2017ApJ...847...20W

We analyze the stellar populations and evolutionary history of bulge-dominated field galaxies at redshifts 0.3 z 1.2 as part of the Gemini/Hubble Space Telescope (HST) Galaxy Cluster Project (GCP). High signal-to-noise optical spectroscopy from the Gemini Observatory and imaging from the HST are used to analyze a total of 43 galaxies, focusing on the 30 passive galaxies in the sample. Using the size-mass and velocity dispersion-mass relations for the passive field galaxies we find no significant evolution of sizes or velocity dispersions at a given dynamical mass between z ~ 1 and the present. We establish the Fundamental Plane and study mass-to-light (M/L) ratios. The M/L versus dynamical mass relation shows that the passive field galaxies follow a relation with a steeper slope than the local comparison sample, consistent with cluster galaxies in the GCP at z = 0.86. This steeper slope indicates that the formation redshift is mass dependent, in agreement with "downsizing," meaning that the low-mass galaxies formed their stars more recently while the high-mass galaxies formed theirs at higher redshift. The zero-point differences of the scaling relations for the M/L ratios imply a formation redshift of z_form=1.35_–0.07^+0.10 for the passive field galaxies. This is consistent with the (Hδ_A+Hγ_A)^′ line index which implies a formation redshift of z_form=1.40_–0.18^+0.60.