Gemini Deep Deep Survey. VI. Massive Hδ-strong Galaxies at z ≃ 1

We show that there has been a dramatic decline in the abundance of massive galaxies with strong Hδ stellar absorption lines from z ~ 1.2 to the present. These "Hδ-strong" (HDS) galaxies have undergone a recent and rapid break in their star formation activity. Combining data from the Gemini Deep Deep and Sloan Digital Sky surveys to make mass-matched samples (M_⋆ > 10^10.2 M_☉; with 25 and 50,255 galaxies, respectively), we find that the fraction of galaxies in an HDS phase has decreased from about 50% at z = 1.2 to a few percent today. This decrease in fraction is due to an actual decrease in the number density of massive HDS systems by a factor of 2-4, coupled with an increase in the number density of massive galaxies by ~30%. We show that this result depends only weakly on the threshold chosen for the Hδ equivalent width (EW) to define HDS systems (if greater than 4 Å) and corresponds to a (1 + z)^2.5±0.7 evolution. Spectral synthesis studies of the high-redshift population using the PÉGASE code, treating Hδ_A, EW[O II], D_n4000, and rest-frame colors, favor models in which the Balmer absorption features in massive HDS systems are the echoes of intense episodes of star formation that faded ≃1 Gyr prior to the epoch of observation. The z = 1.4-2 epoch appears to correspond to a time at which massive galaxies are in transition from a mode of sustained star formation to a relatively quiescent mode with weak and rare star formation episodes. We argue that the most likely local descendants of the distant massive HDS galaxies are passively evolving massive galaxies in the field and small groups.