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Quiescent Compact Galaxies at Intermediate Redshift in the COSMOS Field II. The Fundamental Plane of Massive Galaxies

We examine the relation between surface brightness, velocity dispersion and size$-$the fundamental plane$-$for quiescent galaxies at intermediate redshifts in the COSMOS field. The COSMOS sample consists of $\sim150$ massive quiescent galaxies with an average velocity dispersion $σ\sim 250$ km s$^{-1}$ and redshifts between $0.2<z<0.8$. More than half of the galaxies in the sample are compact. The COSMOS galaxies exhibit a tight relation ($\sim0.1$ dex scatter) between surface brightness, velocity dispersion and size. At a fixed combination of velocity dispersion and size, the COSMOS galaxies are brighter than galaxies in the local universe. These surface brightness offsets are correlated with the rest-frame $g-z$ color and $D_n4000$ index; bluer galaxies and those with smaller $D_n4000$ indices have larger offsets. Stellar population synthesis models indicate that the massive COSMOS galaxies are younger and therefore brighter than similarly massive quiescent galaxies in the local universe. Passive evolution alone brings the massive compact quiescent COSMOS galaxies onto the local fundamental plane at $z = 0$. Therefore, evolution in size or velocity dispersion for massive compact quiescent galaxies since $z\sim1$ is constrained by the small scatter observed in the fundamental plane. We conclude that massive compact quiescent galaxies at $z\lesssim1$ are not a special class of objects but rather the tail of the mass and size distribution of the normal quiescent galaxy population.