The Evolution of Balmer Absorption Line Strengths in E/S0 Galaxies from z=0 to z=0.83

We present new results from a systematic study of absorption line strengths of galaxies in clusters approaching redshifts of unity. In this paper, we specifically compare the strengths of the high-order Balmer absorption features of Hgamma and Hdelta in E/S0s in the four clusters Abell 2256 (z=0.06), CL1358+62 (z=0.33), MS2053--04 (z=0.58), and MS1054--03 (z=0.83). By comparing the correlation of Balmer line strength with velocity dispersion for E/S0s in the three clusters, we find moderate evolution in the zero-point of the (Hdelta_A+Hgamma_A)-sigma relation with redshift. The trend is consistent with passive evolution of old stellar populations. Under the assumption that the samples can be compared directly, we use single-burst stellar population synthesis models to constrain the last major occurrences of star-formation in the observed E/S0s to be z_f > 2.5 (95% confidence). We have compared the evolution of the Balmer absorption with the evolution of the B-band fundamental plane and find that simple stellar population models agree very well with the data. While the best agreement occurs with a low value for Omega_m, the data provide strong confirmation of the time-evolution in recent stellar population models.