the dynamical $M/L$ evolution instead shows little trend with $\sigma_{\rm
    eff}$. 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.
author:
- 'Roeland P. van der Marel'
- 'Pieter G. van Dokkum'
title: |
    Dynamical Models of Elliptical Galaxies in $z = 0.5$ Clusters:\
    II. Mass-to-Light Ratio Evolution without Fundamental Plane Assumptions
---

Introduction {#s:intro}
============

A stellar population of fixed mass fades as it ages. The predicted mass-to-light ratio $M/L$ of a galaxy therefore depends strongly on its age (e.g., Bruzual & Charlot 2003) and $M/L$ measurements can constrain galaxy formation times. The $M/L$ of a galaxy cannot generally be accurately constrained from characteristics of the observed light (e.g., broad-band colors and line-strength indices) alone. This is because low-mass stars contribute the bulk of the mass, but only a 