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 galaxy size, stellar mass and specific star-formation rate (sSFR) at redshifts 1.3 < z < 1.5. Based on a mass complete (M 6×10 10 M ), spectroscopic sample from the UKIDSS Ultradeep Survey (UDS), with accurate stellar-mass measurements derived from spectrophotometric fitting, we find that at z 1.4 the location of massive galaxies on the sizemass 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 g = 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 sizemass relation is a function of stellar population age. In contrast, we find that massive star-forming galaxies at this epoch lie closer to the local late-type size-mass relation and are only a factor f g = 1.6 ± 0.2 smaller than observed locally. Based on a subsample with dynamical mass estimates, which consists of both passive and star-forming objects, we also derive an independent estimate of f g = 2.3±0.3 for the typical growth in half-light radius between z 1.4 and the present day. Focusing on the passive subsample, 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 comfortably produce the necessary evolution, whereby an increase in stellar mass of only a factor of 2, accompanied by size growth of a factor of 3.5, is required to reconcile the sizemass 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 morphological transformation in massive galaxies.

Galaxy and Mass Assembly (GAMA): merging galaxies and their properties

“…This is also the conclusion of several studies in this field (e.g., Lopez-Sanjuan et al 2012;Huang et al 2013;Kaviraj et al 2013;McLure et al 2013;Fritz et al 2014) and is broadly in agreement with more recentmodels where minor mergers and secular evolution may play more important roles in galaxy evolution in the past 1/2 of the Hubble time (e.g., Guo & White 2008;Parry et al 2009). …”

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…This is consistent with the idea that compact SFGs are formed from extended SFGs, which are mostly disk dominated at low z, by increasing their central densities. The evolution toward disk-like morphologies with progressively bigger cores is consistent with the prominence of disks with bulges among massive SFGs at z 1  , as reported in previous works (e.g., Bruce et al 2012Bruce et al , 2014aBuitrago et al 2013;McLure et al 2013;Mortlock et al 2013;Lang et al 2014;Huertas-Company et al 2015). Note that rejuvenation and/or the re-growth of a star-forming disk in compact SFGs is also a possibility, although the increasing number of quiescent galaxies with time suggests that the net flow of galaxies is preferentially toward compaction and quenching.…”

Evolution induced by dry minor mergers onto fast-rotator S0 galaxies

“…Naab et al 2009;Lotz et al 2011;Oser et al 2012;McLure et al 2013). Therefore, an evolutionary scenario in which major mergers at earlier epochs give rise to a population of initial S0s, that later minor mergers can evolve into different dynamical systems is quite plausible (e.g., the scenario proposed by Sil'chenko et al 2012, to explain the age and metallicity properties of S0 galaxies).…”