The Fundamental Plane in the LEGA-C Survey: unraveling the $M/L$
  variations of massive star-forming and quiescent galaxies at $z\sim0.8$
  We explore the connection between the kinematics, structures and stellar
populations of massive galaxies at $0.6<z<1.0$ using the Fundamental Plane
(FP). Combining stellar kinematic data from the Large Early Galaxy Astrophysics
Census (LEGA-C) survey with structural parameters measured from deep Hubble
Space Telescope imaging, we obtain a sample of 1419 massive ($\log(M_*/M_\odot)
>10.5$) galaxies that span a wide range in morphology, star formation activity
and environment, and therefore is representative of the massive galaxy
population at $z\sim0.8$. We find that quiescent and star-forming galaxies
occupy the parameter space of the $g$-band FP differently and thus have
different distributions in the dynamical mass-to-light ratio ($M_{\rm
dyn}/L_g$), largely owing to differences in the stellar age and recent star
formation history, and, to a lesser extent, the effects of dust attenuation. In
contrast, we show that both star-forming and quiescent galaxies lie on the same
mass FP at $z\sim 0.8$, with a comparable level of intrinsic scatter about the
plane. We examine the variation in $M_{\rm dyn}/M_*$ through the thickness of
the mass FP, finding no significant residual correlations with stellar
population properties, S\'ersic index, or galaxy overdensity. Our results
suggest that, at fixed size and velocity dispersion, the variations in $M_{\rm
dyn}/L_g$ of massive galaxies reflect an approximately equal contribution of
variations in $M_*/L_g$, and variations in the dark matter fraction or initial
mass function.
