The Sizes of Massive Quiescent and Star-forming Galaxies at z ∼ 4 with ZFOURGE and CANDELS

We study the rest-frame ultraviolet (UV) sizes of massive (∼ 0.8× {10}¹¹{M}_⊙ ) galaxies at 3.4≤slant z\lt 4.2, selected from the FourStar Galaxy Evolution Survey, by fitting single Sérsic profiles to Hubble Space Telescope/WFC3/F160W images from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. Massive quiescent galaxies are very compact, with a median circularized half-light radius {r}_{{e}}=0.63+/- 0.18 kpc. Removing 5/16 (31%) sources with signs of active galactic nucleus activity does not change the result. Star-forming galaxies have {r}_{{e}}=2.0+/- 0.60 kpc, 3.2+/- 1.3× larger than quiescent galaxies. Quiescent galaxies at z∼ 4 are on average 6.0+/- 1.7× smaller than at z∼ 0 and 1.9+/- 0.7× smaller than at z∼ 2. Star-forming galaxies of the same stellar mass are 2.4+/- 0.7× smaller than at z∼ 0. Overall, the size evolution at 0\lt z\lt 4 is well described by a power law, with {r}_{{e}}=5.08+/- 0.28{(1+z)}^{-1.44+/- 0.08} kpc for quiescent galaxies and {r}_{{e}}=6.02+/- 0.28{(1+z)}^{-0.72+/- 0.05} kpc for star-forming galaxies. Compact star-forming galaxies are rare in our sample: we find only 1/14 (7%) with {r}_{{e}}/{(M/{10}¹¹{M}_⊙ )}^0.75\lt 1.5, whereas 13/16 (81%) of the quiescent galaxies are compact. The number density of compact quiescent galaxies at z∼ 4 is 1.8+/- 0.8× {10}^-5 {{Mpc}}^-3 and increases rapidly, by \gt 5× , between 2\lt z\lt 4. The paucity of compact star-forming galaxies at z∼ 4 and their large rest-frame UV median sizes suggest that the formation phase of compact cores is very short and/or highly dust obscured.

This paper contains data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.