First results from the 3D-HST survey: The striking diversity of massive galaxies at z > 1

We present first results from the 3D-HST program, a near-IR spectroscopic survey performed with the Wide Field Camera 3 (WFC3) on the HST. We have used 3D-HST spectra to measure redshifts and Hα equivalent widths (EW _Hα) for a complete, stellar mass-limited sample of 34 galaxies at 1 < z < 1.5 with M _star > 10¹¹M _⊙ in the COSMOS, GOODS, and AEGIS fields. We find that a substantial fraction of massive galaxies at this epoch are forming stars at a high rate: the fraction of galaxies with EW_Hα >10 is 59%, compared to 10% among Sloan Digital Sky Survey galaxies of similar masses at z = 0.1. Galaxies with weak Hα emission show absorption lines typical of 2-4Gyr old stellar populations. The structural parameters of the galaxies, derived from the associated WFC3 F140W imaging data, correlate with the presence of Hα; quiescent galaxies are compact with high Sérsic index and high inferred velocity dispersion, whereas star-forming galaxies are typically large two-armed spiral galaxies, with low Sérsic index. Some of these star-forming galaxies might be progenitors of the most massive S0 and Sa galaxies. Our results challenge the idea that galaxies at fixed mass form a homogeneous population with small scatter in their properties. Instead, we find that massive galaxies form a highly diverse population at z > 1, in marked contrast to the local universe.