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 $\sigma \sim 250$ 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.
