Dynamical Models of Elliptical Galaxies in Z = 0.5 Clusters: Ii. Mass-to-light Ratio Evolution without Fundamental Plane Assumptions

We study M/L evolution of early-type galaxies using dynamical modeling of resolved internal kinematics. This makes fewer assumptions than Fundamental Plane (FP) studies and provides a powerful new approach for studying galaxy evolution. We focus on the sample of 25 galaxies in clusters at z ≈ 0.5 modeled in Paper I. For comparison we compile and homogenize M/L literature data for 60 nearby galaxies that were modeled in comparable detail. The nearby sample obeys log(M/L)B = Z + S log(σeff/[200 kms ]), with Z = 0.896± 0.010, S = 0.992± 0.054, and σeff the effective velocity dispersion. The z ≈ 0.5 sample follows a similar relation but with lower zeropoint. The implied M/L evolution is ∆ log(M/L)/∆z = −0.457± 0.046 (random)± 0.078 (systematic), consistent with passive evolution following high-redshift formation. This agrees with the FP results for this sample by van Dokkum & van der Marel. This confirms that FP evolution tracks M/L evolution, which is an important verification of the assumptions that underly FP studies. However, while we find more FP evolution for galaxies of low σeff (or low mass), the dynamical M/L evolution instead shows little trend with σeff . We argue that this difference can be plausibly attributed to a combination of two effects: (a) evolution in structural galaxy properties other than M/L; and (b) the neglect of rotational support in studies of FP evolution. The results leave the question open whether the low-mass galaxies in the sample have younger population ages than the high-mass galaxies. This highlights the general importance in the study of population ages for complementing dynamical measurements with broad-band colors or spectroscopic population diagnostics. Subject headings: galaxies: clusters: individual (CL3C295, CL0016+16, CL1601+42) — galaxies: evolution — galaxies: formation — galaxies: kinematics and dynamics.