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

We use >9400 log(m/M-circle dot)>10 quiescent and star-forming galaxies at z less than or similar to 2 in COSMOS/UltraVISTA to study the average size evolution of these systems, with focus on the rare. ultra-massive population at log(m/M-circle dot)>11.4. The large 2. square degree survey area delivers a sample of similar to 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 log(m/M-circle dot)> 11.0 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-circle dot, and that major mergers likely play a major role in the final steps of their evolution.