Population Synthesis Models for Late Build-Up of the Red Sequence

We present population synthesis models designed to represent the star formation histories of L* red sequence galaxies (RSGs). Earlier work has shown that single-burst stellar populations (SSPs) are unable to match Balmer line strengths simultaneously at high and low redshift. We therefore consider alternative star formation histories in which RSGs contain intermediate-aged stars even at late epochs. The models are compared to Balmer Hdelta absorption strengths, U-B color data, and the number density of red sequence galaxies from z=1 to z=0. We find that quenched models (which consist of constant star formation histories truncated at regularly-spaced intervals) average to an RSG population that matches the data well, showing slow evolution in color and Balmer line strength and a rise in number density by a factor of a few after z=1. The data are best fit by a turn-on of quenching at redshifts z=1.5-2.