The Stellar Mass Fundamental Plane and Compact Quiescent Galaxies at z <
  0.6
  We examine the evolution of the relation between stellar mass surface
density, velocity dispersion and half-light radius$-$the stellar mass
fundamental plane$-$for quiescent galaxies at $z<0.6$. We measure the local
relation from galaxies in the Sloan Digital Sky Survey and the intermediate
redshift relation from $\sim500$ quiescent galaxies with stellar masses $10
\lesssim \mathrm{log}(M_\ast/M_\odot) \lesssim 11.5$. Nearly half of the
quiescent galaxies in our intermediate redshift sample are compact. After
accounting for important selection and systematic effects, the velocity
dispersion distribution of galaxies at intermediate redshifts is similar to
galaxies in the local universe. Galaxies at $z<0.6$ appear to be smaller
($\lesssim0.1$ dex) than galaxies in the local sample. The orientation of the
stellar mass fundamental plane is independent of redshift for massive quiescent
galaxies at $z<0.6$ and the zero-point evolves by $\sim 0.04$ dex. Compact
quiescent galaxies fall on the same relation as the extended objects. We
confirm that compact quiescent galaxies are the tail of the size and mass
distribution of the normal quiescent galaxy population.
