  We present first results from the 3D-HST program, a near-IR spectroscopic
survey performed with the Wide Field Camera 3 on the Hubble Space Telescope. We
have used 3D-HST spectra to measure redshifts and Halpha equivalent widths for
a stellar mass-limited sample of 34 galaxies at 1<z<1.5 with M(stellar)>10^11
M(sun) 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 Halpha equivalent widths >10 A is 59%, compared
to 10% among SDSS galaxies of similar masses at z=0.1. Galaxies with weak
Halpha emission show absorption lines typical of 2-4 Gyr old stellar
populations. The structural parameters of the galaxies, derived from the
associated WFC3 F140W imaging data, correlate with the presence of Halpha:
quiescent galaxies are compact with high Sersic index and high inferred
velocity dispersion, whereas star-forming galaxies are typically large
two-armed spiral galaxies, with low Sersic 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.
