2018ApJ...867....1K

We present the rest-frame optical sizes of massive quiescent galaxies (QGs) at z ∼ 4 measured at the K'-band with the Infrared Camera and Spectrograph and adaptive optics (AO) facility, AO188, on the Subaru telescope. Based on a deep multiwavelength catalog in the Subaru XMM-Newton Deep Survey Field, covering a wide wavelength range from the u-band to the IRAC 8.0 µm over 0.7 deg², we evaluate the photometric redshift to identify massive (M_* ∼ 10¹¹ M_☉) galaxies with suppressed star formation. These galaxies show a prominent Balmer break feature at z ∼ 4, suggestive of an evolved stellar population. We then conduct follow-up K'-band imaging with AO for the five brightest galaxies (K_AB,total = 22.5 ∼ 23.4). Compared to lower redshift ones, QGs at z ∼ 4 have smaller physical sizes of effective radii r_eff = 0.2-1.7 kpc. The mean size measured by stacking the four brightest objects, a more robust measurement, is r_eff = 0.5 kpc. This is the first measurement of the rest-frame optical sizes of QGs at z ∼ 4. We evaluate the robustness of our size measurements using simulations and find that our size estimates are reasonably accurate with an expected systematic bias of ∼0.2 kpc. If we account for the stellar mass evolution, massive QGs at z ∼ 4 are likely to evolve into the most massive galaxies today. We find their size evolution with cosmic time in the form of log(r_e/kpc)=-0.56+1.91log(t/Gyr). Their size growth is proportional to the square of stellar mass, indicating that size-stellar mass growth is driven by minor dry mergers.