nsize-mass relation is a function of stellar population age. Based on a\nsub-sample with dynamical mass estimates we also derive an independent estimate\nof f=2.3+/-0.3 for the typical growth in half-light radius between z~1.4 and\nthe present day. Focusing on the passive sub-sample, we conclude that to\nproduce the necessary evolution predominantly via major mergers would require\nan unfeasible number of merger events and over populate the high-mass end of\nthe local stellar mass function. In contrast, we find that a scenario in which\nmass accretion is dominated by minor mergers can produce the necessary\nevolution, whereby an increase in stellar mass by a factor of ~2, accompanied\nby an increase in size by a factor of ~3.5, is sufficient to reconcile the\nsize-mass relation at z~1.4 with that observed locally. Finally, we note that a\nsignificant fraction (44+/-12%) of the passive galaxies in our sample have a\ndisk-like morphology, providing additional evidence that separate physical\nprocesses are responsible for the quenching of star-formation and the\nmorphological transformation of massive galaxies (abridged).\n