The reversal of the star formation-density relation in the distant universe

Aims. We study the relationship between the local environment of galaxies and their star formation rate ( SFR) in the Great Observatories Origins Deep Survey, GOODS, at z similar to 1. Methods. We use ultradeep imaging at 24 mu m with the MIPS camera onboard Spitzer to determine the contribution of obscured light to the SFR of galaxies over the redshift range 0.8=<= z <= 1.2. Accurate galaxy densities are measured thanks to the large sample of similar to 1200 spectroscopic redshifts with high (similar to 70%) spectroscopic completeness. Morphology and stellar masses are derived from deep HST-ACS imaging, supplemented by ground based imaging programs and photometry from the IRAC camera onboard Spitzer. Results. We show that the star formation-density relation observed locally was reversed at z similar to 1: the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at earlier epochs (z>2) and at a lower level. We present a remarkable structure at z similar to 1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density and shows that it is the similar to 1-2 Mpc scale that affects most the star formation in galaxies at z similar to 1. The SFR of z similar to 1 galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/M-star) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at z similar to 1 present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Conclusions. These findings constrain the influence of the growth of large-scale structures on the star formation history of galaxies. Reproducing the SFR-density relation at z similar to 1 is a new challenge for models, requiring a correct balance between mass assembly through mergers and in-situ star formation at early epochs.