CANDELS: THE PROGENITORS OF COMPACT QUIESCENT GALAXIES AT z ∼ 2

We combine high-resolution Hubble Space Telescope /WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive ( M ⋆ > 10 10 M ☉ ) galaxies at redshifts z = 1.4–3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5–3. At z ≳ 2, cSFGs present SFR = 100–200 M ☉ yr −1 , yet their specific star formation rates (sSFR ∼ 10 −9 yr −1 ) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30 times (∼30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10 8 yr). The cSFGs are continuously being formed at z = 2–3 and fade to cQGs down to z ∼ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early ( z ≳ 2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival ( z ≲ 2) path in which larger SFGs form extended QGs without passing through a compact state.