Constraints on Quenching of Z ≲ 2 Massive Galaxies from the Evolution of the Average Sizes of Star-forming and Quenched Populations in COSMOS

Abstract

We use >9400 quiescent and star-forming galaxies at z  ≲ 2 in COSMOS/UltraVISTA to study the average size evolution of these systems, with focus on the rare ultra-massive population at . The large 2 square degree survey area delivers a sample of ∼400 such ultra-massive systems. Accurate sizes are derived using a calibration based on high-resolution images from the Hubble Space Telescope . We find that at these very high masses, the size evolution of star-forming and quiescent galaxies is almost indistinguishable in terms of normalization and power-law slope. We use this result to investigate possible pathways of quenching massive m  >  M * galaxies at z  < 2. We consistently model the size evolution of quiescent galaxies from the star-forming population by assuming different simple models for the suppression of star formation. These models include an instantaneous and delayed quenching without altering the structure of galaxies and a central starburst followed by compaction. We find that instantaneous quenching reproduces the observed mass-size relation of massive galaxies at z  > 1 well. Our starburst+compaction model followed by individual growth of the galaxies by minor mergers is preferred over other models without structural change for galaxies at z  > 0.5. None of our models is able to meet the observations at m  >  M * and z  < 1 without significant contribution of post-quenching growth of individual galaxies via mergers. We conclude that quenching is a fast process in galaxies with m  ≥ 10 11 M ⊙ , and that major mergers likely play a major role in the final steps of their evolution.