A Comparison of the Most Massive Quiescent Galaxies from z ∼ 3 to the Present: Slow Evolution in Size, and spheroid-dominated

We use Hubble Space Telescope imaging to study the structural properties of 10 of the most massive (M (3) 10(11.25) M-circle dot) quiescent galaxies (QGs) in the UKIDSS UDS at 2.5 < z < 3.0. The low spatial density of these galaxies required targeted WFC3 H160 imaging, as such systems are rare in existing surveys like CANDELS. We fit Sersic models to the 2D light profiles and find that the median half-light radius is Re similar to 3 kpc, a factor of similar to 3 smaller than QGs with similar masses at z similar to 0. Complementing our sample with similarly massive QGs at lower redshifts, we find that the median size evolves as Re proportional to H(z)(-0.85 perpendicular to 0.12) (or alternatively, Re proportional to H(1 + z)(-0.90 perpendicular to 0.12) . This rate of evolution is slower than that for lower mass QGs. When compared to low-redshift QGs, the axis ratio distribution for our high-redshift massive QG sample is most consistent with those in which spheroids are dominant. These observations point to earlier size growth among massive QGs that also resulted in spheroidal systems. Finally, we measured residual-corrected surface brightness profiles for our sample. These show that the Sersic parameterization is generally representative out to several effective radii and does not miss excess low surface brightness light. The sizes inferred from the light profiles therefore confirm the compactness of these most massive high-redshift QGs.